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Microbiology and Biotechnology Letters

Review(총설)

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Environmental Microbiology (EM)  |  Biodegradation and Bioremediation

Microbiol. Biotechnol. Lett. 2024; 52(4): 343-357

https://doi.org/10.48022/mbl.2408.08003

Received: August 5, 2024; Revised: September 16, 2024; Accepted: October 15, 2024

Potential of Freshwater Cyanobacterial Harmful Algal Bloom Biomass for Biomethane Production via Anaerobic Digestion

Jong Myong Park1, Young-Hyun You2, Nam Seon Kang3, Eunsue Cho4,5, Chang-Gi Back6, and Ji Won Hong4,7*

1Water Quality Research Institute, Waterworks Headquarters Incheon Metropolitan City, Incheon, Republic of Korea
2Species Diversity Research Division, National Institute of Biological Resources, Incheon 22689, Republic of Korea
3Department of Taxonomy and Systematics, National Marine Biodiversity Institute of Korea, Seocheon 33662, Republic of Korea
4Department of Hydrogen and Renewable Energy, Kyungpook National University, Daegu 41566, Republic of Korea
5Daesung Eco-Energy, Daegu 42926, Republic of Korea
6Department of Environmental Horticulture and Landscape Architecture, Environmental Horticulture, Dankook University, Cheonan 31116, Republic of Korea
7Advanced Bio-Resource Research Center, Kyungpook National University, Daegu 41566, Republic of Korea

Correspondence to :
Ji Won Hong,         jwhong@knu.ac.kr

Abstract

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The increasing incidence of freshwater cyanobacterial harmful algal blooms (cyanoHABs), driven by anthropogenic activities, poses significant environmental challenges and risks to water quality management. However, these blooms also represent an untapped resource for renewable energy production, particularly through the conversion of biomass to biomethane via anaerobic digestion (AD). AD not only converts cyanoHAB biomass into renewable energy but also degrades harmful microcystins, transforming environmental hazards into energy resources. This review explores the potential of cyanobacterial biomass as a substrate for biomethane production highlighting the dual benefits of alleviating environmental impacts and contributing to the renewable energy sector. It discusses the composition and characteristics of cyanobacterial biomass, the process and efficiency of anaerobic digestion, and the practical challenges and opportunities in integrating this approach into existing waste management and energy systems. This paper aims to bridge the gap between managing environmental hazards and generating renewable energy offering insights into research advancements, commercial scalability, and policy implications.

Keywords: Cyanobacterial bloom, biomethane, anaerobic digestion, microcystin, renewable energy, environmental management

Graphical Abstract


Introduction

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Freshwater ecosystems worldwide are increasingly plagued by cyanobacterial harmful algal blooms (cyano-HABs) which pose threats to water quality, aquatic life, and public health [1−3]. Similarly, the frequency and intensity of cyanoHABs in Korea have dramatically increased due to human activities, urbanization, and industrialization. This rise has created serious social and environmental issues, ultimately affecting the supply of clean drinking water [4−6]. Thus, removing harmful cyanobacterial blooms from freshwater bodies is crucial for maintaining water quality and ecosystem health. Various strategies have been developed to address this issue, each with its own advantages and limitations, and vary depending on the specific environmental conditions and the scale of the bloom [7−9]. The most common methods include physical and chemical treatments, and nutrient reduction [10].

Physical removal of cyanoHABs is generally not favored on a large scale due to logistical challenges and high costs. Although effective for immediate localized control in smaller water bodies, such as ponds or near the shorelines of larger lakes, it becomes impractical for larger water bodies or managing widespread blooms. However, physical removal can be considered in specific scenarios where immediate water quality improvement is necessary, such as in swimming areas, around water intake locations, or in small, contained water bodies where other methods might be less feasible. Methods like skimming and suction harvesting are difficult to scale up in large lakes or reservoirs, where blooms can cover vast areas and thus require expensive equipment and extensive labor. Despite these challenges, physical removal is often practiced near water intake areas in Korea during severe cyanoHAB periods. Therefore, proper disposal of the removed biomass is essential to prevent the decomposed material from releasing nutrients back into the water body, which can exacerbate the problem.

Landfilling is often considered a viable disposal method for physically harvested HAB biomass. However, it carries significant environmental considerations, such as the potential release of microcystins (MCs) and green-house gases. Nevertheless, recent research highlights the potential of anaerobic digestion (AD) of cyanoHAB biomass to convert this into a clean energy opportunity by producing biomethane [11−13]. Methane (CH4), recognized as one of the cleanest hydrocarbon fuels, holds substantial promise as a renewable energy source poised to enhance a sustainable future. Anaerobic biodegradation is also reported as an effective method for breaking down MCs into non-toxic products [14−16]. Specifically, methanogenic biodegradation in this process converts these toxins into non-hazardous compounds by utilizing them as a nitrogen source. Additionally, AD has the potential not only to degrade MCs but also to produce CH4 from cyanobacterial biomass.

Biomethane offers considerable potential as a renewable energy source that can contribute to a more sustainable future [17]. As global energy demands increase and environmental challenges intensify, the shift toward renewable energy has become a pressing global priority. Among the various solutions, biomethane production stands out for its ability to simultaneously reduce green-house gas emissions, manage waste, and support sustainable energy systems. Biomethane, generated through methanogenesis, converts agricultural, industrial, and municipal waste into a clean, renewable fuel source [18]. This process not only minimizes reliance on fossil fuels but also integrates seamlessly into existing natural gas infrastructure providing a scalable and reliable alternative for economies at all stages of development. Unlike intermittent renewable sources like wind and solar, biomethane offers a consistent energy supply making it particularly valuable for decarbonizing sectors that are difficult to electrify, such as heavy industry, transportation, and heating [19].

Beyond energy production, biomethane plays a critical role in supporting a circular economy by repurposing waste into valuable resources. The AD process not only generates CH4 but also produces digestate which can be used as an organic fertilizer enhancing soil health and reducing the need for chemical inputs [20]. This closed-loop system effectively tackles both energy and waste management challenges promoting sustainability and reducing environmental degradation. In freshwater eco-systems affected by cyanoHABs, biomethane production offers a unique opportunity to transform environmental hazards into energy assets, with the potential to mitigate the harmful effects of blooms while producing renewable energy. Hence, despite the current restriction on the direct landfilling of organic wastes in Korea [21, 22], landfilling of harvested cyanoHABs should still be considered as a disposal method.

This review paper explores the feasibility of using cyanobacterial biomass as a substrate for biomethane production in Korea. It delves into the potential of this approach to bolster energy sustainability, supported by an in-depth examination of relevant scientific literature. By exploring how biomethane can be integrated into national energy strategies and used to address both energy and waste challenges, this research underscores its transformative potential in advancing global climate goals. It is hoped to highlight the multifunctional benefits of biomethane in creating a more sustainable and resilient energy future emphasizing its capacity to turn environmental pollutants, such as cyanoHAB biomass, into a clean energy solution.

Insights into AD of CyanoHAB Biomass for Biomethane Production

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Environmental impacts of cyanoHABs

Depending on the species involved, cyanobacterial blooms can produce a variety of toxins, posing significant risks to wildlife, domestic animals, and humans [2, 23, 24]. These blooms occur when cyanobacteria proliferate excessively in water bodies, often due to nutrient pollution, particularly nitrogen and phosphorus from agricultural runoff, wastewater discharge, and other anthropogenic activities.

Ecosystem disruption. CyanoHABs drastically alter aquatic ecosystems by reducing light penetration and decreasing oxygen levels through their metabolic processes and subsequent decay. This leads to hypoxic or anoxic conditions, which can cause fish kills, disrupt food chains, and reduce biodiversity [1]. Additionally, cyanoHABs can outcompete other phytoplankton species, creating monocultures that disrupt the balance of the aquatic food web [25]. This results in a loss of biodiversity and harms organisms that depend on diverse phytoplankton populations for sustenance.

Toxin production. Many cyanobacteria produce harmful toxins, known as cyanotoxins, including anatoxins, microcystins, saxitoxins etc. As shown in Table 1, these toxins pose serious risks to human and animal health, leading to issues such as liver damage, neurotoxicity, and gastrointestinal distress in both humans and wild-life [26, 27]. Cyanotoxins can contaminate drinking water supplies, necessitating costly water treatment processes [28, 29]. In extreme cases, exposure to these toxins has led to the closure of water bodies used for recreation and drinking water reservoirs.

Table 1 . Cyanobacterial toxins, mechanisms, effects, and key genera (reproduced from [30]).

Toxin categoryKey effects on organismsCompound typeMechanism of actionSymptoms/effects in organismsMain genera
NeurotoxinsDisruption of nerve functionAlkaloidBlocks nicotinic acetylcholine receptors, preventing depolarizationRespiratory arrest, convulsions, rapid deathAnabaena, Aphanizomenon, Oscillatoria
Blocks sodium channels on nerve axonsParalysis, respiratory failureAnabaena, Lyngbya, Cylindrospermopsis
HepatotoxinsLiver tissue damageCyclic peptideInhibits protein phosphatases, causing oxidative stressLiver necrosis, jaundice, liver dysfunctionMicrocystis, Nodularia, Anabaena
General cellular toxicityAlkaloidInhibits glutathione synthesis, damaging cellular organellesGeneral toxicity, liver dysfunctionLyngbya, Aphanizomenon, Anabaena
IrritantsTissue irritationLipopolysaccharideTriggers inflammatory responsesSkin irritation, mucosal discomfortAll cyanobacteria
Dermal ToxinsSkin and mucosal irritationAmideActivates inflammatory pathwaysRash, blistering, irritationLyngbya
Gastrointestinal ToxinsDigestive distressAlkaloidAlters intestinal lining, causing irritationAbdominal pain, vomiting, diarrheaAnabaena, Microcystis


Economic and social impacts. The environmental degradation caused by cyanoHABs has significant economic ramifications [31]. Fisheries, tourism, and recreational activities can suffer severe losses in regions affected by blooms. Costs are often incurred due to the need for water treatment, wildlife restoration, and the negative impact on property values along affected water bodies. In some areas, the presence of cyanoHABs can also have social implications, especially in communities that rely on fishing or tourism as a primary source of income. Additionally, local authorities are often forced to invest in bloom monitoring and control efforts, diverting resources from other critical areas.

Climate change exacerbation. Climate change is expected to intensify the frequency and severity of cyanoHABs [32]. Warmer water temperatures, increased CO2 levels, and more intense precipitation events leading to nutrient runoff all contribute to more favorable conditions for cyanobacteria proliferation [33, 34]. As a result, we are likely to see more frequent and wide-spread blooms in the coming years. Given these significant environmental impacts, cyanoHABs represent a pressing challenge that requires coordinated efforts in environmental management, regulation, and mitigation strategies to preserve water quality and protect ecosystems.

In Korea, according to the Enforcement Decree of the Water Environment Conservation Act [23], a cyanobacterial bloom is defined based on the cell counts of four specific cyanobacteria genera including Anabaena, Aphanizomenon, Microcystis, and Oscillatoria (Table 2).

Table 2 . Algal bloom alert levels in Korea (reproduced from [23]).

LevelCautionWarningOutbreak
Number of cyanobacteria (cells/ml)11,000-10,00010,000-1,000,000≥1,000,000
Concentration of cyanotoxins (μg/l)2N.A.≥10N.A.

1Anabaena, Aphanizomenon, Microcystis, & Oscillatoria

2Microcystin-LR, RR, LA, YR, LF, & LY

Biochemical Characteristics of cyanoHABs Relevant to AD

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Anabaena

Anabaena is characterized by long chains of filamentous cells, which include specialized heterocysts crucial for nitrogen fixation [35]. It predominantly inhabits nutrient-rich freshwater environments and often produces neurotoxins and hepatotoxins, such as anatoxin-a and MCs. These toxins adversely affect the nervous systems and livers of both animals and humans [36, 37]. Its biomass is high in proteins, attributable to its nitrogen-fixing ability, and is rich in carbohydrates, chlorophyll-a, and phycobiliproteins, which are essential for photosynthesis and contribute to its distinctive blue-green color [38, 39]. Anabaena plays a significant role in the nitrogen cycling of aquatic ecosystems and poses a risk to water safety, impacting aquatic life and potentially entering the human food chain. Additionally, under stress conditions, such as nutrient limitation, Anabaena accumulates lipids that are vital for the cell membrane and considered for biofuel production due to their high fatty acid content [40, 41]. This adaptive mechanism allows Anabaena to store energy in the form of lipids, aiding its survival in varying conditions when external resources are limited, making its understanding crucial for ecological studies, water quality monitoring, and managing public health risks associated with water-borne toxins.

Aphanizomenon

Aphanizomenon is a filamentous cyanobacterium that forms long, branched chains and extensive mats on water surfaces. It thrives in freshwater, brackish, and marine environments, often forming blooms. Like Anabaena, Aphanizomenon contains heterocysts [42, 43], enriching the nitrogen content of aquatic ecosystems. It produces cyanotoxins, including the neurotoxin anatoxin-a and the hepatotoxin cylindrospermopsin, which pose serious threats to both aquatic and terrestrial life, including humans [44, 45]. The biomass of Aphanizomenon, rich in nitrogenous compounds, proteins, carbohydrates, and lipids, plays a crucial role in nutrient cycling and supports the aquatic food web [46− 48]. Given its ecological significance and potential hazards, Aphanizomenon is a critical focus in the study of aquatic biology and environmental management, particularly concerning water quality and public health.

Microcystis

Microcystis is a genus of freshwater cyanobacteria known for forming colonies that contribute to harmful cyanobacterial blooms, often visible as green scum on water surfaces. These blooms are prevalent in warm, nutrient-rich environments such as lakes, rivers, and reservoirs, posing significant environmental concerns due to their impact on water quality and ecosystem health worldwide [49]. The colonies, composed of small, spherical cells, form dense mats on water surfaces. Their buoyancy, regulated by gas vesicles, optimizes their position for light exposure and nutrient uptake. The primary ecological impact of these blooms is eutrophication, a process where nutrient enrichment leads to excessive algae growth and oxygen depletion, creating dead zones inhospitable to aquatic life. This phenomenon disrupts biodiversity and food webs [50]. The most significant concern associated with Microcystis is the production of MCs, cyclic heptapeptide toxins that inhibit phosphatases essential for cellular regulation. These toxins primarily affect the liver, potentially causing acute and chronic damage, tumorigenic effects, and gastrointestinal issues [51−54]. Addressing the challenges posed by Microcystis requires integrated approaches that consider environmental, health, and socio-economic factors, emphasizing nutrient management and research into effective, sustainable control methods to mitigate the impact of these harmful algal blooms. The biomass of Microcystis, predominantly composed of carbohydrates and proteins with a high proportion of mucilage, helps the colonies to float and form scums on water surfaces. The biomass composition varies according to environmental factors and strain differences [55, 56]. Understanding the biomass composition of Microcystis is vital for ecological studies, control of harmful algal blooms through nutrient management, and potential biotechnological applications such as biofuel production and bioremediation.

Oscillatoria

Oscillatoria, present in various aquatic habitats including both freshwater and marine environments [57−59], can move via its filaments and plays a key role in nutrient cycling. While it produces toxins less frequently than other cyanobacteria like Microcystis, Oscillatoria can also generate MCs under certain nutrient levels and environmental stressors [60−62]. These toxins can contaminate water supplies and pose health risks. The biomass of Oscillatoria mainly consists of proteins and carbohydrates [63−65], with some lipid content [66], essential for cell membrane integrity and energy storage.

In summary, the toxicity and biomass compositions of cyanobacteria such as Anabaena, Aphanizomenon, Microcystis, and Oscillatoria are essential for understanding their ecological impacts and the associated risks of their blooms. Each genus possesses unique traits that affect their composition and potential hazards as shown in Table 3. Understanding these characteristics is crucial for managing risks related to cyanobacterial blooms in water bodies, particularly in water treatment and ecological balance maintenance. These traits also inform the development of strategies for harnessing beneficial aspects of cyanobacteria, such as bioremediation and biofuel production.

Table 3 . Damage or harm caused by exposure to cyanotoxins produced from four major cyanobacteria (reproduced from [29, 30]).

GeneraHepatotoxicityNeurotoxicityDermatotoxicityIrritation
AnabaenaOO-O
Aphanizomenon-O-O
MicrocystisO--O
OscillatoriaOOOO

Potential for Biomethane Production from cyanoHABs

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Using cyanobacterial bloom biomass for biomethane production offers a promising approach to managing harmful algal blooms and generating renewable energy [16, 67]. This method involves processing the biomass through AD, where microorganisms decompose organic matter in an oxygen-free environment. The general lipid, protein, and carbohydrate composition of the four major cyanobacterial genera is shown in Table 4.

Table 4 . General carbohydrate, lipid, and protein composition of four major harmful cyanobacteria.

GenusCarbohydratesLipidsProteinsReference
Anabaena25-304-743-56[68-72]
Aphanizomenon32362[68, 71]
Microcystis10-3010-2540-60[55]
Oscillatoria15-3510-2030-55[55]


The usual composition of the genus Anabaena is reported to be 25−30% carbohydrates, 4−7% lipids, and 43−56% proteins. However, under specific environmental conditions, it has been reported that the lipid and carbohydrate contents in Anabaena can reach as high as 46.9% and 40.7%, respectively [40, 73]. Although research on the biochemical composition of the genus Aphanizomenon is scarce, it is typically reported to contain 3% carbohydrates, 23% lipids, and 62% proteins (Table 4). A recent study, however, indicated that the protein content of Aphanizomenon is only about 10.3%[74]. It is generally accepted that the average composition of the genus Microcystis is approximately 10−30% carbohydrates, 10−25% lipids, and 40−60%proteins, although variations exist based on species [55, 74−78]. Typically, Oscillatoria species contain approximately 15−35% carbohydrates, 10−20% lipids, and 30− 55% proteins [59, 79].

Cyanobacteria demonstrate significant variation in their chemical composition due to the significant variety within the phylum [80, 81]. However, certain general distinctions can be observed when comparing the cellular content of cyanobacteria to that of eukaryotic microalgae and plants [82]. Under optimal growth conditions, cyanobacteria typically have relatively low carbohydrate content (10−30%), intermediate lipid levels (5− 10%), and high protein levels (40−79%). In contrast, microalgae contain 5−60% carbohydrates, 1−40% lipids, and 20−50% proteins, while plants consist of 70−80%carbohydrates, 1−15% lipids, and 1−10% proteins [77, 83].

Although relatively few publications have reported on AD of prokaryotic cyanobacteria species, it is reported that common cyanobacterial genera such as Anabaena, Aphanizomenon, Arthrospira, and Synechocystis are considered suitable substrates for AD, offering reasonable specific methane yields [84]. Likewise, Table 5 shows the CH4 yields from four major harmful cyanobacteria ranging from 108.0 to 522.0 ml CH4/g (volatile solid) CH4/g VS (volatile solid). Cyanobacteria store carbon in various forms to manage energy and nutrient availability [82], with the dominant carbon sinks being glycogen [85−87] and exopolysaccharide layers, which protect cells, aid in biofilm formation, and serve as carbon reserves. Additionally, some cyanobacteria are known to accumulate polyhydroxyalkanoates, biodegradable polymers used as carbon reserves [87]. Carbon storage also extends to proteins and lipids, which not only function structurally and functionally within the cell but also act as carbon and energy reserves.

Table 5 . Methane yields from four major cyanobacterial genera.

SpeciesMethane yield (ml CH4/g VS)Reference
Anabaena flos-aquae312.0-318.0[88]
Anabaena planctonica284.2[89]
Anabaena planctonica362.0-522.0[90]
Aphanizomenon ovalisporum287.7[89]
Aphanizomenon ovalisporum388.0-436.0[90]
Microcystis sp.189.9[14]
Microcystis sp. UTEX B 2678241.3[91]
Microcystis spp.140.5[92]
Microcystis spp.108.0-160.0[93]
Oscillatoria sp. SAG 76.79356.0[94]


Numerous researchers have linked CH4 yield from eukaryotic microalgae to their composition, particularly the content of lipids, carbohydrates, and proteins. However, experimental data do not show a strong correlation between these macromolecules and CH4 yield across different microalgal species [95]. Theoretically, lipids should have the most significant impact on CH4 yield, but practical data suggest that lipids are not the primary source of CH4 resulting in a weak correlation. This indicates that the ratio of macromolecules alone is not the key determinant of methane yield. Instead, the content of inert organic matter, such as the cell wall, may play a more critical role [96]. These findings suggest that the simple composition of microalgae biomass cannot be the main factor when selecting the best microalgal strain for CH4 production. More important factors include the biomass production rate and cell wall content.

Practical Challenges and Future Outlook

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Practical applications of biomethane from cyanoHABs

The potential for cyanobacteria-based biomethane production holds significant promise for the renewable energy sector, but its real-world application faces several challenges. The use of cyanoHABs for biomethane production presents several feasibility challenges.

One major issue is the intermittent supply of biomass throughout the year as cyanobacterial blooms are seasonal and not consistently available. Ensuring a year-round biomass supply would require either large-scale storage or complementary biomass sources, both of which increase logistical complexity and cost. Additionally, the large-scale collection of bloom biomass requires specialized equipment and incurs significant operational costs [97−98]. Harvesting cyanobacteria from extensive freshwater systems can be resource-intensive which may limit the scalability of this approach.

Another concern is the potential release of toxins during harvesting and processing, as certain cyanobacteria produce harmful substances. Managing these toxins safely adds another layer of complexity to the process. The presence of MCs can inhibit microbial activity and pose health risks. MCs are a group of monocyclic heptapeptide hepatotoxins characterized by a common structural motif. The general structure of these compounds can be described as cyclo-(D-Ala-X-D-MeAsp-Z-Adda-D-Glu-Mdha) where X and Z represent variable L-amino acids, and Adda is the β-amino acid residue of 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid [99]. To date, over 100 analogs of MCs have been identified with MC-LR being the most toxic and abundant variant [100−102]. Recent studies have shown that MCs can be biodegraded in both oxic and anoxic environments, with specific microorganisms capable of utilizing MCs as their sole nitrogen source [14, 16]. Despite this, the understanding of MC biodegradation mechanisms under anaerobic conditions remains limited, with only a few mechanisms reported to date [15, 103]. Notably, both biomethane production and MC biodegradation have been successfully demonstrated under methanogenic conditions highlighting the potential for integrated cyanoHABs treatment and bioenergy recovery processes. These findings, detailed in Table 6, emphasize the importance of further research to fully elucidate the anaerobic biodegradation pathways of MCs.

Table 6 . Removal of microcystin in cyanobacterial biomass through anaerobic digestion.

SubstratesMicrocystin removal efficiencyReference
Cyanobacterial bloom mixture1,220.19 (initial) to 35.17 mg/l (effluent)[14]
Mixture of algae bloom (Microcystis dominant)748.69 to 304.29 μg/g TS (intracellular)
102.93 to 61.37 μg/g TS (intracellular)		[104]
Microcystis sp.1.20 to 0.20-0.35 μg/l (liquid phase)
1,393.33 μg/L to 4.16-11.51 μg/l (sludge phase)		[105]


In general, microalgae pose challenges for biogas generation in the AD process for two main reasons including their high recalcitrance to microbial hydrolysis due to a rigid cell wall and their unfavorable low carbon-to-nitrogen (C/N) ratio resulting from a high protein content [106]. The cell wall resistance of microalgae can be reduced through various pretreatments. However, these methods can lead to increased investment costs for biomass processing [107]. Nonetheless, the cell walls of cyanobacteria are generally less rigid due to their simpler peptidoglycan-based structure [84, 108, 109], whereas eukaryotic microalgae have more complex and rigid cell walls due to the presence of additional polysaccharides and other strengthening components [110−112]. Therefore, cyanobacteria offer several advantages, such as easier decomposition and reduced processing costs, over eukaryotic microalgae in AD for biogas generation. However, the low C/N ratio of cyanobacteria in AD remains an unavoidable issue, as cyanobacteria maintain a ratio of approximately 5.0 under balanced growth conditions [113, 114]. Similarly, relatively low values compared to other common AD substrates have been reported for four cyanobacterial genera (Table 7). The optimal ratio for AD typically ranges from 20:1 to 30:1 [115]. This range provides a balanced environment for the microbial communities involved in the digestion process, ensuring the efficient breakdown of organic material and optimal CH4 production.

Table 7 . C/N ratios of four major cyanobacteria.

SpeciesC/N ratioReference
Anabaena cylindrica4.5-8.0[116]
Anabaena sp.7.0[117]
Anabaena sp. KAC 63.43-4.95[118]
Anabaena sp. PCC 71204.37[119]
Aphanizomenon flos-aquae KAC 155.95-6.48[118]
Microcystis aeruginosa3.8-7.4[120]
Microcystis sp.6.0[117]
Microcystis sp.7.3-10.6[121]
Oscillatoria sp.7.0[122]


Due to the high protein content of cyanobacteria, toxic substances such as ammonia (NH3+), hydrogen sulfide (H2S), volatile fatty acids (VFAs), phenolic compounds, and organic acids can be released. These substances inhibit microbial activity and reduce biogas production. NH3+ and H2S, resulting from the degradation of protein and sulfur-containing amino acids respectively, are particularly harmful to methanogens [123, 124]. The accumulation of VFAs and organic acids can lower pH and disrupt the AD process [125]. Managing the C/N ratio [126] and employing pretreatment methods like NH3+ stripping [127] are essential to mitigate these toxic effects and ensure stable AD operation. Codigestion, which involves the simultaneous digestion of multiple feedstocks, offers a promising solution to address the challenges posed by the high protein content of cyanobacteria in AD. This approach can balance the C/N ratio by mixing cyanobacteria with co-substrates that have higher carbon content, thereby enhancing microbial activity and biogas production [128]. Additionally, co-digestion dilutes toxic substances, mitigating their inhibitory effects on methanogens and other microbes. The synergy of combined substrates often results in higher biogas yields, improved process stability, and consistent feedstock composition, which helps maintain stable pH levels and reduces the risk of disruptions. The effectiveness of co-digestion relies on the availability and consistent supply of suitable co-substrates. Common co-substrates used in AD for biogas production include various agricultural wastes such as manure from cows, pigs, and chickens, crop residues like corn stalks and rice husks, and fruit and vegetable wastes, as well as different types of seaweed [129−132]. Industrial wastes, including brewery waste, dairy processing waste, slaughterhouse waste, and food processing waste, are also utilized. Municipal wastes, such as sewage sludge, the organic fraction of municipal solid waste, and food waste, play a significant role as well. Additionally, energy crops like maize silage, grass silage, sorghum, and alfalfa, along with other organic wastes like fats, oils, and grease, glycerol, and bakery waste, are valuable co-substrates. Animal by-products, including blood, offal, and fish waste, further enhance the anaerobic digestion process, optimizing and maximizing CH4 production when combined in various proportions. Although the co-digestion of microalgae with carbon-rich feedstock has been proposed as a cost-effective and efficient approach, this has mainly been investigated with eukaryotic microalgae [128]. To the best of our knowledge, only a few investigations have been conducted with cyanobacteria, mostly focusing on Arthrospira and Microcystis, as shown in Table 8. This is likely due to the limited availability of sufficient biomass. It should be noted that Arthrospira, commonly known as Spirulina, is not considered harmful. In fact, it is widely regarded as a safe and highly nutritious cyanobacterium. Spirulina is often cultivated as a dietary supplement and food ingredient due to its high protein content (55−70%), as well as its vitamins and minerals [133]. Proteins are rich in nitrogen which contributes to a lower C/N ratio compared to other biomass sources that have higher carbohydrate or lipid content. Since Spirulina contains a high proportion of N-containing compounds, such as amino acids, its C/N ratio tends to be lower. Consequently, it is frequently chosen as a co-digestion substrate because of its availability from large-scale commercial cultivation.

Table 8 . Co-digestion of different substrates with cyanobacteria.

SpeciesCo-substratesReference
Arthrospira maximaSewage sludge[134]
Arthrospira platensisBarley straw, beet silage, & brown seaweed[135]
Arthrospira platensisSewage sludge[136]
Microcystis aeruginosaFood waste[137]
Microcystis spp.Corn straw[138]
Oscillatoria tenuisPig manure[139]


Various pretreatment methods have been proposed to enhance biogas productivity [84, 140]. Unlike eukaryotic microalgae, prokaryotic cyanobacteria have weaker cell walls making them more attractive as feedstock for AD. Since these cyanobacteria can avoid thermal, physical, chemical, and enzymatic treatments, cost reductions in AD can be achieved without risking CH4 yields.

Finally, ensuring the consistency and uniformity of the harvested biomass is difficult, as bloom composition can vary significantly based on location and environmental conditions, making the conversion process less predictable and efficient.

Economic challenges and scalability

The economic viability of cyanoHABs-based biomethane production faces several significant obstacles, particularly when considering scalability due to the challenges mentioned earlier. A high initial capital investment is required for infrastructure, including specialized equipment for harvesting, drying, and processing cyanobacteria. These costs are compounded by ongoing operational expenses, such as maintenance, energy consumption, and labor [141, 142]. The seasonal nature of cyanoHABs further complicates the economics, necessitating either large storage facilities for a year-round biomass supply or the use of supplementary biomass sources, both of which increase costs. The high cost of equipment needed to efficiently collect and process cyanobacterial blooms often outweighs potential savings from renewable energy production, further straining economic feasibility [143].

Another major challenge is the unpredictable nature of biomass yields. The volume and composition of cyano-HABs can vary significantly, making it difficult to accurately forecast production outputs, leading to inconsistent biomethane yields. This variability reduces the return on investment. Additionally, cyanobacteria’s low C/N ratio hampers CH4 production, often requiring supplementation with higher-carbon substrates, which adds further complexity and cost to the process.

Market competition from more established renewable energy sources, such as solar, wind, and traditional bioenergy, presents a significant hurdle. These alternatives benefit from mature supply chains, lower costs, and broader adoption. Without substantial financial incentives, technological advancements, or government subsidies, cyanobacteria-based biomethane production may struggle to compete in this crowded energy market.

Finally, scalability remains a critical issue. Ensuring a year-round supply of biomass is essential for utilizing existing anaerobic digesters in wastewater treatment plants or landfills for resource recovery. This would require either consistent production or reliable biomass supplementation, both of which pose logistical and economic challenges.

Policy implications of biomethane from cyanoHABs

Governments and policymakers play a pivotal role in shaping the future of biomethane production from cyanoHABs. One of the key areas where their support is essential is in research and development (R&D). Increased funding for R&D is crucial for advancing cyanoHABs-based biomethane production. This investment would help in identifying the most productive strains of cyanobacteria and refining anaerobic digestion technology, which is vital for efficiently converting organic material into biomethane. By prioritizing these advancements, governments can foster technological breakthroughs that make biomethane production more feasible and scalable.

In addition to R&D, financial incentives are critical for stimulating growth in this emerging industry. Biomethane production, especially from innovative sources like cyanoHABs, often involves substantial upfront costs. Subsidies, carbon credits, and tax breaks for companies investing in biomethane can help offset these expenses. Such incentives would also help reduce the price gap between biomethane and fossil fuels making renewable energy sources more competitive in the market. With the right financial framework, governments can encourage more businesses to enter the biomethane sector, driving both innovation and large-scale production.

Another important responsibility for policymakers is the development of clear and comprehensive regulations. These regulations need to address various aspects of biomethane production including waste management, carbon emissions, and integration with the existing energy grid. Establishing a well-defined regulatory framework will ensure that biomethane production is safe, environmentally responsible, and capable of contributing to national and global energy goals. Such policies would also provide companies with the certainty they need to invest in long-term projects, further solidifying biomethane as a viable alternative to fossil fuels.

Finally, fostering partnerships between industry and academia is crucial for translating scientific discoveries into practical applications. Governments and policymakers can encourage these collaborations by creating platforms for knowledge exchange and co-funding joint initiatives. Through such partnerships, academic research can be directly aligned with industry needs, speeding up the development of effective solutions and technologies. By promoting these collaborations, policy-makers can ensure that scientific progress in cyanobacteria-based biomethane production leads to real-world implementation and contributes to broader energy sustainability efforts.

Additional Cyanobacterial Candidates for CH4 Production

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Several genera of cyanobacteria are commonly associated with toxic blooms. Among these, Cylindrospermopsis, Lyngbya, Nodularia, Phormidium, and Synechococcus are of particular concern, in addition to the four main genera previously mentioned. To our knowledge, no studies have been conducted on CH4 production from these cyanobacterial genera via AD. However, when the category of cyanobacteria is broadened, a few studies on Arthrospira spp., Pseudanabaena sp., and Synechocystis sp. have been conducted (Table 9) which demonstrated their potential as AD substrates for CH4 production.

Table 9 . Methane yields from additional candidate cyanobacteria.

SpeciesMethane yield (ml CH4/g VS)Reference
Arthrospira maxima90-350[144]
Arthrospira maxima190.0[134]
Arthrospira platensis293.0[144]
Arthrospira platensis278.0[136]
Pseudanabaena sp. CY14-1322.5-451.8[84]
Synechocystis sp.380.0[89]

Concluding Remarks

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Biomethane production through AD involves microorganisms breaking down organic matter without O2 generating CH4 for renewable energy. Cyanobacterial blooms, abundant in many freshwater systems, offer a potential biomass source for this process. Utilizing cyanobacterial biomass for energy production not only helps mitigate the environmental impact of these blooms but also provides an incentive for their removal. The feasibility of using cyanobacterial biomass for AD to produce CH4 has been supported by research, though several challenges remain. Toxin degradation, process optimization, and economic viability are critical issues that need addressing. Effective degradation of cyanobacterial toxins is essential to ensure safety, while optimizing digestion conditions like pH, temperature, and retention time is crucial for maximizing CH4 yield. Additionally, the economic aspects of harvesting, transporting, and processing the biomass require thorough evaluation. Harnessing cyanobacterial blooms for biomethane production presents significant environmental and economic benefits. It can help control harmful blooms, improving water quality, and provides a clean, renewable energy source, reducing fossil fuel dependency. Moreover, AD can facilitate nutrient recycling, aiding in nutrient management in water bodies. Further research and pilot projects are necessary to optimize the process, enhance methane yield, and address the challenges posed by cyanobacterial toxins, turning an environmental threat into a valuable resource.

Integrating this biomass conversion into water management systems can help control bloom events while producing energy, thus improving water quality. However, this innovative solution requires robust policy support, technological investment, and ongoing research to optimize conversion processes and ensure the economic viability of large-scale applications. Further research into the detailed characteristics of different cyanobacterial strains will aid in designing more effective biomass-to-energy conversion systems, addressing challenges such as ammonia inhibition from protein breakdown, and optimizing pre-treatment processes to enhance the biodegradability of the biomass. The process not only generates biomethane but also produces a nutrient-rich digestate that serves as a biofertilizer, recycling essential nutrients like nitrogen and phosphorus, and reducing reliance on synthetic fertilizers.

In addition, stringent regulations in Korea strongly discourage and frequently prohibit the direct landfilling of any type of biomass, including organic materials like food waste and biodegradable substances such as seaweed. These rigorous waste management policies advocate for the recycling and repurposing of organic waste and apply similar restrictions to biomass from physically harvested cyanoHABs. However, landfilling cyanobacterial biomass could pose environmental risks if the methane generated during decomposition is not effectively captured and utilized. Consequently, it is recommended that any landfilling of cyanoHABs only occurs at sanitary landfills equipped with energy recovery systems. Korea is actively developing regulations to enhance the production and use of biogas focusing on reducing organic waste and promoting renewable energy sources. The Korean Ministry of Environment has introduced laws that require industries producing significant amounts of organic waste, such as livestock manure and food waste, to transition to biogas production [21]. Effective from December 31, 2023, these regulations specify that pig farms with more than 25,000 pigs, manure treatment facilities processing over 100 metric tons daily, and food waste generators handling more than 1,000 tons annually must comply with biogasification requirements. Additionally, the government has set ambitious targets for both the public and private sectors to increase their biogas production by up to 80% by 2045 with interim targets set for the coming years. This initiative is part of a broader strategy to increase the share of renewable energy and achieve carbon neutrality by 2050 aligning with global environmental goals.

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