{{Short description|For killing and preventing the growth of algae}} thumb|Spraying algicide into a pond '''Algaecide''' or '''algicide''' is a biocide used for killing and preventing the growth of algae, often defined in a loose sense that, beyond the biological definition, also includes cyanobacteria ("blue-green algae").<ref>{{cite web |title=Algae in Turf |url=https://content.ces.ncsu.edu/algae-in-turf |website=NC State Extension Publications}}</ref> An algaecide may be used for controlled bodies of water (reservoirs, golf ponds, swimming pools), but may also be used on land for locations such as turfgrass.<ref>{{cite web |title=Stomping out algae |url=https://www.golfcourseindustry.com/article/product-application-algae-turf/ |website=Golf Course Industry |language=en}}</ref>
== Types ==
=== Inorganic compounds === Some inorganic compounds are known since antiquity for their algicidal action due to their simplicity.
* Copper(II) sulfate remains "the most effective algicidal treatment".<ref>{{cite journal |doi=10.1051/jp4:20030547|title=Fate and forms of Cu in a reservoir ecosystem following copper sulfate treatment (Saint Germain les Belles, France) |year=2003 |last1=Van Hullebusch |first1=E. |last2=Chatenet |first2=P. |last3=Deluchat |first3=V. |last4=Chazal |first4=P. M. |last5=Froissard |first5=D. |last6=Lens |first6=P. N.L. |last7=Baudu |first7=M. |journal=Journal de Physique IV (Proceedings) |volume=107 |pages=1333–1336 }}</ref> A related traditional use is the Bordeaux mixture, used to control fungus on fruits.<ref>{{cite journal|title = Uses of Copper Compounds: Copper Sulfate's Role in Agriculture|journal = Annals of Applied Biology|volume = 20|issue = 2|pages = 342–363|access-date = 2007-12-31|url = http://www.copper.org/applications/compounds/copper_sulfate02.html|doi = 10.1111/j.1744-7348.1933.tb07770.x|year = 1933|last1 = Martin|first1 = Hubert|archive-date = 2013-09-07|archive-url = https://web.archive.org/web/20130907165844/http://www.copper.org/applications/compounds/copper_sulfate02.html|url-access = subscription}}</ref> * Hydrated lime, as a biocide, is allowed in the production of organic foods.<ref>{{Cite web |url=https://www.ams.usda.gov/sites/default/files/media/Lime%20Hydrated%20TR%202015.pdf |title=Hydrated Lime: Technical Evaluation Report |last=Pesticide Research Institute for the USDA National Organic Program |date=23 March 2015 |website=Agriculture Marketing Services |access-date=17 July 2019}}</ref>
===Barley straw === Barley straw, in England, is placed in mesh bags and floated in fish ponds or water gardens to help reduce algal growth without harming pond plants and animals. Barley straw has not been approved by the United States Environmental Protection Agency (EPA) for use as a pesticide and its effectiveness as an algaecide in ponds has produced mixed results during university testing in the United States and England. It is unclear how straw actually works.<ref>{{cite web |title=Barley Straw for Algae Control |url=https://extension.psu.edu/barley-straw-for-algae-control |website=PennState Extension |access-date=3 December 2022 |date=January 22, 2013}}</ref> [[File:Peinture algicide et fongicide.jpg|thumb|Algicidal and fungicidal paint applied to a building]]
===Synthetic algicides=== Synthetic algicides include: * Benzalkonium chloride – "quat" disinfectant that attacks membranes * Bethoxazin – "new broad spectrum industrial microbicide" in 2012,<ref>{{cite journal |last1=Chee |first1=GL |last2=Bhattarai |first2=B |last3=Daniel Gietz |first3=R |last4=Alrushaid |first4=S |last5=Nitiss |first5=JL |last6=Hasinoff |first6=BB |title=Chemical reactivity and microbicidal action of bethoxazin. |journal=Bioorganic & Medicinal Chemistry |date=15 February 2012 |volume=20 |issue=4 |pages=1494–501 |doi=10.1016/j.bmc.2011.12.051 |pmid=22264763}}</ref> noted as "Canceled in U.S." in 2022 PubChem-EPA query<ref>{{cite web |title=Bethoxazin |url=https://pubchem.ncbi.nlm.nih.gov/compound/Bethoxazin#section=EPA-Ecotoxicity&fullscreen=true |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> * Cybutryne – banned since 2023 in ship paint<ref name="bimco">{{cite web |title=International Maritime Organization bans toxic paint substance cybutryne |url=https://www.bimco.org/news/environment-protection/20210618-imo-bans-toxic-paint-substance-cybutryne |website=www.bimco.org |language=en}}</ref> * Dichlone – quinone fungicide/algaecide, not persistent in soil * Dichlorophen – also kills invertebrate animals and bacteria * Diuron – herbicide/algaecide, inhibits photosynthesis * Endothal – herbicide/algaecide, inhibits protein phosphatase 2A * Fentin – quinone fungicide/algaecide, discontinued * Isoproturon – selective substituted urea herbicide, discontinued * Methabenzthiazuron – substituted urea herbicide, discontinued * Nabam – fungicide/algicide discontinued in the EU over cancer * Oxyfluorfen – herbicide, "very toxic to aquatic life with long lasting effects"<ref>{{cite web |title=Substance Information - ECHA |url=https://echa.europa.eu/substance-information/-/substanceinfo/100.050.876 |website=echa.europa.eu}}</ref> * Pentachlorophenyl laurate * Quinoclamine – herbicide/algicide, not used in most of the EU<ref>{{cite web |last1=Hertfordshire |first1=University of |title=Quinoclamine |url=http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/579.htm |website=sitem.herts.ac.uk}}</ref> * Quinonamid * Simazine – herbicide/algaecide, inhibits photosynthesis * Terbutryn * Tiodonium
== Algicidal Bacteria == thumb|Algae bloom in the Baltic Sea Algicidal bacteria are a diverse group of bacteria that either directly or indirectly inhibit the growth of algae.<ref name=":13" /> They play a key role in maintaining ecological stability in fresh water and marine environments by mitigating the negative effects of harmful algal blooms (HAB).<ref name=":7" /> Algicidal bacteria interactions with algal cells can be broadly split into two categories: direct<ref name=":8" /> and indirect.<ref name=":9" /><ref name=":1" /> Direct methods typically involve the algicidal bacteria physically attacking the algal cell to induce cell lysis.<ref name=":10" /><ref name=":8" /> Indirect methods of attack by algicidal bacteria typically involve releasing algae inhibiting chemicals into the surrounding water.<ref name=":7" /><ref name=":4" />
Algicidal bacteria can detect targeted algal cells via chemotaxis<ref name=":0" /> and quorum sensing.<ref name=":11" /> Different species of algicidal bacteria can have varying degrees of specificity and many have co-evolved along their target algae.<ref name=":0" />
Algicidal bacteria have been used as a biological control method against HABs. They tend to be more environmentally friendly than traditional algicides.<ref name=":12" />
== Effects of Algicidal Bacteria on Algae == Algicidal bacteria can cause harm to algae in multiple different ways. Depending on the type of algae and type of bacteria, the mechanisms and resulting effects can vary.
=== Cell integrity loss === Cell integrity loss is caused by bacteria releasing dissolved algicidal compounds, which are toxic to algae, into the surrounding water, causing rapid cell lysis of algae cells.<ref>{{Cite journal |last1=Doucette |first1=Gregory J. |last2=McGovern |first2=Elizabeth R. |last3=Babinchak |first3=John A. |date=December 1999 |title=Algicidal Bacteria Active Against Gymnodinium Breve (Dinophyceae). I. Bacterial Isolation and Characterization of Killing Activity |url=https://onlinelibrary.wiley.com/doi/10.1046/j.1529-8817.1999.3561447.x |journal=Journal of Phycology |language=en |volume=35 |issue=6 |pages=1447–1454 |doi=10.1046/j.1529-8817.1999.3561447.x |issn=0022-3646|url-access=subscription }}</ref> Cell lysis occurs when algicidal enzymes or small molecules degrade cell walls and membranes, causing structural collapse.<ref name=":1">{{Cite journal |last1=Wang |first1=Jiaxin |last2=Xu |first2=Binfu |last3=Huang |first3=Lixing |date=April 2026 |title=Molecular Mechanisms of Algicidal Bacteria in Controlling Harmful Algal Blooms: Advances in Bacteria-Algae Interactions |journal=Environmental Microbiology Reports |language=en |volume=18 |issue=2 |article-number=e70305 |doi=10.1111/1758-2229.70305 |issn=1758-2229 |pmc=12951363 |pmid=41769730}}</ref> While most compounds released by algicidal bacteria cause cell lysis by damaging the algae cells walls, other lysis inducing mechanisms can include ion permeability disruption, and acidification of the cytoplasm by small molecules passing through the cell wall.<ref name=":1" />
An example of a bacterium which causes cell integrity loss is a strain of Alteromonas abrolhosensis, named JY‑JZ1, which when cultured, showed strong algicidal activity.<ref name=":2">{{Cite journal |last1=Jia |first1=Yang |last2=Lu |first2=Jiazhan |last3=Wang |first3=Min |last4=Qin |first4=Wenli |last5=Chen |first5=Binbin |last6=Xu |first6=Hanqing |last7=Ma |first7=Zengling |date=2023-09-15 |title=Algicidal bacteria in phycosphere regulate free-living Symbiodinium fate via triggering oxidative stress and photosynthetic system damage |url=https://www.sciencedirect.com/science/article/pii/S0147651323008734 |journal=Ecotoxicology and Environmental Safety |volume=263 |article-number=115369 |doi=10.1016/j.ecoenv.2023.115369 |pmid=37586194 |bibcode=2023EcoES.26315369J |issn=0147-6513|doi-access=free }}</ref> Through microscopy, researchers found that algal cells exposed to this bacterium had damaged cell membranes and leaked intracellular contents which indicates cell death by lysis.<ref name=":2" />
=== Photosynthesis impairment === thumb|150px|Chemical structure of the compound 4-BP Photosynthesis impairment occurs when bacteria release toxins that disrupt photosynthetic electron transport, lowering energy production and weakening algae cells.<ref name=":1" /> A specific example includes the production of 3,3′,5,5′‑tetrabromo‑2,2′‑biphenyldiol (4‑BP), which is a small molecule secreted by a certain strain of marine bacteria called Gammaproteobacteria.<ref name=":3">{{Cite journal |last1=Zhang |first1=Zenghu |last2=Li |first2=Dehai |last3=Xie |first3=Ruize |last4=Guo |first4=Ruoyu |last5=Nair |first5=Shailesh |last6=Han |first6=Huan |last7=Zhang |first7=Guojian |last8=Zhao |first8=Qun |last9=Zhang |first9=Lihua |last10=Jiao |first10=Nianzhi |last11=Zhang |first11=Yongyu |date=2023-11-01 |title=Plastoquinone synthesis inhibition by tetrabromo biphenyldiol as a widespread algicidal mechanism of marine bacteria |journal=The ISME Journal |language=en |volume=17 |issue=11 |pages=1979–1992 |doi=10.1038/s41396-023-01510-0 |issn=1751-7362 |pmc=10579414 |pmid=37679430 |bibcode=2023ISMEJ..17.1979Z }}</ref> The algicidal effect of 4‑BP occurs due to interference with plastoquinone synthesis, which is a key molecule in the photosynthetic electron transport chain of phytoplankton.<ref name=":3" /> 4‑BP competitively binds to the active site of the enzyme responsible for plastoquinone synthesis, blocking its function and disrupting the phytoplankton's ability to photosynthesize.<ref name=":3" /> This disruption causes pigment loss, reduces photosynthetic efficiency, and leads to cell damage or death (lysis).<ref name=":3" /> This algicidal compound has been found to kill a range of different phytoplankton, including diatoms, chlorophytes, dinoflagellates and cyanobacteria.<ref name=":3" />
=== Oxidative stress === Many algicides produced by bacteria increase reactive oxygen species (ROS), which are highly reactive molecules produced during cell processes such as photosynthesis and respiration inside algal cells.<ref name=":1" /> This damages DNA, proteins, and lipids, due to an oxidative chain reaction which produces toxic products that lead to cell death.<ref name=":1" /><ref name=":7">{{Cite journal |last1=Zhu |first1=Xiaoying |last2=Chen |first2=Shuangshuang |last3=Luo |first3=Guiying |last4=Zheng |first4=Wei |last5=Tian |first5=Yun |last6=Lei |first6=Xueqian |last7=Yao |first7=Luming |last8=Wu |first8=Caiming |last9=Xu |first9=Hong |date=2022-02-23 |title=A Novel Algicidal Bacterium, ''Microbulbifer'' sp. YX04, Triggered Oxidative Damage and Autophagic Cell Death in ''Phaeocystis globosa'' , Which Causes Harmful Algal Blooms |journal=Microbiology Spectrum |volume=10 |issue=1 |article-number=e00934-21 |doi=10.1128/spectrum.00934-21 |doi-access=free |pmid=35019679 |pmc=8754136 |issn=2165-0497}}</ref> Algal cells can usually counteract these elevated ROS levels with antioxidants, however excessive or prolonged ROS levels can overwhelm the cell and lead to its death.<ref name=":1" /> Oxidative stress can be detected in algae based on elevated levels of malondialdehyde (MDA),<ref name=":4">{{Cite journal |last1=Jia |first1=Yang |last2=Lu |first2=Jiazhan |last3=Zhang |first3=Jingxia |last4=Jin |first4=Yuyang |last5=Ren |first5=Lei |last6=Xu |first6=Hanqing |last7=Zhao |first7=Min |last8=Ma |first8=Zengling |date=2025-06-15 |title=Algicidal mechanism and algicidal active metabolites of Alteromonas abrolhosensis against harmful dinoflagellates Karenia mikimotoi |url=https://www.sciencedirect.com/science/article/pii/S0304389425007290 |journal=Journal of Hazardous Materials |volume=490 |article-number=137815 |doi=10.1016/j.jhazmat.2025.137815 |pmid=40048794 |bibcode=2025JHzM..49037815J |issn=0304-3894|url-access=subscription }}</ref><ref name=":1" /> which is a marker of lipid peroxidation (where lipids are attacked by oxidants), as well as elevated levels of antioxidants and defence enzymes.<ref name=":4" /><ref name=":1" />
=== Calcium signalling disruption === Calcium ions (Ca²⁺) are essential signalling molecules for important cell functions such as membrane stability, cell growth and metabolism.<ref name=":1" /> Some algicidal bacterial compounds disrupt intracellular Ca²⁺ balance by causing unusual increases in Ca²⁺ levels.<ref name=":1" /> These heightened levels trigger signalling chains that eventually lead to cell death.<ref name=":1" />
== Mechanisms of Algicidal Bacteria == Various mechanisms are employed by bacteria to facilitate their algicidal processes. There are two established methods of algicidal activity: direct and indirect. However, some bacteria are capable of both.<ref name=":1" /><ref name=":5">{{Cite journal |last1=Yang |first1=Xiaohong |last2=Liu |first2=Zijian |last3=Zhang |first3=Yanwen |last4=Shi |first4=Xinguo |last5=Wu |first5=Zhen |date=2024-07-31 |title=Dinoflagellate–Bacteria Interactions: Physiology, Ecology, and Evolution |journal=Biology |language=en |volume=13 |issue=8 |page=579 |doi=10.3390/biology13080579 |doi-access=free |issn=2079-7737 |pmc=11351557 |pmid=39194517}}</ref><ref name=":0" /> Some of these mechanisms are enhanced under different environmental conditions, such as varying nutrient availability.<ref>{{Cite journal |last1=Ray |first1=S. |last2=Bagchi |first2=S. N. |date=2002-01-04 |title=Nutrients and pH regulate algicide accumulation in cultures of the cyanobacterium Oscillatoria laetevirens |url=https://nph.onlinelibrary.wiley.com/doi/10.1046/j.1469-8137.2001.00061.x |journal=New Phytologist |language=en |volume=149 |issue=3 |pages=455–460 |doi=10.1046/j.1469-8137.2001.00061.x |pmid=33873328 |issn=0028-646X}}</ref>
=== Direct algicidal mechanisms === Direct algicidal mechanisms include methods in which bacterial cells physically interact with algal cells.<ref name=":1" /><ref name=":0" /> For example, ''Streptomyces globisporus'' strain G9 utilizes hyphae to entrap ''Microcystis aeruginosa'', shading the phytoplankton from sunlight, causing photoinhibition.<ref name=":10">{{Cite journal |last1=Zeng |first1=Yudie |last2=Wang |first2=Jiayu |last3=Yang |first3=Chunyan |last4=Ding |first4=Mengyue |last5=Hamilton |first5=Paul B. |last6=Zhang |first6=Xiaohui |last7=Yang |first7=Caiyun |last8=Zhnag |first8=Lei |last9=Dai |first9=Xianzhu |date=2021-05-15 |title=A Streptomyces globisporus strain kills Microcystis aeruginosa via cell-to-cell contact |url=https://www.sciencedirect.com/science/article/pii/S0048969720380207 |journal=Science of the Total Environment |volume=769 |article-number=144489 |doi=10.1016/j.scitotenv.2020.144489 |pmid=33465632 |bibcode=2021ScTEn.76944489Z |issn=0048-9697|url-access=subscription }}</ref> Another example is compound production triggered by direct cell contact. For instance, ''Chitinimonas prasina'' strain LY03 attaches to diatoms that contain chitin in their cell walls directed by their bacterial flagellum, and then produce chitinases that cause diatom lysis.<ref name=":8">{{Cite journal |last1=Li |first1=Yi |last2=Lei |first2=Xueqian |last3=Zhu |first3=Hong |last4=Zhang |first4=Huajun |last5=Guan |first5=Chengwei |last6=Chen |first6=Zhangran |last7=Zheng |first7=Wei |last8=Fu |first8=Lijun |last9=Zheng |first9=Tianling |date=2016-02-23 |title=Chitinase producing bacteria with direct algicidal activity on marine diatoms |journal=Scientific Reports |language=en |volume=6 |issue=1 |article-number=21984 |doi=10.1038/srep21984 |issn=2045-2322 |pmc=4763246 |pmid=26902175 |bibcode=2016NatSR...621984L }}</ref> In some instances, locally concentrated compounds have to reach specific levels in order to become algicidal.<ref name=":1" />
=== Indirect algicidal mechanisms === [[File:Bacillus subtilis.jpg|thumb|Algicidal bacteria ''Bacillus subtilis'']] Indirect algicidal mechanisms involve no physical contact. Instead, bacteria secrete compounds into the environment that result in the death of algal cells.<ref name=":1" /><ref name=":0" /> There is a wide variety of chemical compounds that may have algicidal capabilities, including alkaloids, amino acids, peptides, proteins, carbohydrates, lipids, enzymes, polyketides, terpenoids, and fatty acids.<ref name=":9">{{Cite journal |last1=Yang |first1=Shuxin |last2=Williams |first2=Spencer J. |last3=Courtney |first3=Myles |last4=Burchill |first4=Laura |date=2025-04-16 |title=Warfare under the waves: a review of bacteria-derived algaecidal natural products |url=https://pubs.rsc.org/en/content/articlelanding/2025/np/d4np00038b |journal=Natural Product Reports |language=en |volume=42 |issue=4 |pages=681–719 |doi=10.1039/D4NP00038B |pmid=39749862 |issn=1460-4752|url-access=subscription }}</ref><ref name=":1" /> Secretion can be constitutive, in which chemicals are released continuously, or inducible, in which the detection of other molecules or cells trigger their release.<ref name=":1" /> There is also evidence that algicidal chemicals act additively. For example, ortho-tyrosine and urocanic acid, both produced by ''Bacillus'' sp. B1, had algicidal effects on ''Heterosigma akashiwo'', where ortho-tyrosine affected its photosynthetic system, and urocanic acid damaged its mitochondria.<ref>{{Cite journal |last1=Quan |first1=Honglin |last2=Zhang |first2=Yuan |last3=Yin |first3=Pinghe |last4=Zhao |first4=Ling |date=2021-09-01 |title=Effects of two algicidal substances, ortho-tyrosine and urocanic acid, on the growth and physiology of Heterosoigma akashiwo |url=https://www.sciencedirect.com/science/article/pii/S0269749121005868 |journal=Environmental Pollution |volume=284 |article-number=117004 |doi=10.1016/j.envpol.2021.117004 |pmid=33906037 |bibcode=2021EPoll.28417004Q |issn=0269-7491|url-access=subscription }}</ref>
=== Regulation of algicidal activity === To perform algicidal processes, bacteria require mechanisms to detect and regulate their responses.<ref name=":1" /><ref name=":0" /> Some common bacterial mechanisms associated with algicidal activities include chemotaxis, attachment and biofilm formation, and quorum sensing.
==== Chemotaxis ==== Chemotaxis is the ability to direct movement through sensing a gradient of chemoattractants or chemorepellents using flagella, allowing bacteria to locate algal cells to lyse.<ref name=":0" /><ref name=":1" /> Some algal cells can have intrinsic attractant properties, whereas others produce attractants.<ref name=":1" /><ref name=":6">{{Cite journal |last1=Seymour |first1=Justin R. |last2=Simó |first2=Rafel |last3=Ahmed |first3=Tanvir |last4=Stocker |first4=Roman |date=2010-07-16 |title=Chemoattraction to Dimethylsulfoniopropionate Throughout the Marine Microbial Food Web |url=https://www.science.org/doi/10.1126/science.1188418 |journal=Science |language=en |volume=329 |issue=5989 |pages=342–345 |doi=10.1126/science.1188418 |pmid=20647471 |bibcode=2010Sci...329..342S |issn=0036-8075|url-access=subscription }}</ref> A common chemoattractant is dimethylsulfoniopropionate (DMSP).<ref name=":6" /> Additionally, chemotaxis allows for preferential "predation", as some organisms may prefer specific attractants. For instance, amino acids tend to be preferentially taken up by ''Polaribacter marinivivus'' and ''Lentibacter algarum'', whereas nucleotides are preferentially taken up by ''Litoricola marina''.<ref name=":5" />
==== Attachment and biofilm formation ==== Attachment is another important method for regulating algicidal activity, as it can elongate the duration of interactions between bacteria and algae.<ref name=":1" /> It was observed that the number of attached bacteria per diatom was positively correlated with the lysis rate of algal cells, and the lysis rate increased when attachment rates were elevated.<ref>{{Cite journal |last1=Cai |first1=Guanjing |last2=Yu |first2=Xiaoqi |last3=Wang |first3=Hui |last4=Zheng |first4=Tianling |last5=Azam |first5=Farooq |date=2023-09-29 |title=Nutrient-dependent interactions between a marine copiotroph Alteromonas and a diatom Thalassiosira pseudonana |journal=mBio |volume=14 |issue=5 |pages=e00940–23 |doi=10.1128/mbio.00940-23 |doi-access=free |pmc=10653928 |pmid=37772817}}</ref> Attachment can be strengthened through biofilm formation, which consists of extracellular polymeric substances (EPS), adhesins, and other components capable of aggregating cells together. Biofilms allow for elongated interactions between bacteria and algal cells, and maintain the exchange of nutrients and gases.<ref name=":1" />
==== Quorum sensing ==== Quorum sensing is a method that depends on detecting cell or molecule densities to trigger specific responses. As cell density increases, signalling molecule concentrations also increase, eventually reaching a threshold that activates a response.<ref name=":11">{{Cite journal |last1=Williams |first1=Paul |last2=Winzer |first2=Klaus |last3=Chan |first3=Weng C |last4=Cámara |first4=Miguel |date=2007-07-29 |title=Look who's talking: communication and quorum sensing in the bacterial world |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=362 |issue=1483 |pages=1119–1134 |doi=10.1098/rstb.2007.2039 |issn=0962-8436 |pmc=2435577 |pmid=17360280}}</ref> This mechanism is useful to lower functioning costs for bacteria, as this allows high-energy responses to only occur when they are triggered as necessary by the signalling molecule.<ref name=":0" /> Known signalling molecules observed in algicidal responses include acyl-homoserine lactones (AHLs), quinolones, and diketopiperazines.<ref name=":13">{{Cite journal |last1=Coyne |first1=Kathryn J. |last2=Wang |first2=Yanfei |last3=Johnson |first3=Gretchen |date=2022-04-07 |title=Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms |journal=Frontiers in Microbiology |language=English |volume=13 |article-number=871177 |doi=10.3389/fmicb.2022.871177 |doi-access=free |issn=1664-302X |pmc=9022068 |pmid=35464927 |bibcode=2022FrMic..1371177C }}</ref> Taxa such as ''Pseudomonas'' sp., ''Pseudoalteromonas'' sp., ''Vibrio'' sp., and ''Alteromonas'' sp. all demonstrate algicidal activity linked to quorum sensing mechanisms.<ref name=":1" />
== Coevolution == Coevolution is the long-term outcome of the algae-bacteria interaction. Algae and bacteria interact through mutualism, competition, and antagonistic interactions.<ref>{{Cite journal |last1=Wang |first1=Jiaxin |last2=Xu |first2=Binfu |last3=Huang |first3=Lixing |date=April 2026 |title=Molecular Mechanisms of Algicidal Bacteria in Controlling Harmful Algal Blooms: Advances in Bacteria-Algae Interactions |journal=Environmental Microbiology Reports |language=en |volume=18 |issue=2 |article-number=e70305 |doi=10.1111/1758-2229.70305 |issn=1758-2229 |pmc=12951363 |pmid=41769730}}</ref> Bacteria have evolved mechanisms to utilize algal organic matter or lyse algal cells, while algae have developed defence mechanisms such as resting stages, induced resistance, and chemical signalling responses. This reciprocal adaptation is an evolutionary arms race between algae and bacteria.
== Specificity == Algicidal activity can be highly strain-specific and sometimes appears random or unexplained. Current studies have found that particle-associated bacteria tend to have broader algicidal activity, while free-living bacteria are often more species-specific.<ref>{{Cite journal |last1=Meyer|first1=Nils|last2=Bigalke|first2=Arite|last3=Kaulfuß|first3=Anett|last4=Pohnert|first4=Georg|date=2017-11-01|title=Strategies and ecological roles of algicidal bacteria|url=http://academic.oup.com/femsre/article/41/6/880/4091024|journal=FEMS Microbiology Reviews|language=en|volume=41|issue=6|pages=880–899|doi=10.1093/femsre/fux029|pmid=28961821 |issn=1574-6976}}</ref> Algicidal bacteria with low host specificity may occupy a broader ecological niche as they can interact with and utilize organic matter from multiple algal species. However, highly specific bacteria may be more efficient in targeting particular algal hosts. Specificity does not necessarily determine bacterial abundance, but rather reflects different ecological strategies such as generalist versus specialist lifestyles.
== Algal Defence ==
=== Evasive strategies === Algae may resist algicidal bacteria through evasive strategies, such as forming morphologically distinct resting stages. These resting stages can help algae avoid direct contact with algicidal bacteria and survive unfavourable conditions. When the conditions improve, the resting stages can germinate and re-establish growing populations.<ref name=":0">{{Cite journal |last1=Meyer |first1=Nils |last2=Bigalke |first2=Arite |last3=Kaulfuß |first3=Anett |last4=Pohnert |first4=Georg |date=2017-11-01 |title=Strategies and ecological roles of algicidal bacteria |url=http://academic.oup.com/femsre/article/41/6/880/4091024 |journal=FEMS Microbiology Reviews |language=en |volume=41 |issue=6 |pages=880–899 |doi=10.1093/femsre/fux029 |pmid=28961821 |issn=1574-6976}}</ref>
=== Induced resistance === Induced resistance is another mechanism that algae develop to defend against algicidal bacteria. One example, ''Chaetoceros didymus,'' was found to be resistant to the algicidal bacterium ''Kordia algicida'' by producing its own proteases.<ref name=":0" /> These proteases are believed to counteract the lytic enzymes produced by the bacteria. Such a resistance response can be triggered not only by direct contact with bacteria but also by chemical signals present in bacterial culture filtrates.
== Applications ==
=== Biological control of harmful algal blooms (HABs) === [[File:Microcystis aeruginosa.jpeg|thumb|''Micocystis aeruginosa.'' Can form harmful algae blooms]] Harmful effects on the ecosystem caused by HABs include water deoxygenation, production of toxins that deter competing species and grazers.<ref name=":13" /> Certain species of algae have been observed to produce paralytic toxins targeting shellfish which can greatly disrupt coastal ecosystems as well as commercial fisheries.<ref name=":13" />
Algicidal bacteria have been studied as biological control agents for inhibiting the formation of harmful algal blooms. They may also help protect aquaculture operations, improve water treatment processes, and contribute to the restoration of phytoplankton community balance in aquatic ecosystems.<ref name=":14">{{Cite journal |last1=Calvo-Bayo |first1=Isabel |last2=Bolívar-Galiano |first2=Fernando |last3=Romero-Noguera |first3=Julio |date=2025-11-26 |title=Algicidal Bacteria: A Sustainable Proposal to Control Microalgae in the Conservation and Restoration of Stone Cultural Heritage |journal=Sustainability |language=en |volume=17 |issue=23 |article-number=10610 |doi=10.3390/su172310610 |doi-access=free |bibcode=2025Sust...1710610C |issn=2071-1050}}</ref>
=== Antifouling material for industry === Some algicidal bacterial species produce bioactive compounds that have potential applications in antifouling coatings for ship hulls and aquaculture facilities.<ref name=":12">{{Cite journal |last=Bowman |first=John P. |date=2007-12-18 |title=Bioactive Compound Synthetic Capacity and Ecological Significance of Marine Bacterial Genus Pseudoalteromonas |journal=Marine Drugs |language=en |volume=5 |issue=4 |pages=220–241 |doi=10.3390/md504220 |doi-access=free |issn=1660-3397 |pmc=2365693 |pmid=18463726}}</ref> These compounds can inhibit the growth and attachment of algae and other microorganisms on submerged surfaces, reducing biofouling. They are still under investigation and can potentially be an environmentally friendly alternative to the traditional chemical anti-fouling paints.
== Taxonomy == {| class="wikitable sortable mw-collapsible" |+Table 1. Target algae species and corresponding algicidal bacteria<ref name=":14" /> !Target Algae !Algicidal Bacteria |- | colspan="2" |<big>Unicellular Cyanobacteria</big> |- |''Microcystis sp.'' |''Achromobacter, Acinetobacter, Aeromonas, Agrobacterium, Aquimarina, Bacillus, Brevibacillus, Brevundimonas, Chryseobacterium, Cytophaga, Enterobacter, Exiguobacterium, Halobacillus, Leuconostoc, Lysobacter, Morganella, Ochrobactrum, Paenibacillus, Paucibacter, Pedobacter, Pseudomonas, Raoultella, Rhizobium, Rhodococcus, Serratia, Saprospira, Stenotrophomonas, Streptomyces, Xanthobacter'' |- |''Chroococcus sp.'' |''Aeromonas, Bacillus, Chryseobacterium, Exiguobacterium'' |- | colspan="2" |<big>Fillamentus Cyanobacteria</big> |- |''Anabaea sp'' |''Pseudomonas, Serratia, Streptomyces, Bacillus, Aeromonas, Lysobacter, Rhodococcus, Cytophaga'' |- |''Leptolyngbya sp.'' |''Pseudomonas, Bacillus, Exiguobacterium, Streptomyces'' |- |''Nostoc sp.'' |''Aeromonas, Bacillus, Chryseobacterium, Enterobacter, Exiguobacterium, Flexibacter, Pseudomonas, Stenotrophomonas, Streptomyces'' |- |''Oscillatoria sp.'' |''Bacillus, Myxococcus, Streptomyces, Pseudomonas, Aquimarina'' |- |''Phormidium sp.'' |''Streptomyces, Pseudomonas, Lysobacter'' |- |''Pseudanabaena sp.'' |''Streptomyces, Pseudomonas, Lysobacter'' |- | colspan="2" |<big>Unicellular Green Algae</big> |- |''Chlamydomonas sp.'' |''Aeromonas, Aquimarina, Bacillus, Exiguobacterium, Chryseobacterium, Stenotrophomonas'' |- |''Chlorella sp.'' |''Bacillus, Aeromonas, Aquimarina, Bowmanella, Enterobacter, Flammeovirga, Flavobacterium, Microbacterium, Pseudomonas, Stenotrophomonas, Streptomyces, Deinococcus'' |- |''Scenedesmus sp.'' |''Microbacterium, Aeromonas, Enterobacter, Pseudomonas, Bacillus'' |- | colspan="2" |<big>Fillamentus Green Algae</big> |- |''Spirogyra sp.'' |''Bacillus'' |- | colspan="2" |<big>Diatom</big> |- |''Ulnaria'' |''Brevundimonas, Sphingomonas, Agrobacterium, Methylobacterium, Acinetobacter, Bacillus, Deinococcus'' |}
==References== {{reflist}}
==External links== * [http://npic.orst.edu National Pesticide Information Center (NPIC)] Information about pesticide-related topics.
Category:Algaecides