Editing Cyanobacteria (section)

=== Collective behaviour ===
{{further|Algal bloom}}

[[File:Collective behaviour and lifestyle choices in single-celled cyanobacteria.webp|thumb|upright=2| {{center|Collective behaviour and  buoyancy strategies in single-celled cyanobacteria{{hsp}}<ref name=Mullineaux2021>{{cite journal | vauthors = Mullineaux CW, Wilde A | title = The social life of cyanobacteria | journal = eLife | volume = 10 | date = June 2021 | pmid = 34132636 | pmc = 8208810 | doi = 10.7554/eLife.70327 | doi-access = free }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>}}]]

Some cyanobacteria – even single-celled ones – show striking collective behaviours and form colonies (or [[algal bloom|blooms]]) that can float on water and have important ecological roles. For instance, billions of years ago, communities of marine [[Paleoproterozoic]] cyanobacteria could have helped create the [[biosphere]] as we know it by burying carbon compounds and allowing the initial build-up of oxygen in the atmosphere.<ref>{{cite journal | vauthors = Kamennaya NA, Zemla M, Mahoney L, Chen L, Holman E, Holman HY, Auer M, Ajo-Franklin CM, Jansson C | display-authors = 6 | title = High pCO<sub>2</sub>-induced exopolysaccharide-rich ballasted aggregates of planktonic cyanobacteria could explain Paleoproterozoic carbon burial | journal = Nature Communications | volume = 9 | issue = 1 | pages = 2116 | date = May 2018 | pmid = 29844378 | pmc = 5974010 | doi = 10.1038/s41467-018-04588-9 | bibcode = 2018NatCo...9.2116K }}</ref> On the other hand, [[Harmful algal bloom|toxic cyanobacterial bloom]]s are an increasing issue for society, as their toxins can be harmful to animals.<ref name=Huisman2018>{{cite journal | vauthors = Huisman J, Codd GA, Paerl HW, Ibelings BW, Verspagen JM, Visser PM | title = Cyanobacterial blooms | journal = Nature Reviews. Microbiology | volume = 16 | issue = 8 | pages = 471–483 | date = August 2018 | pmid = 29946124 | doi = 10.1038/s41579-018-0040-1 }}</ref> Extreme blooms can also deplete water of oxygen and reduce the penetration of sunlight and visibility, thereby compromising the feeding and mating behaviour of light-reliant species.<ref name=Mullineaux2021 />

As shown in the diagram on the right, bacteria can stay in suspension as individual cells, adhere collectively to surfaces to form biofilms, passively sediment, or flocculate to form suspended aggregates. Cyanobacteria are able to produce sulphated [[polysaccharide]]s (yellow haze surrounding clumps of cells) that enable them to form floating aggregates. In 2021, Maeda et al. discovered that oxygen produced by cyanobacteria becomes trapped in the network of polysaccharides and cells, enabling the microorganisms to form buoyant blooms.<ref name=Maeda2021>{{cite journal | vauthors = Maeda K, Okuda Y, Enomoto G, Watanabe S, Ikeuchi M | title = Biosynthesis of a sulfated exopolysaccharide, synechan, and bloom formation in the model cyanobacterium ''Synechocystis'' sp. strain PCC 6803 | journal = eLife | volume = 10 | date = June 2021 | pmid = 34127188 | pmc = 8205485 | doi = 10.7554/eLife.66538 | doi-access = free }}</ref> It is thought that specific protein fibres known as [[Pilus|pili]] (represented as lines radiating from the cells) may act as an additional way to link cells to each other or onto surfaces. Some cyanobacteria also use sophisticated intracellular [[gas vesicle]]s as floatation aids.<ref name=Mullineaux2021 />

[[File:Model of a clumped cyanobacterial mat.webp|thumb|upright=1.35|left| {{center|Model of a clumped cyanobacterial mat{{hsp}}<ref name=Sim2012>{{cite journal |doi=10.3390/geosciences2040235 |doi-access=free |title=Oxygen-Dependent Morphogenesis of Modern Clumped Photosynthetic Mats and Implications for the Archean Stromatolite Record |year=2012 | vauthors = Sim MS, Liang B, Petroff AP, Evans A, Klepac-Ceraj V, Flannery DT, Walter MR, Bosak T | display-authors = 6 |journal=Geosciences |volume=2 |issue=4 |pages=235–259 |bibcode=2012Geosc...2..235S|hdl=1721.1/85544 |hdl-access=free }} {{Creative Commons text attribution notice|cc=by3|from this source=yes}}</ref>}}]]

[[File:Cyanobacteria guerrero negro.jpg|thumb|upright=1.15| Light microscope view of cyanobacteria from a [[microbial mat]]]]

{{clear}}

The diagram on the left above shows a proposed model of microbial distribution, spatial organization, carbon and O<sub>2</sub> cycling in clumps and adjacent areas. (a) Clumps contain denser cyanobacterial filaments and heterotrophic microbes. The initial differences in density depend on cyanobacterial motility and can be established over short timescales. Darker blue color outside of the clump indicates higher oxygen concentrations in areas adjacent to clumps. Oxic media increase the reversal frequencies of any filaments that begin to leave the clumps, thereby reducing the net migration away from the clump. This enables the persistence of the initial clumps over short timescales; (b) Spatial coupling between photosynthesis and respiration in clumps. Oxygen produced by cyanobacteria diffuses into the overlying medium or is used for aerobic respiration. [[Dissolved inorganic carbon]] (DIC) diffuses into the clump from the overlying medium and is also produced within the clump by respiration. In oxic solutions, high O<sub>2</sub> concentrations reduce the efficiency of CO<sub>2</sub> fixation and result in the excretion of glycolate. Under these conditions, clumping can be beneficial to cyanobacteria if it stimulates the retention of carbon and the assimilation of inorganic carbon by cyanobacteria within clumps. This effect appears to promote the accumulation of [[particulate organic carbon]] (cells, sheaths and heterotrophic organisms) in clumps.<ref name=Sim2012 />

It has been unclear why and how cyanobacteria form communities. Aggregation must divert resources away from the core business of making more cyanobacteria, as it generally involves the production of copious quantities of extracellular material. In addition, cells in the centre of dense aggregates can also suffer from both shading and shortage of nutrients.<ref name=Conradi2019>{{cite journal | vauthors = Conradi FD, Zhou RQ, Oeser S, Schuergers N, Wilde A, Mullineaux CW | title = Factors Controlling Floc Formation and Structure in the Cyanobacterium ''Synechocystis'' sp. Strain PCC 6803 | journal = Journal of Bacteriology | volume = 201 | issue = 19 | date = October 2019 | pmid = 31262837 | pmc = 6755745 | doi = 10.1128/JB.00344-19 }}</ref><ref>{{cite journal | vauthors = Enomoto G, Ikeuchi M | title = Blue-/Green-Light-Responsive Cyanobacteriochromes Are Cell Shade Sensors in Red-Light Replete Niches | journal = iScience | volume = 23 | issue = 3 | pages = 100936 | date = March 2020 | pmid = 32146329 | pmc = 7063230 | doi = 10.1016/j.isci.2020.100936 | bibcode = 2020iSci...23j0936E }}</ref> So, what advantage does this communal life bring for cyanobacteria?<ref name=Mullineaux2021 />

[[File:Cell death in eukaryotes and cyanobacteria.jpg|thumb|upright=2| {{center|'''Cell death in eukaryotes and cyanobacteria'''{{hsp}}<ref name=Aguilera2021>{{cite journal | vauthors = Aguilera A, Klemenčič M, Sueldo DJ, Rzymski P, Giannuzzi L, Martin MV | title = Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature | journal = Frontiers in Microbiology | volume = 12 | pages = 631654 | year = 2021 | pmid = 33746925 | pmc = 7965980 | doi = 10.3389/fmicb.2021.631654 | doi-access = free }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>}} Types of cell death according to the [[Nomenclature Committee on Cell Death]] (upper panel;<ref>{{cite journal | vauthors = Galluzzi L, Bravo-San Pedro JM, Vitale I, Aaronson SA, Abrams JM, Adam D, Alnemri ES, Altucci L, Andrews D, Annicchiarico-Petruzzelli M, Baehrecke EH, Bazan NG, Bertrand MJ, Bianchi K, Blagosklonny MV, Blomgren K, Borner C, Bredesen DE, Brenner C, Campanella M, Candi E, Cecconi F, Chan FK, Chandel NS, Cheng EH, Chipuk JE, Cidlowski JA, Ciechanover A, Dawson TM, Dawson VL, De Laurenzi V, De Maria R, Debatin KM, Di Daniele N, Dixit VM, Dynlacht BD, El-Deiry WS, Fimia GM, Flavell RA, Fulda S, Garrido C, Gougeon ML, Green DR, Gronemeyer H, Hajnoczky G, Hardwick JM, Hengartner MO, Ichijo H, Joseph B, Jost PJ, Kaufmann T, Kepp O, Klionsky DJ, Knight RA, Kumar S, Lemasters JJ, Levine B, Linkermann A, Lipton SA, Lockshin RA, López-Otín C, Lugli E, Madeo F, Malorni W, Marine JC, Martin SJ, Martinou JC, Medema JP, Meier P, Melino S, Mizushima N, Moll U, Muñoz-Pinedo C, Nuñez G, Oberst A, Panaretakis T, Penninger JM, Peter ME, Piacentini M, Pinton P, Prehn JH, Puthalakath H, Rabinovich GA, Ravichandran KS, Rizzuto R, Rodrigues CM, Rubinsztein DC, Rudel T, Shi Y, Simon HU, Stockwell BR, Szabadkai G, Tait SW, Tang HL, Tavernarakis N, Tsujimoto Y, Vanden Berghe T, Vandenabeele P, Villunger A, Wagner EF, Walczak H, White E, Wood WG, Yuan J, Zakeri Z, Zhivotovsky B, Melino G, Kroemer G | display-authors = 6 | title = Essential versus accessory aspects of cell death: recommendations of the NCCD 2015 | journal = Cell Death and Differentiation | volume = 22 | issue = 1 | pages = 58–73 | date = January 2015 | pmid = 25236395 | pmc = 4262782 | doi = 10.1038/cdd.2014.137 }}</ref> and proposed for cyanobacteria (lower panel). Cells exposed to extreme injury die in an uncontrollable manner, reflecting the loss of structural integrity. This type of cell death is called "accidental cell death" (ACD). "Regulated cell death (RCD)" is encoded by a genetic pathway that can be modulated by genetic or pharmacologic interventions. [[Programmed cell death]] (PCD) is a type of RCD that occurs as a developmental program, and has not been addressed in cyanobacteria yet. RN, regulated necrosis.]]

New insights into how cyanobacteria form blooms have come from a 2021 study on the cyanobacterium ''[[Synechocystis]]''. These use a set of genes that regulate the production and export of sulphated [[polysaccharide]]s, chains of sugar molecules modified with [[sulphate]] groups that can often be found in marine algae and animal tissue. Many bacteria generate extracellular polysaccharides, but sulphated ones have only been seen in cyanobacteria. In ''Synechocystis'' these sulphated polysaccharide help the cyanobacterium form buoyant aggregates by trapping oxygen bubbles in the slimy web of cells and polysaccharides.<ref name=Maeda2021 /><ref name=Mullineaux2021 />

Previous studies on ''Synechocystis'' have shown [[type IV pili]], which decorate the surface of cyanobacteria, also play a role in forming blooms.<ref>{{cite journal | vauthors = Allen R, Rittmann BE, Curtiss R | title = Axenic Biofilm Formation and Aggregation by ''Synechocystis'' sp. Strain PCC 6803 Are Induced by Changes in Nutrient Concentration and Require Cell Surface Structures | journal = Applied and Environmental Microbiology | volume = 85 | issue = 7 | date = April 2019 | pmid = 30709828 | pmc = 6585507 | doi = 10.1128/AEM.02192-18 | bibcode = 2019ApEnM..85E2192A }}</ref><ref name=Conradi2019 /> These retractable and adhesive protein fibres are important for motility, adhesion to substrates and DNA uptake.<ref>{{cite journal | vauthors = Schuergers N, Wilde A | title = Appendages of the cyanobacterial cell | journal = Life | volume = 5 | issue = 1 | pages = 700–715 | date = March 2015 | pmid = 25749611 | pmc = 4390875 | doi = 10.3390/life5010700 | bibcode = 2015Life....5..700S | doi-access = free }}</ref> The formation of blooms may require both type IV pili and Synechan – for example, the pili may help to export the polysaccharide outside the cell. Indeed, the activity of these protein fibres may be connected to the production of extracellular polysaccharides in filamentous cyanobacteria.<ref name=Khayatan2015>{{cite journal | vauthors = Khayatan B, Meeks JC, Risser DD | title = Evidence that a modified type IV pilus-like system powers gliding motility and polysaccharide secretion in filamentous cyanobacteria | journal = Molecular Microbiology | volume = 98 | issue = 6 | pages = 1021–1036 | date = December 2015 | pmid = 26331359 | doi = 10.1111/mmi.13205 | doi-access = free }}</ref> A more obvious answer would be that pili help to build the aggregates by binding the cells with each other or with the extracellular polysaccharide. As with other kinds of bacteria,<ref>{{cite journal | vauthors = Adams DW, Stutzmann S, Stoudmann C, Blokesch M | title = DNA-uptake pili of Vibrio cholerae are required for chitin colonization and capable of kin recognition via sequence-specific self-interaction | journal = Nature Microbiology | volume = 4 | issue = 9 | pages = 1545–1557 | date = September 2019 | pmid = 31182799 | pmc = 6708440 | doi = 10.1038/s41564-019-0479-5 }}</ref> certain components of the pili may allow cyanobacteria from the same species to recognise each other and make initial contacts, which are then stabilised by building a mass of extracellular polysaccharide.<ref name=Mullineaux2021 />

The bubble flotation mechanism identified by Maeda et al. joins a range of known strategies that enable cyanobacteria to control their buoyancy, such as using gas vesicles or accumulating carbohydrate ballasts.<ref>{{cite journal |doi=10.1093/plankt/12.1.161 |title=A computer model of buoyancy and vertical migration in cyanobacteria |year=1990 | vauthors = Kromkamp J, Walsby AE |journal=[[Journal of Plankton Research]] |volume=12 |issue=1 |pages=161–183}}</ref> Type IV pili on their own could also control the position of marine cyanobacteria in the water column by regulating viscous drag.<ref>{{cite journal | vauthors = Aguilo-Ferretjans MD, Bosch R, Puxty RJ, Latva M, Zadjelovic V, Chhun A, Sousoni D, Polin M, Scanlan DJ, Christie-Oleza JA | display-authors = 6 | title = Pili allow dominant marine cyanobacteria to avoid sinking and evade predation | journal = Nature Communications | volume = 12 | issue = 1 | pages = 1857 | date = March 2021 | pmid = 33767153 | pmc = 7994388 | doi = 10.1038/s41467-021-22152-w | bibcode = 2021NatCo..12.1857A }}</ref> Extracellular polysaccharide appears to be a multipurpose asset for cyanobacteria, from floatation device to food storage, defence mechanism and mobility aid.<ref name=Khayatan2015 /><ref name=Mullineaux2021 />