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Thephycosphere is a microscale mucus region that is rich inorganic matter surrounding aphytoplankton cell. This area is high in nutrients due to extracellular waste from the phytoplankton cell and it has been suggested thatbacteria inhabit this area to feed on these nutrients. This high nutrient environment creates amicrobiome and a diversefood web for microbes such asbacteria andprotists.[1] It has also been suggested that the bacterial assemblages within the phycosphere are species-specific and can vary depending on differentenvironmental factors.[2]
In terms of comparison, the phycosphere inphytoplankton has been suggested analogous to therhizosphere inplants, which is the root zone important for nutrient recycling. Both plant roots and phytoplankton exude chemicals which alter their immediate surrounds drastically – including altering the pH and oxygen levels. In terms of community construction, chemotaxis is used in both environments in order to propagate the recruitment of microbes. In the rhizosphere, chemotaxis is used by the host – the plant – to mediate the motility of the soil which allows for microbial colonization. In the phycosphere, the phytoplankton release of specific chemical exudates elicits a response from bacterial symbionts who exhibit chemotaxis signaling, thereby enabling the recruitment of microbes and subsequent colonization. The interfaces also have a few similar microbes, chemicals, and metabolites involved in the host – symbiont interactions. This includes microbes such as Rhizobium, which in the phycospheres of green algae was found to be the foremost microbe when compared to other abundant community members. Chemicals such as dimethylsuloniopropionate (DMSP) and 2,3-dihydroxypropane-1-sulfonate (DHPS) and metabolites such as sugars and amino acids are implicated in the mechanisms of action of both microbiomes.
Themicroscale interactions between thephytoplankton andbacteria are complex. The phytoplankton-bacteria interactions have the potential to beparasitism, competition ormutualism.
Interactions between phytoplankton and bacteria in the phycosphere could be potentially important in low-nutrient regions of the ocean and an example of mutualism. In marineecosystems that are low in nutrients (i.e.oligotrophic regions of the oceans), it could be potentially beneficial for the phytoplankton to have remineralizingbacteria in the phycosphere fornutrientrecycling. It has been suggested that while the bacterial activity may be low, thetaxonomicdiversity and nutritionaldiversity is high.[3] This can possibly suggest that the phytoplankton species may rely on a diverse array of bacterial interactions for recyclednutrients in theseoligotrophic regions and the bacteria rely on organic matter surrounding the phycosphere for a source of food.
However, bacterial-phytoplankton interactions in the phycosphere could beparasitic. In the same low nutrient oligotrophic regions of the ocean,phytoplankton that arenutrient stressed may not be able to produce this protective mucus layer or its associatedantibiotics. Thebacteria, who are also food stressed, could kill thephytoplankton and use it as a food substrate.[4]
Also,bacteria metabolize theorganic matter throughaerobic respiration, which depletes oxygen from the water and can lower thepH of the water column. If enoughorganic matter is produced, the bacteria could potentially harm the phytoplankton by causing the water to become moreacidic. (See alsoeutrophication).
Phycosphere profiles and composition also seem to be based on the type of host species the bacteria surrounds. While most hosts are dominated by the families Rhodobacteraceae and Flavobacteriaceae, differences in ratios and other bacteria can be observed on a host species basis. This can be due to a number of factors such as a species habitat, stressors, and nutrient exchange throughout the microbiome. Some host species such asC. weissflogii uniquely exhibit anaerobic pathways despite the phycosphere being an aerobic environment and other species such asI. galbana andT. suecica where such pathway is absent.[5]
In reality, the actualbacterialdiversity of the phycosphere is extremely diverse and is dependent environmental factors, such asturbulence in the water (so the bacteria can attach to the mucus or the phytoplankton cell) or the concentrations of nutrients. Also, the bacteria tend to be highly specialized when associated with this region. Nevertheless, here are some examples ofbacteriumgenera associated with the phycosphere.
5. Seymour, Justin R., et al. “Zooming in on the Phycosphere: the Ecological Interface for Phytoplankton–Bacteria Relationships.” Nature Microbiology, vol. 2, no. 7, 2017, pp. 1–13., doi:10.1038/nmicrobiol.2017.65.
6. Kim, B.-H., Ramanan, R., Cho, D.-H., Oh, H.-M., & Kim, H.-S. (2014). Role of Rhizobium, a plant growth promoting bacterium, in enhancing algal biomass through mutualistic interaction. Biomass and Bioenergy, 69, 95–105. doi: 10.1016/j.biombioe.2014.07.015
7. Geng, H., & Belas, R. (2010). Molecular mechanisms underlying roseobacter–phytoplankton symbioses. Current Opinion in Biotechnology, 21(3), 332–338. doi: 10.1016/j.copbio.2010.03.0138. Ramanan, R., Kang, Z., Kim, B.-H., Cho, D.-H., Jin, L., Oh, H.-M., & Kim, H.-S. (2015). Phycosphere bacterial diversity in green algae reveals an apparent similarity across habitats. Algal Research, 8, 140–144. doi: 10.1016/j.algal.2015.02.003
9. Scharf, B. E., Hynes, M. F., & Alexandre, G. M. (2016). Chemotaxis signaling systems in model beneficial plant–bacteria associations. Plant Molecular Biology, 90(6), 549–559. doi: 10.1007/s11103-016-0432-4
