Simulated ocean acidification altered community composition and growth of a coastal phytoplankton assemblage (South West coast of India, eastern Arabian Sea).

Marine phytoplankton can be highly sensitive to ocean acidification; however, their responses are diverse and therefore, phytoplankton response study on the regional scale is of high research priority. The present study documented the community shift and growth responses of a natural phytoplankton assemblage from the South West coastal water of India (South Eastern Arabian Sea) under ambient CO (A-CO ≈ 400 µatm) and high CO (H-CO ≈ 830 µatm) levels in microcosms during the winter monsoon. A doubling of pCO resulted in increased cell density, particulate organic carbon and nitrogen (POC, PON) contents, and C:N ratios. The depleted values of δC in the H-CO-incubated cells indicated a higher diffusive CO influx. HPLC marker pigment analysis revealed that the community was microphytoplankton dominated (mostly diatoms); nanoplanktonic prymnesiophytic algae and picoplanktonic cyanobacteria showed insignificant response to the simulated ocean acidification. A high CO-induced increased growth rate was noticed in 6 diatoms (Leptocylindrus danicus; Rhizosolenia setigera; Navicula sp., Asterionella glacialis, Dactyliosolen fragilissimus, and Thalassiosira sp.). The cell volumes of Thalassionema frauenfeldii, Asterionella glacialis, and Cylindrotheca closterium increased significantly, whereas Rhizosolenia setigera and Thalassiosira sp. showed decreased cell volume at the elevated CO levels. These changes in growth rate, cell volume, and elemental stoichiometry could be related to CO acquisition and the nutritional status of the cells. Some phytoplankton genera from this region are probably acclimatized to pCO fluctuations and are likely to benefit from the future increase in CO levels. Higher POC production and increased C:N ratio along with variable cell volume may impact the trophic transfer and cycling of organic carbon in this coastal water. However, a multi-stressor approach in a longer experimental exposure should be considered in future research.