Séminaire

The Arabian Sea (AS) and Bay of Bengal (BoB) are distinctly different from biogeochemical perspective. In both the study areas diatoms are the dominant phytoplankton species observed in the coastal stations. A series of incubation experiments were conducted under elevated CO2 levels to understand the responses of coastal diatoms from Indian coastal waters. These studies constitute the first base line data on the responses of coastal phytoplankton to increasing CO2 levels from the tropical Indian coastal waters. The results from BoB and AS coastal waters clearly showed that increasing CO2 levels resulted in higher growth response and they probably exhibit a carbon concentrations mechanisms (CCMs) under CO2 limitation.  Depletion in δ13CPOM values, enhanced POC content and C:N ratios in the high CO2 treated cells indicated increased diffusive influx of dissolved CO2 in response to external CO2 supply. Further, under CO2 limitation the zinc (Zn) induced positive growth response, suggested the possible usage of Zn-carbonic anhydrase activity for enzymatic conversion of HCO3-1 to CO2. Impacts of solar light variability on biomass accumulation showed that the CCMs in tropical phytoplankton require high amount of light energy and therefore, under CO2 and light limitation, growth can be considerably hindered. Estuarine phytoplankton from the BoB showed a community shift from diatom to cyanobacteria under silicate limitation and elevated CO2 levels. In the AS experiment considerable increase in carbohydrate to protein ratio was consistent with increased rate of carbon fixation followed by macromolecule formation. This may have considerable impact on food chain and organic matter mineralization process in the coastal waters. However, when the similar experiment was conducted on a phytoplankton community collected from the upwelling area of the AS, the responses were very much different. A nutrient enriched diatom bloom was initiated on board and was grown under ambient and high CO2 levels in combination with different copper and zinc addition. There was a significant change noticed in the phytoplankton community composition both in response to CO2 and trace metals. But no considerable change in the per cell quota of carbon or BSi was seen between the treatments. Some dominant diatom species from this upwelling systems might have some strong adaptability to these variability and could be less susceptible to the ongoing/future changes. Some more analyses from this study are in progress.

damien.cardinal@sorbonne-universite.fr