Increased CO2 availability promotes growth of a tropical coastal diatom assemblage (Goa coast, Arabian Sea, India)

Abstract

Responses of tropical coastal phytoplankton assemblages to increasing CO2 levels are poorly known. The Arabian Sea (AS), the western part of the north Indian Ocean, is an upwelling-induced, highly productive region, but there are virtually no studies from this area documenting the responses of the natural phytoplankton communities to increasing CO2 concentrations. CO2-induced growth responses of a diatom-dominated phytoplankton community from the coastal water of Arabian Sea (Dona Paula, Goa) were investigated in a set of three experiments (one long-term and two short-term) in nutrient-replete, semi-continuous batches. Significant increases in photosynthetic rate, growth rate, carbohydrate content and carbohydrate to protein ratio were observed in the cells grown under elevated CO2. Trace amounts of zinc (Zn) addition to low CO2 grown cells also resulted in a marked increase in growth rate, carbohydrate content and carbohydrate to protein ratio. Zn addition at elevated CO2 only induced increased cell division. These results show that the experimental diatom-dominated phytoplankton community, (1) responded positively to CO2 enrichment, (2) probably possesses a carbon-concentration mechanism (CCM) and (3) uses a Zn-carbonic anhydrase in carbon acquisition. It is likely that the experimental diatom communities downregulated CCM activity with increased CO2 but grew faster. Future increases in coastal water CO2 concentrations may significantly impact phytoplankton primary production and biochemical composition and thus have large biogeochemical consequences.