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.

Keywords:

• increasing CO₂
• phytoplankton
• diatom
• Arabian Sea
• tropical ocean

Acknowledgements

The authors thank the director, CSIR National Institute of Oceanography (NIO, Goa) for his continuous support and encouragement. We are also thankful to Dr Ramaiah (Head, Biological Oceanography Division) for his kind support to start this work. We would like to greatly acknowledge Dr Ingole, Dr M. Gauns and Dr Anil for providing laboratory facilities to complete this work. The valuable contribution of three anonymous reviewers in improving this manuscript is gratefully acknowledged. We would like to acknowledge Council of Scientific and Industrial Research for providing financial support. NIO contribution number for this manuscript is (6079).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Council for Scientific and Industrial Research (CSIR), India.