Physiological and biochemical responses of diatoms to projected ocean changes

Abstract

With progressive future global change, marine phytoplankton in surface oceans will be subjected to ocean acidification, as well as to increased solar exposures and decreased vertical transport of nutrients from depth due to increasing stratification. We employed a simultaneous multivariate treatment approach to investigate the physiological and biochemical responses of the diatoms Thalassiosira pseudonana and Skeletonema costatum to these projected ocean changes. Diatoms were grown under different 'clustered' regimes of solar radiation, nutrients, and pCO2 (pH), reflecting present-day (2011) and potential mid-century (2050) and end-of-century (2100) scenarios. Growth rates, chlorophyll a contents and maximal photochemical quantum yield all decreased from the present to 2100 scenarios. While cellular particulate organic carbon significantly increased in both species along with increased cellular organic C:N ratios, biogenic silica content showed species-specific differences among the cluster treatments. Our results suggest that reduced thickness of the upper mixed layer with enhanced stratification may interact with ocean acidification to influence diatom-related biogeochemical processes by affecting their growth and biochemical composition in species-specific ways.