Ocean Acidification

To predict effects of climate change and possible feedbacks, it is crucial to understand the mechanisms behind pCO₂ responses of biogeochemically relevant phytoplankton species. Previous experiments on the abundant N₂-fixer Trichodesmium demonstrated strong pCO₂ responses, which were attributed to an energy reallocation between its carbon and nitrogen acquisition. Pursuing this hypothesis, we manipulated the cellular energy budget by growing Trichodesmium erythraeum IMS101 under different pCO₂ levels (180, 380, 980 and 1400 µatm) and nitrogen sources (N₂ and NO₃^–). Subsequently, biomass production and the main energy-generating processes (photosynthesis and respiration) and energy-consuming processes (N₂-fixation and carbon acquisition) were measured. While oxygen fluxes and chlorophyll fluorescence indicated that energy generation and its diurnal cycle was neither affected by pCO₂ nor nitrogen source, cells differed in production rates and composition. Elevated pCO₂ increased N₂-fixation and organic carbon and nitrogen contents. The degree of stimulation was higher for nitrogenase activity than for cell contents, indicating a pCO₂ effect on the transfer efficiency from N₂ to biomass. pCO₂-dependent changes in the diurnal cycle of N₂-fixation correlated well with carbon affinities, confirming the interactions between nitrogen and carbon acquisition. Regarding effects of the nitrogen source, production rates were enhanced in NO₃^– grown cells, which we attribute to the higher N retention and lower ATP demand compared to N₂-fixation. pCO₂ effects on carbon affinity were less pronounced in NO₃^– users than N₂-fixers. Our study illustrates the necessity to understand energy budgets and fluxes under different environmental conditions for explaining indirect effects of rising pCO₂.

Eichner M., Kranz S. A. & B. Rost, in press. Combined effects of different CO2 levels and N sources on the diazotrophic cyanobacterium Trichodesmium. Physiologia Plantarum. Article. Article (subscription required).