Paper Reviewed
Hong, H., Li, D., Lin, W., Li, W. and Shi, D. 2017. Nitrogen nutritional condition affects the response of energy metabolism in diatoms to elevated carbon dioxide. Marine Ecology Progress Series 567: 41-56.

Diatoms are important unicellular phytoplankton that play a significant role in sequestering carbon into the deep ocean, accounting for approximately 40 percent of all marine primary production (Falkowski et al., 2004). This is due, in part, to their possession of efficient carbon concentrating mechanisms (CCMs) that elevate the CO2 concentration at the site of carbon fixation by the enzyme rubisco, thus improving their ability to photosynthesize at lower atmospheric CO2 concentrations. Consequently, it has been predicted that diatoms, like other marine phytoplankton, will benefit from rising atmospheric CO2 concentrations in the future by down-regulating their CCMs, which will save energy resources that can be utilized for other growth-related processes.

But is this hypothesis correct?

In a recent test of this premise, Hong et al. (2017) cultured three diatom species (Thalassiosira pseudonana, Phaeodactylum tricornutum and Thalassiosira weissflogii) at ambient (400 ppm) and elevated (750 ppm) CO2 levels under both nitrogen-replete (100 µM of NO3^-) and nitrogen-limiting (15 µM of NO3^-) conditions, whereupon they examined key metabolic processes (light harvesting, carbon fixation, photorespiration, respiration and nitrogen assimilation) and incorporated their findings into an energy budget analysis to determine if such energy reallocation did indeed take place to benefit this marine species at elevated concentrations of atmospheric CO2. Their experiment was conducted in a controlled laboratory setting after a pre-acclimation period of approximately 50 generations.

Results of their analysis revealed that under high CO2 and replete nitrogen (N) conditions, down-regulation of the CCM stimulated diatom photosynthesis and growth and was the primary factor responsible for that stimulation. Under high CO2 and nitrogen-limiting conditions, down-regulation of the CCM was also a factor in stimulating growth and reducing photorespiration, but elevated CO2 additionally significantly impacted the photosynthetic photon flux and respiration, which "resulted in an increase of the C:N ratio in all three diatom species." And as a result, Hong et al. state that "the N-limited diatoms therefore could fix more C per unit of N in response to elevated CO2, which could potentially provide a negative feedback to the ongoing increase in atmospheric CO2."

In further commenting about the significance of their findings, the five Chinese researchers note that although the growth enhancements they observed were "mostly on the order of 10 to 20%, they could be of consequence as diatoms are known to contribute ~40% of marine primary productivity," and they add that "in vast regions of the ocean that are N-limited, the export of C is effectively determined by the C:N ratio of the sinking particles and the input of new N to the surface water." Therefore, they continue, "if N input remains unchanged and diatoms fix more C per unit of N in response to elevated pCO2, it could potentially result in increased C export and thus a negative feedback to the ongoing increase in atmospheric CO2." Consequently, in light of all of the above findings, it would appear that future ocean acidification could very well increase the sequestration of carbon into the deep ocean and enhance the overall marine biological carbon pump.

Reference
Falkowski, P.G., Katz, M.E., Knoll, A.H., Quigg, A., Raven, J.A., Schofield, O. and Taylor, F.J. 2004. The evolution of modern eukaryotic phytoplankton. Science 305: 354?360.