The Response of Nitrogen-limited Marine Phytoplankton to Increasing Carbon Dioxide

Abstract

With atmospheric carbon dioxide (CO2) concentrations rising rapidly as a result of anthropogenic emissions, understanding the potential of the ocean to store carbon is more important than ever. Nitrogen (N) limits primary production in large parts of the ocean, and hence the export of inorganic carbon to deep waters. The response of marine phytoplankton to this ongoing increase in CO2 is unknown, especially whether the critical C:N ratio of phytoplankton biomass will deviate from canonical values at high CO2. In regions of the ocean where nitrogen supply is limited, the C:N ratio of the phytoplankton biomass determines how much organic carbon can be exported from the surface to the deep, i.e. the efficiency of the biological pump. A change in C:N at high CO2 could lead to a significant feedback to increasing atmospheric CO2.

In this thesis, I address the question of how C:N ratios of N-limited phytoplankton may change with CO2 concentration by conducting a combination of field work with natural populations of phytoplankton and laboratory experiments with model organisms under well defined conditions. The primary goals of this work were to identify trends in C:N ratios with CO2 under N-limiting conditions and to elucidate the underlying mechanisms responsible for any such trends.

In the field, the C:N ratios of phytoplankton biomass showed a modest increase from low to high CO2 in 4 out of 5 N-limited experiments due chiefly to an increase in particulate organic carbon (POC). In contrast, parallel experiments under N-replete conditions showed no change in C:N ratios. Diatoms and coccolithophores accounted for an important fraction of the ambient phytoplankton population in the N-limited experiments that showed an effect of CO2 on the C:N ratio of the biomass, while cyanobacteria were dominant in the experiment that showed no effect. The concentration of Rubisco enzyme decreased at high CO2 in several N-limited experiments, but this decrease did not account for the change in C:N since it was only a small fraction of total protein.

Contrary to the long-held assumption that Rubisco may account for up to half of total protein in phytoplankton, Rubisco concentrations represented less than 6% of total protein in laboratory cultures of eight species of microalgae and also in field experiments. Rubisco concentrations increased with growth rates. Theoretical calculations using our data suggest that phytoplankton contain the minimum amount of enzyme necessary to support their growth rates. Rubisco was also observed to decrease with increasing CO2 in some experiments, implying that the enzyme may not be completely saturated at ambient conditions. However, the low concentration of Rubisco in phytoplankton makes such a response to CO2 insignificant in terms of the overall C:N composition of the organisms.

To follow-up on the field results, I investigated the effects of varying CO2 in continuous cultures of one coccolithophore and two diatoms under N-limitation. The C:N ratio of all species increased with decreasing growth rate as expected. As previously reported, the C:N ratio of the coccolithophore Emiliania huxleyi increased at high CO2 due to an increase in cellular carbon. Unexpectedly, I observed a significant increase in the C:N ratio of the two diatoms, Thalassiosira weissflogii and Thalassiosira oceanica, at very low CO2. However the C:N ratio remained essentially constant as the CO2 concentration was increased from current to greater values. Because diatoms are the major contributors to the biological pump and coccolithophores minor contributors, my results, if confirmed and generalizable to other species, imply that little change should be expected in the stoichiometry of the sinking biomass as CO2 increases in surface seawater. Contrary to land plants, marine phytoplankton are unlikely to increase the sequestration of CO2 and provide a negative feedback to the ongoing increase in atmospheric CO2. I note however, that the surprising increase in the C:N ratio of diatoms at low CO2 may be relevant to glacial/interglacial processes when the atmospheric CO2 concentration decreased below 200 parts per million (ppm), about half of what it is today.