MEPS 468:31-42 (2012)  -  DOI: https://doi.org/10.3354/meps09981

ABSTRACT: In a fall 2008 cruise off the coast of California, on-deck incubations with surface seawater were conducted to compare the effect of increasing carbon dioxide (CO₂) on nitrogen (N)-limited versus N-replete (+ NO₃⁻) phytoplankton. Based on previous laboratory studies, we expected that under N-limitation C:N ratios would be more sensitive to CO₂ availability. In particular, we wanted to test whether down-regulation of RuBisCO at high CO₂ leads to an increased C:N ratio, since RuBisCO is thought to be a large pool of N in many autotrophs. In 4 out of 5 Nlimited experiments, the C:N ratio of phytoplankton biomass increased slightly from low to high CO₂, and this increase was due chiefly to an increase in particulate organic carbon (POC), with little change in particulate organic nitrogen (PON). In contrast, the N-replete experiments did not show a change in C:N ratio, but some experiments exhibited higher rates of carbon fixation and growth at high CO₂. In the one N-limited experiment that did not show a change in C:N ratio with CO₂, the ambient population had a higher proportion of cyanobacteria compared with the other experiments. The concentration of RuBisCO decreased at high CO₂ in several N-limited experiments, but this reduction only accounted for a minor proportion of the overall change in C:N ratio. An increase in C:N ratios at high CO₂ in N-limited phytoplankton could provide negative feedback to increasing atmospheric CO₂.

KEY WORDS: CO₂ · Phytoplankton · C:N ratios · N-limitation · RuBisCO