Physiological control on carbon isotope fractionation in marine phytoplankton

One of the great challenges in biogeochemical research over the past half
a century has been to quantify and understand the mechanisms underlying
stable carbon isotope fractionation (εp) in phytoplankton in response to
changing CO2 concentrations. Partly, this interest is grounded in the use
of fossil photosynthetic organism remains as a proxy for past atmospheric
CO2 levels. Phytoplankton organic carbon is depleted in 13C compared to
its source because of kinetic fractionation by the enzyme RubisCO during
photosynthetic carbon fixation, as well as through physiological pathways
upstream of RubisCO. Moreover, other factors such as nutrient limitation,
variations in light regime as well as phytoplankton culturing systems and
inorganic carbon manipulation approaches may confound the influence of
aquatic CO2 concentration ([CO2]) on εp. Here, based on experimental data
compiled from the literature, we assess which underlying physiological
processes cause the observed differences in εp for various phytoplankton
groups in response to C-demand/C-supply (i.e., POC production/[CO2]) and
test potential confounding factors. Culturing approaches and methods of
carbonate chemistry manipulation were found to best explain the
differences in εp between studies, although daylength was an important
predictor for εp in haptophytes. Extrapolating results from culturing
experiments to natural environments and for proxy applications therefore
requires caution, and it should be carefully considered whether culture
methods and experimental conditions are representative of natural
environments.