Influence of the carbon concentrating mechanism on carbon stable isotope discrimination by the marine diatom Thalassiosira pseudonana

There is no clear explanation why phytoplankton delta(13)C values are more negative in colder waters, but one current theory suggests that because colder waters hold more CO2, there is less diffusional limitation of CO2. This results in more discrimination against C-13 and more negative phytoplankton delta(13)C values. However, many species are able to actively take up CO2 or HCO3-, the latter being the major inorganic carbon species present in the dissolved inorganic carbon (DIC) pool of modern oceans. A previous study suggests that carbon concentrating mechanism (CCM) induction would affect carbon isotope discrimination, and this study confirms the presence of a relationship between discrimination and induction of a CCM in the marine diatom Thalassiosira pseudonana. CCM induction was measured by determining the half-saturation constant of photosynthesis (K0.5DIC). Values of K0.5DIC increased from 85 to 470 mu M DIC over a range of ambient DIC levels from 0.2 to 2.7 mM. The fractionation factor increased from 10 to 21.3 parts per thousand over this Same range. There was a significant relationship between K0.5DIC and the fractionation factor suggesting that CCM induction state influences carbon isotope discrimination. Other factors that influence discrimination may act through CCM induction.