Short- and long-term effects of irradiance and CO2 availability on carbon fixation by two marine diatoms

Unbuffered and nutrient-replete dilute batch cultures of Skeletonema costatum Cleve and Phaeodactylum tricornutum Bohlin were grown at high and low CO2 availability conditions and two incident irradiances, 150 and 30 µmol photons·m–2·s–1. Long-term combined effects of such light and CO2 availability conditions on carbon fixation rates of both diatoms were compared. At saturating light, P. tricornutum showed higher photosynthetic rates than S. costatum at both CO2 conditions. However, under subsaturating light, carbon fixation rates of P. tricornutum were higher than observed for S. costatum only at low CO2. Skeletonema costatum showed a strong reduction in photosynthetic rates only when both resources, irradiance and CO2, were low. Short-term alterations of light and CO2 availability on carbon fixation showed that the response of S. costatum differed considerably from long-term trends: the short-term reduction in CO2 availability at both light levels resulted in a considerable decrease in the maximum photosynthetic rates. This effect was much less noticeable in P. tricornutum. The results show that, at saturating light, both diatoms maintain maximum photosynthetic rates under low CO2 levels, but only P. tricornutum is well adapted to rapid changes in this resource. This capacity of adaptation seems to be light dependent, since light limitation altered the responses of both diatoms to low CO2 availability conditions.Key words: CO2, 14C fixation, irradiance, Phaeodactylum tricornutum, Skeletonema costatum.