Pathways of carbon assimilation in phytoplankton from the Antarctic Ocean1

Abstract

The assimilation of [14C]bicarbonate into the major end‐products of photosynthesis was measured at five stations in the Southern Ocean. Under conditions of low light intensity and low temperature (<0°C) phytoplankton incorporated as much as 80% of the fixed carbon into lipid, with insigificant incorporation into protein. At higher temperatures (>0°C) and higher light intensities incorporation into lipid accounted for <20% of the fixed carbon, the synthesis of polysaccharide and (to a lesser extent) protein being more prominent. Populations showing high rates of incorporation of 14C into lipid have high PC:Chl ratios, high PC:PN ratios, and low C‐specific rates of 14C assimilation. All populations were dominated by diatoms. High concentrations of nitrate, phosphate, and silicate argue against the concept of nutrient limitation. This pattern of photosynthesis differs from that observed in many other parts of the oceans. It is suggested that the prominent synthesis of lipid is a response to extremely low temperatures and is enhanced by the additional stress of low light intensities.