LINKING TIME-DEPENDENT CARBON-FIXATION EFFICIENCIES IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) TO UNDERLYING METABOLIC PATHWAYS

The chl-specific short-term 14C-based production (Pb) measurement is a widely used tool to understand phytoplankton responses to environmental stresses. However, among the metabolic consequences of these stresses is variability in lifetimes of newly fixed carbon that cause Pb to range between chl-specific net primary production (NPP*) and chl-specific gross photosynthetic electron flow that is available for carbon reduction (GPP*(ARC)) depending on growth rate. To investigate the basis for this discrepancy, photosynthate utilization was characterized in Dunaliella tertiolecta Butcher grown at three different growth rates in N-limited chemostats. Pb was measured throughout a 2 min to 24 h time course and showed clear growth-rate-dependent differences in lifetimes of newly fixed carbon. C-14 pulse-chase experiments revealed differences in patterns of carbon utilization between growth rates. At high growth rate, the majority of C-14 was initially fixed into polysaccharide and lipid, but the relative contribution of each labeled biochemical pool to the total label changed over 24 h. In fast-growing cells, labeled polysaccharides decreased 50%, while labeled lipids increased over the first 4 h. At low growth rate, C-14 was initially incorporated primarily into protein, but the contribution of labeled protein to the total label increased over the next 24 h. Together, time-resolved measurements of Pb and cellular NAD and NADP content suggest an enhanced role for alternative dissipation pathways at very low growth rate. Findings of this study contribute to an integrated understanding of growth-rate-dependent shifts in metabolic processes from photosynthesis to net growth.