LINKING TIME‐DEPENDENT CARBON‐FIXATION EFFICIENCIES IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) TO UNDERLYING METABOLIC PATHWAYS¹

Abstract

The chl-specific short-term (14) C-based production (P(b) ) 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 P(b) to range between chl-specific net primary production (NPP*) and chl-specific gross photosynthetic electron flow that is available for carbon reduction () 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. P(b) was measured throughout a 2 min to 24 h time course and showed clear growth-rate-dependent differences in lifetimes of newly fixed carbon. (14) C pulse-chase experiments revealed differences in patterns of carbon utilization between growth rates. At high growth rate, the majority of (14) C 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, (14) C 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 P(b) 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.

Photosynthetic parameters in the Beaufort Sea in relation to the phytoplankton community structure

“…In the case of short-term incubations, the most limiting problem lies in the interpretation of the measurements, in particular whether the gross or net primary production was measured. To know where the measurement falls between these two extremes, the growth rates must be known (Dring and Jewson, 1982;Halsey et al, , 2011; we did not have such measurements during the cruise. Previous measurements performed in the Arctic, however, suggest that with the right model, calculations made with PvsE curves of the dailyintegrated primary production obtained from 2 to 4 h incubation provide equivalent results to deck incubations for 24 h (Harrison et al, 1985).…”

Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

“…Incubations lasted for 3.5 h and were ended by gentle filtration onto pre-combusted 25 mm GF/F filters. Given the length of the incubations and the likely slow growth rate of the iron-limited phytoplankton assemblage sampled, our approach probably reflects a rate closer to net rather than gross primary productivity (e.g., Halsey et al, 2011;Pei and Laws, 2013).…”

“…Furthermore, data presented in Schuback et al (2015) included phytoplankton assemblages sampled over a range of iron-limited and iron-replete conditions. The resulting variability in phytoplankton growth rates influences the balance between net and gross carbon fixation captured in 3 h 14 C-uptake experiments (Halsey et al, 2011;Milligan et al, 2015;Pei and Laws, 2013) and affects the derived conversion factor K c /n PSII .…”

Long-Term Acclimation to Iron Limitation Reveals New Insights in Metabolism Regulation of Synechococcus sp. PCC7002

“…PCC7002 long-term acclimated to three levels of Fe limitation and Fe replete conditions. Acclimation processes to nutrient limitation involve shifts in metabolic pathways associated to (1) the variation of carbon flux between the major biogeochemical pools (carbohydrates, proteins and lipids) and (2) the variation in the lifetimes of these pools (Halsey et al, 2011). Therefore, while cyanobacteria cells subjected to abrupt-changes in nutrient availability regulate their homeostasis toward a short-term equilibrium, cells acclimated to Fe limitation are expected to operate though different metabolic pathways that are still poorly understood.…”