Does the 14C method estimate net photosynthesis? Implications from batch and continuous culture studies of marine phytoplankton

We carried out batch culture studies with seven species of marine phytoplankton and chemostat studies with two of the seven species to determine whether and to what extent C-14 uptake approximated net photosynthesis. In two of seven cases, Isochrysis galbana and Dunaliella tertiolecta, cells uniformly labeled with C-14 lost no activity when they were transferred to a C-14-free medium and allowed to grow in the light. In similar experiments with four other species, uniformly labeled cells lost activity when incubated in the light, but the loss rates were only a few percent per day. Thus these six species appear to respire primarily recently fixed carbon. In the case of the remaining species, Chlorella kessleri, loss rates of C-14 in the light from uniformly labeled cells were about 29% per day, the apparent ratio of respiration to net photosynthesis being 0.4. Follow-up chemostat studies with I. galbana and C kessleri grown under both light- and nitrate-limited conditions produced results consistent with the implications of the batch culture work: uptake of C-14 by I. galbana after incubations of 24 h yielded estimates of photosynthetic carbon fixation equal to the product of the chemostat dilution rate and the concentration of organic carbon in the growth chamber. Similar experiments with C kessleri produced C-14-based estimates of photosynthetic carbon fixation that exceeded the net rates of organic carbon production in the growth chamber by roughly 55%. Time-course studies with both species indicated that at high growth rates recently fixed carbon began to enter the respiratory substrate pool after a time lag of several hours, a result consistent with previous work with D. tertiolecta. The lag time appeared to be much shorter at low growth rates. The results with C kessleri are similar to results previously reported for Chlorella pyrenoidosa and Amphidium carteri. Collectively these results suggest that C-14 uptake by species with relatively high ratios of respiration to photosynthesis may tend to substantially overestimate net photosynthesis, perhaps because a substantial percentage of the carbon respired by such species is old carbon. (C) 2013 Elsevier Ltd. All rights reserved.