Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
The 14C method has been used extensively by both limnologists and oceanographers to measure photosynthetic rates in aquatic systems, and the large database of 14C measurements that now exists is the ground truth with which satellite algorithms for estimating marine photosynthetic rates on a basin and global scale have been calibrated. However, disconcerting uncertainties still remain with respect to whether and under what circumstances the 14C method provides an estimate of net or gross photosynthesis, or something in between. My study combined batch and continuous culture studies to clarify this ambiguous issue. The batch culture work with seven species of marine phytoplankton indicated that the 14C method should estimate net photosynthesis for Isochrysis galbana and Dunaliella tertiolecta, gross photosynthesis for Chlorella kessleri, and a rate in between for the remaining four species. 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. For I. galbana the photosynthetic rates estimated by 14C uptake were in good agreement with the actual net fixation, but for C. kessleri the 14C method overestimated TOC fixation by roughly 50–100%, the degree of overestimation depending on incubation length and growth condition. Time-course studies with C. kessleri indicated that at a high nitrate-limited growth rate recently fixed carbon began to enter the respiratory substrate pool after a time interval of about four hours. Results of 12:12 L:D cycle incubations were not as straightforward to interpret as the continuous culture results, but the calculated photosynthetic rates relative to net carbon fixation were clearly a function of species and growth rate. The fact that the specific activity (SA) of organic carbon respired in the dark period was less than the SA of the inorganic carbon in the growth medium implies that carbon respired in the dark was a combination of recently fixed carbon and old carbon. These results imply that in field studies the uptake of 14C during the photoperiod overestimates net photosynthesis, the degree of overestimation depending on the growth conditions and composition of the phytoplankton community.
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Pei, Shaofeng, "What does the ¹⁴C method for estimating photosynthetic rates in the ocean really measure?" (2012). LSU Doctoral Dissertations. 4036.
Laws, Edward A.