The <sup>14</sup>C method has been used extensively by both limnologists and oceanographers to measure photosynthetic rates in aquatic systems, and the large database of <sup>14</sup>C 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 <sup>14</sup>C 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 <sup>14</sup>C method should estimate net photosynthesis for <em>Isochrysis galbana</em> and <em>Dunaliella tertiolecta</em>, gross photosynthesis for <em>Chlorella kessleri</em>, and a rate in between for the remaining four species. Follow-up chemostat studies with <em>I. galbana</em> and <em>C. kessleri</em> grown under both light- and nitrate-limited conditions produced results consistent with the implications of the batch culture work.
For <em>I. galbana</em> the photosynthetic rates estimated by <sup>14</sup>C uptake were in good agreement with the actual net fixation, but for <em>C. kessleri</em> the <sup>14</sup>C method overestimated TOC fixation by roughly 50100%, the degree of overestimation depending on incubation length and growth condition. Time-course studies with <em>C. kessleri</em> 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 <sup>14</sup>C during the photoperiod overestimates net photosynthesis, the degree of overestimation depending on the growth conditions and composition of the phytoplankton community.
| Identifer | oai:union.ndltd.org:LSU/oai:etd.lsu.edu:etd-08252012-152026 |
| Date | 31 August 2012 |
| Creators | Pei, Shaofeng |
| Contributors | Laws, Edward A., LaRock, Paul A., Hou, Aixin, Cable, Jaye E. |
| Publisher | LSU |
| Source Sets | Louisiana State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lsu.edu/docs/available/etd-08252012-152026/ |
| Rights | restricted, I hereby certify that, if appropriate, I have obtained and attached herein a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to LSU or its agents the non-exclusive license to archive and make accessible, under the conditions specified below and in appropriate University policies, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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