Thymidine is usually incorporated directly into DNA by bacterioplankton, but there are a number of environments where it is extensively metabolized. South San Francisco Bay, California (SSFB) is one of these. Substantial amounts of 3H from methyl- ?3H!-thymidine are found in RNA, protein, and a fraction extracted by 80% ethanol. A much larger proportion of the label from 6-?3H!-thymidine is found in DNA, and none of it appears in the 80% enthanol. A much larger proportion of the label from 6- ?3H!-thymidine is found in DNA, and none of it appears in the 80% ethanol extractable fraction. As a result, bacterial productivity estimates differ by factors of up to 10 for the same sample. Additional data suggest that isotope dilution analysis of precursor pool specific activity doesn't work in SSFB either. These results are in sharp contrast with Monterey Bay experiments where thymidine incorporation is "normal": the majority goes into DNA, there is no difference between methyl- and 6-labeled thymidine, and isotope dilution seems to work. The proposed study will investigate this phenomenon in detail. The overall goal is to determine the physiological or environmental basis for the difference between SSFB and "normal" coastal populations and to use this information to adapt the thymidine method to SSFB and other environments where bacterioplankton have similar physiologies (meso- and bathypelagic waters, other coastal waters, hypolimnia of some lakes). Specific objectives are to test the hypotheses that 1) the methyl group is rapidly exchanged with an unlabeled pool in the cells, 2) ring carbon or 6-tritiated thymidine will provide better estimates of DNA synthesis rates in these environments, and 3) the physiological basis for the difference is due to larger DNA and TdR fluxes in SSFB and greater dependence of SSFB bacterioplankton on thymidine as a carbon, nitrogen and energy source relative to "normal" coastal populations.
