Does adenine incorporation into nucleic acids measure total microbial production?1

Abstract

Incorporation of radioactive adenine into DNA has recently been used as a measure of total microbial production in marine environments, sometimes with unexpected results. We have examined two of the assumptions on which the method is based, particularly regarding the distribution of adenine uptake and nucleic acid content in mixed natural microbial assemblages. Size fractionation, autoradiography, and metabolic inhibition data indicate that the requirement that bacteria, algae, and protozoa have a uniform uptake and metabolism of added dissolved adenine dots not appear to hold. In systems where the biomass and productivity were greatly dominated by large phytoplankton, bacteria took up almost all of the adenine added at nanomolar concentrations. A separate experiment with marine ciliates also showed adenine uptake by eucaryotes to be negligible compared to that by bacteria, The method requires a measurement of microbial adenosine triphosphate (ATP) specific activity. In our experiments this quantity would be dominated by ATP in the eucaryotes that took up negligible adenine. In this situation, mostly eucaryotic biomass and not production would influence the "total microbial production" measurement. Second, the C: DNA ratio of 50 used in reports to date is too high for natural bacteria, which have a ratio closer to 5. We conclude that the method is likely to have produced large overestimates of production and that variations (e.g. with depth) may not reflect growth rates, but rather a complicated combination of activities and ratios of eucaryotic : procaryotic biomasses.

Omnivorous feeding by planktotrophic arvae of the eastern oyster Crassostrea virginica

“…Similar correspondence between size fractioned particulate 3H and bacterial abundance has been documented in other studies using 3H-thymidine (e.g. Fuhrman & Azam 1980, Fuhrman et al 1986). Furthermore, one of these studies (Fuhrman et al 1986) also used microautoradiography to confirm the nearly exculsive uptake of 3H-thymidine and 3H-adenine by heterotrophic bacteria.…”

“…Using microautoradiography, they demonstrated that < 1 % of exposed silver grains were associated with organisms other than bacteria in natural plankton samples that had been labeled for up to 12 h. Similar results have been obtained in a number of other studies that employed microautoradiography to examine 3H-thymidine incorporation into natural plankton assemblages (Bern 1985, Fuhrman et al 1986, Douglas et al 1987) and sedimentary microorganisn~s (Carman 1990). Although the likelihood that planktonic organisms will incorporate 'H-thymidine is undoubtedly species-, time-, and locationspecific, on the basis of the results from the above autoradlographic studies, we assumed (and later confirmed, see 'Discussion') that under our incubation conditions, negligible amounts of %thymidine would be incorporated by organisms other than heterotrophic bacteria.…”

“…Fuhrman & Azam 1980, Fuhrman et al 1986). Furthermore, one of these studies (Fuhrman et al 1986) also used microautoradiography to confirm the nearly exculsive uptake of 3H-thymidine and 3H-adenine by heterotrophic bacteria. Based on the dominant role of 1 3 pm heterotrophic bacteria in the uptake of 3H-thymidine in our study, and similar findings from previously discussed autoradiograghpy studies (Fuhrman & Azam 1982, Bern 1985, Fuhrrnan et al 1986, Douglas et al 1987, Carman 1990), we further conclude that 3 to 30 pm Q-labeled organisms were attached bacteria and, to a lesser extent (see below), phagotrophic protozoans which had consumed 3H-labeled bacteria.…”

Unbalanced growth in natural assemblages of marine bacterioplankton

“…Furthermore, variation of average DNA per cell (Table 2) and average cell biovolume ( Paul et al 1985) environments. The average C : DNA ratio of bactena was 4.58 -t 0.58 (Table 2) suggesting that natural marine bacteria have a higher DNA content than cultured bacteria (Fuhrman et al 1986).…”

Utilization of inorganic and organic nitrogen by bacteria in marine systems1

“…The relative importance of phototrophic picoplankton (as indicated by Chl a levels and primary production) tends to be much less in nearshore than oceanic waters (Azam and Hodson 1977;Herbland et al 1985;Probyn 1985;Fuhrman et al 1986). Furthermore, we can estimate the amount of nitrogen required for heterotrophic bacterial growth and compare this with measured uptake rates in estuarine waters off Sapelo Island.…”