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

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. Within the past year or two, some unexpected and potentially important productivity measurements have been reported for various marine environments, including the water column (both suspended and on sinking particles) and sediments. These reports share the same adenine incorporation methodology aimed at measuring the combined production of all bacteria, algae, and protozoa in the habitats studied. Specifically, Winn and Karl (19 84a) reported that total ’ This work was supported in part by NSF grants OCE 84-10074 and OCE 84-067 12 to J.F. and OCE 84-006524 to H.D., and by the NSF-sponsored MECCAS program. microbial production at the 150-900-m depth of the waters off Hawaii is double that of the O-l 50-m depth range (euphotic zone). Burns et al. (1984) estimated total microbial production in coral reef rubble of Kaneohe Bay, Hawaii, and found rates higher than previous estimates of whole reef production. Karl and Knauer (1984) and Karl et al. (1984) reported an increase in total microbial production within rapidly sinking material at the 700-900-m depth and concluded that it is due to chemolithotrophic production. We believe that some of the key assumptions of the adenine method have not been tested adequately on natural samples under field conditions. Here we ex-