Partitioning radiolabeled thymidine uptake by bacteria and meiofauna using metabolic blocks and poisons in benthic feeding studies*

Techniques exist which allow for the measurement of in situ grazing rates of meiobenthos on sedimentary bacteria and microalgae. Radiolabeled substrates are incorporated into microbes which serve as food for meiofauna and which themselves also become labeled during feeding. However, during in situ grazing experiments, meiofauna may become radiolabeled by a variety of non-feeding processes. Proper controls to correct for these extraneous routes of labeling have been developed in the present study. The use of [methyl-3H] thymidine (3HTdR) in studies of meiofaunal grazing on bacteria has two unique advantages: (1) it is incorporated only into prokaryotic macromolecules, and (2) bacterial incorporation of 3HTdR may be selectively blocked by several inhibitors which are non-toxic to meio faunal grazers. Coupled with formalin-killed control treat ments, the use of these inhibitors makes it possible to accu rately determine the partitioning of radiolabel into meiofauna during grazing into adsorptive, absorptive and grazing components. A saturated solution of nalidixic acid and 5'-deoxythymidine was found to be most effective in inhibiting water-column bacterial uptake and incorporation of 3HTdR, and had no toxic effects on meiofauna. The inhib itor was found to immediately block bacterial incorporation of 3HTdR and was as effective at 20% saturation as at 100%. The presence of sediment reduced the effectiveness of this inhibitor by 50%. Solutions of the inhibitor with excess undissolved material, however, completely blocked sediment bacterial uptake of 3HTdR. Employing these techniques during in situ grazing experiments showed that up to 83% of total meiofaunal uptake of 3H-label may be attributable to non-grazing processes. Experiments conducted in slurried sediments yielded grazing rates which were the same as those from intact cores. Furthermore, meiofaunal grazing rates on multiple food sources (e.g. bacteria and diatoms) may be determined synoptically by adding isotopically-distinct sub strates (e.g. 3HTdR and H14CO3) to the same experimental incubation.