Radioactive labeling of a natural assemblage of marine sedimentary bacteria and microalgae for trophic studies: An autoradiographic study

Autoradiography was used to examine critical questions for trophic studies concerning the uptake of radioactive tracers by a natural assemblage of sedimentary microorganisms. Labeled organic substrates ([³H]-acetate and [³H]-thymidine) were taken up only by heterotrophic bacteria, and [¹⁴C]-bicarbonate was taken up only by microalgae. Only approximately 2% of the bacterial assemblage took up detectable quantities of either [³H]-acetate or [³H]-thymidine, regardless of whether labeled substrates were delivered to sediments via slurries or by injection with a microliter syringe. Significantly more diatoms were labeled when [¹⁴C]-bicarbonate was delivered to sediments by the injection method (75%) as compared to the slurry method (50%). These results indicate that radio-active tracers can be used in natural sediments to selectively label potential microbial food of invertebrate grazers. Only a small proportion of bacteria, however, may actually use a labeled substrate, which introduces a large uncertainty into the conversion of radioactivity in grazers to the number of bacteria consumed. Finally, the use of disruptive methods (e.g., slurries) to deliver labels to sediments does not increase the proportion of microorganisms that become labeled. Thus, given the variety of artifacts that may be associated with the use of sediment slurries, it is probably advisable to use nondisruptive methods to deliver substrates to sediments.