Kevin R. Carman

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.

Evidence that sediment type influences the horizontal and vertical distribution of nematodes at a deep-sea site

Abstract The relationship between sediment type and the nematode fauna was studied at the HEBBLE site (40°27′N, 62°20′W), which is at 4820 m depth on the Scotian Rise in the northeast Atlantic. Because of the hydrodynamic regime, the fine-grained, surficial sediments (brown mud) are eroded and redeposited several times per year, so the properties of brown mud can be expected to be relatively homogeneous, regardless of depth of burial. Brown mud accumulates in topographic lows, so although the 0–1 cm layer in our samples is brown mud, the 1–2 and 2–3 cm layers can be either brown mud or foraminiferan mud. Where both sediment types occur in a single horizontal plane (i.e. the 1–2 cm layer and the 2–3 cm layer), nematodes are more abundant in the brown mud than in the foraminiferan mud, and this pattern occurs across feeding and tail-morphology functional groups. In those samples in which brown mud occurs from 0 to 3 cm, nematodes do not differ significantly in abundance among the three depth layers, suggesting that the usual decrease in abundance with depth in the sediment is not simply a function of proximity to the sediment surface, but relates to a change in the quality of the sediment with depth. Further, in brown mud, nematodes tend to be more abundant in the 1–2 cm layer than in the 0–1 cm layer. At the HEBBLE site, there is reason to suspect that small organisms are at risk on the sediment surface. The unexpected nematode-abundance profile may occur for the same reason.

Comparison of Three Techniques for Administering Radiolabeled Substrates to Sediments for Trophic Studies: Incorporation by Microbes

Three principal methods have been used to administer substrates to sediments: injection, porewater replacement, and slurry. Here we assess how each of these techniques affects incorporation of radiolabels into macromolecules of marine sedimentary microbes. Eighty-five cores of intertidal sand were collected in a randomized-block, factorial design. One set of cores received14C-bicarbonate/3H-thymidine and was incubated in the light; another set received14C-acetate/3H-thymidine and was incubated in the dark. Following a 5-hour incubation, sediments were analyzed for incorporation of radiolabel into lipid fractions (neutral, glyco-, and polar) and DNA. The three methods of isotope administration were also applied to cores subsequently analyzed for polar lipid phosphates and phospholipid fatty-acid (PLFA) profiles. In general, incorporation was greatest when injections were made, consistent with the prediction that incorporation would decrease as specific activity of the radiolabeled substrate was diminished by dilution. The ratio of14C from acetate incorporated into polar and glycolipid fractions indicated that a significant disturbance accompanied the porewater and slurry techniques. Substantial amounts of3H were recovered in the neutral-lipid fraction, indicating that thymidine was catabolized by sedimentary microbes and tritiated products were incorporated by eukaryotes. There were no significant differences in PLFA profiles or estimates of microbial biomass among methods or controls. Incorporation of3H into DNA was similar with all combinations of methods and radiocarbon substrates.14C was extensively incorporated into DNA, indicating that photoautotrophs and heterotrophs utilized radiocarbon from bicarbonate and acetate, respectively, for de novo synthesis of DNA. Injection is suggested as the method of choice, as it presents more flexibility in its application than porewater replacement and disturbs the consortia of gradients in sediments to a significantly lesser degree than porewater replacement and slurry.

Radiotracer Determination of Ingestion and Assimilation of Periphytic Algae, Bacteria, and Adsorbed Amino Acids by Snails

We examined the use of radiotracers to determine if ³H-amino acids (AA) and ¹⁴C-bicarbonate could differentiate grazing by Physella virgata, a pulmonate gastropod, on the bacterial and algal components in periphyton. Large amounts of ³H-AA became associated with periphyton as a result of adsorption to periphyton, rather than microbial uptake. As a consequence of this abiotic adsorption of ³H-AA by periphyton, uptake of ³H by grazers could be due to ingestion of either ³H-labeled bacteria or detritivory on material to which ³H-AA were adsorbed. Physella was able to ingest and assimilate adsorbed ³H-AA directly in the absence of any bacterial uptake of amino acids. In contrast to ³H-AA, relatively little ¹⁴C-bicarbonate adsorbed to periphyton, and essentially all of the uptake could be attributed to photosynthetic activity. ¹⁴C-bicarbonate is, therefore, a reliable tracer for determining grazing activity on the algal component of periphyton assemblages. Calculated assimilation efficiencies for snails grazing on periphyton to which ³H-AA was adsorbed (78%) were greater than those of snails grazing on periphyton in which bacteria had incorporated ³H-AA (50%) or periphyton algae labeled with ¹⁴C-bicarbonate (32%) suggesting that adsorbed organic carbon and detritivory may be important to this snails nutrition.

Comparison of three techniques for administering radiolabeled substrates to sediments for trophic studies: uptake of label by harpacticoid copepods

Microbial grazing by two species of meiofaunal harpacticoid copepods (Heteropsyllus nunni and Thompsonula hyaenae) was determined by uptake of radioactive labels following their introduction into natural sediments from a low-energy intertidal site in Florida (29°54′40″N; 84°31′30″W) in May 1986. Grazing was related to three methods of radioactive-label introduction: injection, porewater replacement, and slurry. Uptake of label by harpacticoids was examined using two dual-label combinations, 3H-thymidine/14C-bicarbonate and 3H-thymidine/14C-acetate. The injection and porewater-replacement methods yielded statistically indistinguishable results. Results obtained by the slurry method differed significantly from the other two methods. We suggest that the unique results obtained in the slurry method were a consequence of the disruption of microbial-meiofaunal spatial relationships.