RESUMO
Tritiated thymidine incorporation (TTI) into DNA was used to estimate bacterial productivity in sediment and water samples from two sites in Langebaan Lagoon, South Africa. Routine analysis of isotope dilution showed seasonal variations of approximately threefold in the thymidine precursor pool sizes for bacterial assemblages from each site. Dual label incorporation of [(3)H]-thymidine and (14)C-leucine into DNA and protein, respectively, showed that pelagic but not sediment assemblages were in a balanced state of growth during TTI. This is the first report of dual label measurements of bacterial production in sediments. Sediments supported bacterial productivities that exceeded those in the water column by factors from five- to 950-fold, whereas bacterial abundance supported by sediments exceeded that in the water column by more than 3 orders of magnitude. Estimates of bacterial productivities in sediments were coincident with levels of organic content in sediments, but not with bacterial abundance. Measurements of TTI activity for 5 different benthic microhabitats at one lagoon site showed highest activity associated with seagrass beds (2.11 ± 0.84 nmol thymidine hours(-1) g-1 dry weight), whereas activities decreased with depth (0.46 ± 0.21 nmol thymidine hours(-1) g(-I) dry weight) below sediment surface.
RESUMO
The presence and digestive capabilities of bacteria associated with the digestive systems and habitats of two saltmarsh-burrowing detritivore thalassinid prawns (Upogebia africana andCallianassa kraussi) was examined.U. africana is a filter-feeding prawn inhabiting muddy deposits, whereasC. kraussi, a deposit feeder, inhabits coarser more sandy deposits. Scanning electron microscopy was used to examine the gut lining and associated microflora and the nature of the ingested food of both prawns. The gut contents of both prawns included plant fragments, fragmented diatoms, partially degraded protozoa, and bacteria attached to organic matter. In bothU. africana andC. kraussi the midgut walls and gut contents were extensively coated by filamentous bacteria which were absent in the hindgut. The hindgut epithelium ofU. africana was coated by mats of rodshaped bacteria, not reported in marine invertebrates previously. The digestive glands of both species contained bacteria in the lumen. Isolation of gut and habitat bacteria suggests that bothU. africana andC. kraussi maintain a gut microflora distinct from the habitat microflora. Bacteria isolated from the guts of both species of prawn differed from those isolated from their respective habitats with regards to both the genera isolated and their digestive capabilities. The dominant genera isolated from the guts of bothU. africana andC. kraussi wereVibrio andPseudomonas, with an unidentified fermenter andPseudomonas, respectively dominating in the digestive glands. Bacteria of the genusAcinetobacter dominated the isolates from the habitats of both species of prawn. Resident gut bacteria isolated from the guts of both species of prawn exhibited lipase, protease, chitinase, and lysozyme, but not cellulase activity, and may contribute to nitrogen aquisition by the prawns. Isolates from the prawns' habitat exhibited alginase, gelatinase, and lipase activity, a few (3%) fromU. africana habitat having cellulases. In this study a distinction between resident gut bacteria and transient gut bacteria was made. Results suggest that some habitat bacteria remain viable in the guts ofU. africana, but not inC. kraussi.
