Detritus and microbial ecology in aquaculture

Proceedings of the 1985 Bellagio Conference on Detrital Systems for Aquaculture held in Bellagio, Como, Italy, to analyze and summarize available information on detrital food chains and the means for their manipulation in aquaculture. Contains 16 papers and related discussions.Detritus, Food chains, Microbial ecology, Aquaculture

A method for estimating the biomass of bacteria in aquatic sediments and its application to trophic studies

A method is described for estimating the biomass of bacteria in aquatic sediments by an enzymic measurement of D-lactic acid derived from hydrolysis of muramic acid. A correlation is shown between muramic acid and biomass. The Gram-negative rod bacteria contain about 20 μg muramic acid/mg carbon whereas the Gram-negative or Gramvariable pleomorphic and Gram-positive bacteria contain about 100 μg muramic acid/mg carbon. Thus to measure biomass, the relative proportions of these bacteria in the population must be measured. The method is limited at present to sediments in which the biomass of blue-green algae is insignificant compared to bacteria. It is particularly suited to measuring the biomass of bacteria in sediments ingested by animals. This is illustrated by analysis of the gut contents of two deposit-feeders, a mullet (Mugil cephalus) and a prawn (Metapenaeus bennettae), in which it is shown that bacteria are an important component of their diet

Quantitative studies on bacteria and algae in the food of the mullet Mugil cephalus L. and the prawn Metapenaeus bennettae (Racek & Dall)

The biomass of bacteria ingested by two deposit-feeding animals has been estimated by measuring the muramic acid in their gut contents. Bacteria comprised about 15 to 30 % of the organic carbon in the stomach of the mullet Mugil cephalus L. feeding on sea-grass flats and about 20 to 35 % of organic carbon in the proventriculus of the prawn Metapenaeus bennettae (Racek & Dall) feeding on muddy estuarine sediments. Diatoms made up about 20 to 30 % of the carbon in the mullet stomachs, but algae were unimportant in the food of most of the prawns examined. In the mullet stomachs, total organic carbon was about 2 to 3 % of ash weight, an increase of 10 to 20 fold over that in the in situ sediments. The prawns were much more selective in their feeding since the organic carbon was about 30 to 45 % of ash weight in the proventriculus. In laboratory experiments, five species of bacteria and one blue-green alga were digested and assimilated by the prawns. The highest percentage assimilation was > 90 % and this is interpreted to indicate that micro-organisms passing into the digestive gland were nearly completely digested whereas those which passed directly from the proventriculus to the mid-gut were poorly digested. Evidence is presented which suggests that the mullet feed only during the day and not at night on the sea-grass flats. A regression equation is given for the dry weight of sediment in the full gut on fresh weight of fish. A calculation of the amount of sediment (and hence bacteria and algae) eaten per day by a given fish, exemplifies the application of the available tentative data

Ultrastructure of bacteria and the proportion of Gram-negative bacteria in marine sediments

Bacteria in sediments from the surface aerobic layer (0-1 cm) and a deeper anaerobic layer (20-21 cm) of a seagrass bed were examined in section by transmission electron microscopy. Bacteria with a Gram-negative ultrastructure made up 90% of bacteria in the surface layer, and Gram-positive bacteria comprised 10%. In the anaerobic zone, Gram-negative bacteria comprised 70% and Gram-positive bacteria 30% of the bacterial population. These differences were highly significant and support predictions of these proportions made from muramic acid measurements and direct counting with fluorescence microscopy. Most cells were enveloped in extracellular slime layers or envelopes, some with considerable structural complexity. The trophic value to animals of these envelopes is discussed. A unique organism with spines was observed

Microbial respiration and diffusive oxygen uptake of deep-sea sediments in the Southern Ocean (ANTARES-I cruise)

Benthic microbial respiration and diffusive oxygen uptake were measured, and used to calculate rates of aerobic mineralization of organic matter and concomitant CO2 production at the sediment water interface in the abyssal region of the Crozet Basin. This study was part of the ANTARES-I cruise on the R.V. Marion Dufresne, in the Permanent Open Ocean Zone of the Southern Ocean on a south to north transect from 52°to 42°S in the Indian Ocean. At all stations, oxygen penetrated much deeper than 10 cm. Aerobic respiration was maximal in the top 1 cm (10-96 nmol cm-3 day-1), was always detectable down to 5 cm depth, and at some stations even to 10 cm depth. Total depth-integrated oxygen consumption corresponded to within ±25% of the diffusive oxygen uptake across the sediment water interface. The shape of the profile indicated that a diffusive downward flow of oxygen occured below 10 cm depth. Thymidine incorporation experiments suggested that bacteria, present at depths of between 15 am and 1 m in the sediment, were in a dormant state or growing extremely slowly. However, rapid DNA-synthesis started within 4 h after thymidine was added, indicating a deep bacterial biosphere in Southern Ocean sediments. It is proposed that the diffusive downward flux of oxygen below 10 cm depth sustains aerobic bacterial metabolism and survival at greater depths. Along the transect, the total depth-integrated oxygen uptake peaked at 48°S close to the Polar Front, and at the Subantarctic and Subtropical Convergence Frontal Zone. Nevertheless, in general, the differences were not very pronounced. The average value of depth-integrated microbial O2-consumption was 0.61 mmol m-2 day-1, which is equivalent to a carbon mineralization rate of 2.3 g Cm-2 year-1. These observations, together with relatively high Electron Transport System (ETS)-values (6.6 μl 02 g-1 h-1) and bacterial numbers (4 x 109 g-1 dry weight) in the top centimeter, imply that this region is less oligotrophic than previously assumed.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Phytoplankton community structure and productivity in relation to the hydrological regime of the Gulf of Carpentaria, Australia, in summer

To describe the influence of hydrology on the phytoplankton communites of the Gulf of Carpentaria, six phytoplankton parameters were measured on five transects: productivity, biomass, community structure, phospholipid-derived fatty acids, ratios of stable carbon isotopes and the relationship between photosynthesis and irradiance. The mean (±s.e.) estimate of depth-integrated phytoplankton productivity was 914 ± 185 mgC m-2 day-1 (n = 9). In the shallow ( 10 fim, with little in between. The highest rates of growth occurred within a very narrow light regime: self-shading limited growth in deeper water (at light intensities less than 125 mol s-1 m-2), and growth was photoinhibited in shallow water (at light intensities greater than 150 mol s-1 m-2). The resulting biomass-specific primary productivity (mgC mgChla-1 day-1) maximum did not coincide with the depth of either the chlorophyll a maximum or the highest nutrient concentration. The natural carbon isotope ratio for the particulate matter showed that little land-derived carbon was exported beyond a narrow coastal fringe (about 10 to 20 km). The profiles of phospholipid fatty acids also showed that particulate organic matter was dominated by algal-derived compounds, which confirms that the bulk of the organic matter in the offshore Gulf of Carpentaria is of marine origin. Therefore, during the summer in this study, the stratified waters of the central gulf were both biologically and hydrologically independent of the coastal water and not influenced by terrestrial runoff. The phytoplankton production in the central gulf is maintained by nutrient supplies from internal sources, such as those that are remineralized in and resuspended from the sediment