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B a c t e r~a l growth rates In sedlments have been calculated fiom measurements of the rate of lncolporatlon of trltlated thymldlne Into d e o x y r~b o n u c l e~c acld (DNA) T h e dllutlon of Isotope In DNA w a s used to estlmate the s u m of the pools of thymldlne In the sedlment and of other cellular precursors of t h y m~d~n eIn DNA Growth rates of bacterla In the surface zone of seagrass bed sedlrnents v a r~e d from 3 7 X 10' cell d l v~s~o n s h -' g -' dry weight of sediment on a hot autumn day to 3 3 X 10" cell d l v l s~o n s h -' g ' In w~n t e r By c o m b~n~n g growth rate measurements uslng the ~s o t o p e d~l u t~o n procedure with biomass measurements ~t IS now p o s s~b l e to obtaln redsondbly r e l~a b l e estimates of b a c t e r~a l product~vity In sedlments Seagrass beds are highly productive plant communltles, in which much of the primary production 1s not u t~l i z e d directly by animals, but enters h~g h e r trophlc levels through microorganisms, especially bacteria ( P h i l l~p s and McRoy, 1980) To quantify this process we need to know not only the blomass of the bacterial population, but also its growth rate Methods are available for measuring biomass in these sedim e n t~, w h~c h show that bacteria in seagrass beds of Moreton Bay Queensland constitute dbout 20 O/o of the sedlment organic matter excluding that which seagrass roots contribute ( M o r l a~t y ,1980) The measurement of growth rates of the whole population has not been possible by classical microb~ological techniques By measuring the rate of synthesis of d e o x y r~b o n u c l e~c acid (DNA), we hoped to be able to estimate the growth rate of bactena In the sedlment Bacteria In sedlments take up [ m e t h~l -~H ] thymidine (thymine-2-deoxyribose Tdr) and use lt for DNA synthesis (Tobln a n d Anthony, 1978) Fuhrman a n d Azam (1980) have used the rate of incorporation of Tdr into DNA in seawater to e s t~m a t e growth rate of planktonic bacterm They assumed that by adding a large excess of ~s o t o p e , the contribution of Tdr from other pools or pathways would be negligible In fact this may not be the case as Rosenbaum-Oliver and Zamenhof (1972) found that exogenous Tdr contributed only a portlon of the Tdr in DNA, varylng from 35 % to 63 % In a normal strain of Escherichia coli, depending on the culture conditions. The highest value of 63 % of exogenous Tdr was obtalned with l mg Tdr m l -' of culture medium. If natural populations of bacteria behave similarly, then to estlmate the rate of DNA synthesis, a n d thus obtain the rate of bacterial division, it is necessary to measure the dilution of a d d e d Tdr by pools in the cells a n d in the sediment. Thymidine-5'-triphosphate (dTTP), the final precursor in DNA synthesis, is synthesised only partly from exogenous Tdr and partly via other pathways within the cell (Rosenbaum-Oliver and Zamenhof, 1972). An isotope dilution experiment can be used to d e t e r m~n e the effect of added 'cold' precursor on the amount of labelled precursor incorporated into ...

Organic carbon and nitrogen and bacterial biomass were measured in the sediments and gut contents of H. atra and S. chloronotuson the Great Barrier Reef. Organic carbon averaged from 3.4 to 4.7 mg g-1, organic nitrogen from 0.20 to 0.31 mg g-1 and muramic acid from 1.4 to 3.3�g g-1 dry weight of surface sandy sediments. Bacterial biomass, determined by muramic acid measurements, averaged 3-8% of organic carbon in the sediments; blue-green algae accounted for 3-7% of muramic acid. Significantly higher values of organic carbon and nitrogen and muramic acid were found in foregut contents of the holothurians, indicating selective feeding on organically rich components of the sediment. Carbon values were 16-34% higher in the foregut than in the sediment. nitrogen values 35-111% higher and muramic acid values 33-300% higher. These values indicate that bacteria and nitrogenous components of the organic matter were selectively eaten. Values for organic carbon and nitrogen and muramic acid were generally lower in the hindgut than in the foregut, due to digestion and assimilation. Assimilation efficiencies averaged 30% for organic carbon, 40% for organic nitrogen and 30-40% for muramic acid (bacteria). Detritus (non-living matter) probably constituted 60-80% of the organic matter in the sediment and thus the food of the holothurians.

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Exopolymer mucus secretions of bacteria and diatoms are potential foods for benthic animals. These secretions are coincidently ingested by animals during consumption of microbial cells and sediments. The utilization of microbial secretions was investigated with exopolymer derived from a marine bacterium (Pseudomonas sp.) from seagrass beds and a harpacticoid copepod Laophonte sp. from the same habitat. A new technique was developed to examine ingestion, absorption, and absorption efficiencies of these bacterial secretions by consumers. Exopolymer mucus (from the bacterium in stationary phase) was labeled with 14C, collected, purified, and bound onto bacteriumsized beads. The exopolymer slime coating mimicked the coatings associated with many marine bacteria. Results from feeding experiments where the coated beads were mixed with sediment demonstrated that the mucus-exopolymer secretions of bacteria were ingested and utilized by Laophonte sp. Absorption efficiencies, determined directly, were >80% in the presence of other food resources, indicating that the exopolymer is potentially a highly labile C resource for this