DNA Synthesis as a Measure of Bacterial Productivity in Seagrass Sediments

Abstract

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 e In 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 com-munltles, 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 bac-teria (P h i l l ~ p s and McRoy, 1980) To quantify this pro-cess 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 sedi-m 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 sea-grass roots contribute (M o r l a ~ t y , 1980) The measure-ment 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 bac-term 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 syn-thesis, is synthesised only partly from exogenous Tdr and partly via other pathways within the cell (Rosen-baum-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 pre-cursor incorporated into a macromolecule (Forsdyke, 1968). In this paper w e show how, using this technique, the total sum of pools contributing thymine bases to DNA synthesis may b e estimated, a n d thus calculate the growth rate of the bacterial population.