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.