Combination of biomolecular and stable isotope techniques to determine theorigin of organic matter used by bacterial communities: application to sediment

Natural isotopic composition is a good tool to trace organic matter in ecos ystems. Recent studies used a combination of molecular and stable isotope t echniques to determine the origin of the organic carbon used by bacteria in the water column. In our study, we show that this procedure can be used fo r analysis of sediment bacterial communities with few modifications. In the water column, bacterial recovery is done before DNA extraction. In the sed iment, we tested qualitatively and quantitatively a direct and indirect ext raction of DNA. The direct extraction was the most efficient. It recovered between 3.1 and 15.8 mu g DNA g(-1). dry sediment and the contamination of field samples by eucaryotic DNA was less than 13%. In this preliminary stud y of the salt marsh ecosystem, the delta(13)C values of DNA (-26 to -24 par ts per thousand) recovered from the sediment were close to the delta(13)C v alues of halophytic plants (-26.4 and -25.3 parts per thousand) showing a r elationship between plants and microorganisms, Thus, this procedure can be used to trace the flow of carbon through the sediment microbial biomass and to understand the variation of bacterial activity according to the inputs of allocthonous and autochtonous organic matter. (C) 1999 Elsevier Science B.V. All rights reserved.