FEMS Microbiology Ecology 14 (1994) 33-44 © 1994 Federation of European Microbiological Societies 0168-6496/94/$07.00 Published by Elsevier 33 FEMSEC 00518 Organic matter mineralization in an organic-rich sediment: Experimental stimulation of sulfate reduction by fish food pellets Marianne Holmer * and Erik Kristensen Institute of Biology, Odense University, Campusvej 55, DK-5230 Odense M, Denmark (Received 15 September 1993; revision received 23 December 1993; accepted 27 December 1993) Abstract." The combined effects of organic matter additions and temperature on short chain fatty acid (SCFA) turnover, sulfate reduction and nutrient accumulation were examined in an organic-rich fish farm sediment. Fish food pellets, which contribute significantly to the organic matter loss from fish farms, were added to surface sediment at three loadings (2.8; 14.0; 28.0 mg ww g-1 ww sediment; equivalent to organic matter loadings measured during fish farming) and incubated for 30 days in anaerobic bags at 5°C and 15°C. SCFA accumulated to high levels (acetate up to 85 mM, propionate up to 17 raM, butyrate up to 25 mM) in sediments amended with food pellets, and sulfate reduction was stimulated up to 30 times relative to unamended sediments. Sulfate reducers appeared saturated with substrates (SCFA) even in the lowest additions. A low C/N ratio (0.4-1.8) of the major mineralization products (TCO 2 and NH~-) indicated preferential nitrogen mineralization in amended sediment compared with the total particulate pool (C/N = 8.8-11.9) and added food pellets (C/N = 8.4). Key words: Organic stimulation; Sulfate reduction; Short chain fatty acid dynamics; Carbon and nitrogen mineralization Introduction An increased input of organic matter to coastal sediments has significant impact on numerous biogeochemical processes, e.g. sulfate reduction, methanogenesis, ammonium and phosphate pro- duction, as well as on the turnover of small or- ganic molecules such as short chain fatty acids * Corresponding author. Present address: Institute of Life Sciences and Chemistry, 17.2 Roskilde University, PO Box 260, DK-4000 Roskilde, Denmark. Tel: (46) 75 7711; Fax: (46) 75 7721; e-mail: Holmer@mmf.ruc.dK. and amino acids [1-5]. Anaerobic mineralization rates stimulated several orders of magnitude have been found in sediments underlying marine fish farms [6,7]. There is, however, a need for eluci- dating the factors controlling mineralization pro- cesses in such eutrophic sediments, since both the quantity as well as the quality of the sedimentary organic matter affects both reaction rates and pathways of microbial processes [8-12]. Several studies have examined the relationship between organic matter input and anaerobic min- eralization in sediments [10-13], but all have been conducted with loading rates much less than those usually found in fish farm sediments. Sulfate re- SSDI 0168-6496(93)E0078-V