Influence of three contrasting detrital carbon sources on planktonic bacterial metabolism in a mesotrophic lake

Lakes receive organic carbon from a diversity of sources which vary in thei r contribution to planktonic microbial food webs. We conducted a mesocosm s tudy to test the effects of three different detrital carbon sources (algae, aquatic macrophytes, terrestrial leaves) on several measures of microbial metabolism in a small meso-eutrophic lake (DOC approximate to 5 mg/L). Smal l DOC additions (Delta C < 1 mg/L) affected bacterial numbers, growth, and pathways of carbon acquisition. Macrophyte and leaf detritus significantly increased TDP and color, but bacterial densities initially (+12 h) were una ffected. After 168 h, densities in systems amended with terrestrial detritu s were 60% less than in controls, while production rates in mesocosms with macrophyte detritus were 4-fold greater. Detritus treatments resulted in gr eater per-cell production rates either through stable cell numbers and grea ter growth rates (macrophyte-C) or lower densities with stable production r ates (terrestrial-C). After only 12 h, rates of leucine aminopeptidase (LAP ase) activity were 2.5x greater in macrophyte-C systems than in controls, b ut LAPase and beta-N-acetylglucosamindase activities in systems amended wit h terrestrial-C were only 50% of rates in controls. After 168 h, beta-xylos idase rates were significantly greater in communities with terrestrial and phytoplankton detritus. Microbial utilization of >20% of 102 carbon sources tested were affected by at least one detritus addition. Macrophyte-C had p ositive (6% of substrates) and negative (14%) effects on substrate use; ter restrial detritus had mainly positive effects. An ordination based on carbo n-use profiles (+12 h) revealed a duster of macrophyte-amended communities with greater use of psicose, lactulose, and succinamic acid; controls and a lgal-detritus systems were more effective in metabolizing two common sugars and cellobiose. After 168 h, communities receiving terrestrial detritus we re most tightly clustered, exhibiting greater use of raffinose, pyroglutami c acid, and sebacic acid. Results suggest that pelagic bacterial communitie s respond to changes in organic carbon source rapidly and by different rout es, including shifts in per-cell production rates and variations in degrada tion of a variety of compounds comprising the DOC pool.