Effects of Grazing by Estuarine Gammaridean Amphipods on the Microbiota of Allochthonous Detritus

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Abstract
Estuarine gammaridean amphipods grazing at natural population density on detrital microbiota affected the microbial community composition, biomass, and metabolic activity without affecting the physical structure of the leaves. Total microbial biomass estimated by adenosine triphosphate and lipid phosphate or observed by scanning electron microscopy was greater on grazed than on ungrazed detritus. The rates of oxygen consumption, poly-β-hydroxybutyrate synthesis, total lipid biosynthesis, and release of 14 CO 2 from radioactively prelabeled microbiota were higher on grazed than on ungrazed leaves, indicating stimulation of the metabolic activity of grazed detrital microbes. This was true with rates based either on the dry leaf weight or microbial biomass. Alkaline phosphatase activity was lower in the grazed system, consistent with enhanced inorganic phosphate cycling. The loss of 14 C from both total lipid and poly-β-hydroxybutyrate of microorganisms prelabeled with 14 C was greater from grazed than ungrazed microbes. There was a faster decrease in the 14 C-glycolipid than in the 14 C-neutral lipid or 14 C-phospholipid fractions. Analysis of specific phospholipids showed losses of the metabolically stable [ 14 C]glycerolphosphorylcholine derived from phosphatidylcholine and much more rapid metabolism of the bacterial lipid phosphatidylglycerol measured as [ 14 C]glycerolphosphorylglycerol with amphipod grazing. The biochemical data supported scanning electron microscopy observations of a shift as the grazing proceeded from a bacterial/fungal community to one dominated by bacteria.