Perturbations in the biomass, metabolic activity, and community structure of the estuarine detrital microbiota: Resource partitioning in amphipod grazing

With the development of quantitative methods for estimating the biomass, metabolic activity, nutritional status and community structure of the estuarine detrital microbiota, and the indication of marked effects on the microbiota by amphipod grazing, a detailed examination of the effects of grazing by two sympatric amphipods was conducted. The microbiota, which develop on Teflon squares after a 2-wk exposure in a North Florida estuary, showed, when grazed for 2 wk, significantly increased bacterial biomass (short-branched and cis-vaccenic fatty acids) and microeukaryotes (total polyenoic fatty acids). Grazing in the laboratory at field density further increased the bacterial biomass (muramic acid, wall glucosamine), as well as the total photosynthetic biomass (lipid galactose) if compared to the microbiota grazed in the field. Grazing in the laboratory also depressed the algae estimated as alpha linolenic acid but stimulated the fast growing diatoms (20 carbon polyenoic fatty acids of the alpha linolenic series). The field-grazed microbiota showed metabolic stress indicated by the higher adenosine to adenosine triphosphate (ATP) ratio. Two sympatric amphipods Gammarus mucronatus (Say) and Melita appendiculata (Say) having markedly different mouth parts were shown to partition the detrital microbiota. Melita feed on nonphotosynthetic microeukaryotes (lowered triglyceride glycerol) leaving a larger total biomass (extractable lipid phosphate), photosynthetic biomass (lipid galactose) with greater total metabolic activity estimated as the 4-h rate of phospholipid and sulfolipid synthesis. Bacterial biomass increased (muramic acid, short-branched and cis-vaccenic fatty acids) compared with the detritus grazed by Gammarus. Gammarus feeds less specifically on bacteria and photosynthetic microeukaryotes. Bacteria (muramic acid) and microeukaryotes (the ratio of lipid glycerol to lipid phosphate) were significantly decreased. This feeding provokes an intense metabolic activity from the lowered biomass that is reflected in the significantly higher total adenosine nucleotides, ATP, and adenylate energy charge. The amphipods from the field and laboratory microcosms showed similar adenylate energy charges and neutral lipid glycerol to phospholipid ratios, indicating their nutritional status was similar.