Determination of Microbial Activity of Estuarine Detritus by Relative Rates of Lipid Biosynthesis

Microorganisms colonizing allochthonous detritus (live oak leaves) incubated in a north Florida estuary show metabolic activity that can be assayed reproducibly by the incorporation of [32P]H3PO 4and [1-14C] sodium acetate into the lipids in a 2-h period without introducing subculture bias. Relatively uncolonized live oak leaves show only about 1% of the biosynthetic capacity of leaves incubated in the estuary for 1 week. Lipid synthesis is proportional to time for at least 2 h and also to detrital mass. Random sampling from pooled portions of many leaves greatly reduces the variance of activities from individual leaves. Rates of phospholipid synthesis paralleled the total extractable ATP and the alkaline phosphatase activity during a 6-week incubation of live oak leaves in Apalachicola Bay. Rates of14Cincorporation into lipids paraleled the respiratory and the α-D-mannosidase activities over the same period, suggesting that lipid biosynthesis is a reasonable measure of detrital microbial activity.