Biochemical degradation of algal fatty acids in oxic and anoxic sediment–seawater interface systems: effects of structural association and relative roles of aerobic and anaerobic bacteria

To examine microbially mediated degradation of algal fatty acids in marine environments, we conducted a series of microcosm experiments by incubating Emiliania huxleyi cells in simulated oxic/anoxic sediment–water interface systems. Variations in concentration of fatty acids, lipid-degrading enzyme (lipase) activity, and bacterial abundance over 2-month incubations were followed to determine degradation rate constants of major algal fatty acids and responses of bacteria. In the cell-spiked experiments, fatty acids bound in the membrane and intracellular components were separated to examine effects of structural association of fatty acids on their degradation. Experimental results showed that algal fatty acids generally degraded faster (2–4×) under oxic than under anoxic conditions. Most membrane fatty acids seemed to more readily degrade than intracellular ones under anoxic conditions but the two classes degraded at similar rates under oxic conditions. Ratios of oxic to anoxic degradation rate constants were generally higher for intracellular fatty acids than for membrane fatty acids, implying that oxygen might play a more critical role in intracellular fatty acid degradation. Most algal fatty acids degraded almost completely under oxic conditions while a significant fraction (10–40%) of initially added algal fatty acids remained after 2 months under anoxic conditions. By contrast, variations in bacterial abundance during incubations were apparently greater under anoxic conditions compared to oxic conditions, suggesting that the function and relative effectiveness of aerobic vs. anaerobic bacteria rather than total bacterial abundance control biochemical degradation of algal fatty acids. Variations in potential lipase activity followed the same pattern as bacterial abundance in oxic and anoxic systems, indicating that bacteria might be a major source for lipase in these experimental systems. Bacteria-specific fatty acids varied differently during incubations and were not directly linked to bacterial abundance.