Microbial bioavailability of dissolved organic carbon from leachates of freshwater autotrophs

Abstract

The concentration and bioavailability of dissolved organic carbon (DOC) released during senescence of aquatic vegetation varies among autotrophs, and the corresponding response by microbial communities depends on DOC bioavailability. To evaluate microbial response to different leachate sources, experiments that measured O2 consumption, utilization of DOC and monosaccharides, and the specific absorbance of light at 350 nm (SUVA350) were conducted on leachates from 3 primary producers. Specific absorbance at 350 nm (SUVA350) increased in 2 of 3 experiments. Increases in SUVA350 are consistent with the preferential use of labile DOC with a lower SUVA350. However, similar patterns of monosaccharide and DOC utilization do not support the hypothesis of preferential usage of labile carbohydrates over bulk DOC by the microbial community over the 8 d study. Monosaccharides appear to represent only a portion of a larger pool of labile DOC constituents that are found in autotrophic leachates that can be taken up more rapidly than metabolized by microbial communities. O2 consumption by the microbial community varied significantly among leachate types but was relatively high during the early phase of the experiments in all leachates. Similarly, DOC uptake was relatively rapid during the early phase of the experiments. The microbial uptake of DOC relative to the consumption of dissolved oxygen, expressed as a ratio, ranged from 2.31 to 3.95 during the initial 24 h period but approached 1:1 over the duration of the experiment. These ratios suggest that 'luxury uptake' of DOC by microbial communities might have occurred in the initial phase of the experiment.