Patterns of Microbial Heterotrophy Through Changing Environments in a Marine Sediment

Heterotrophic activity was measured through a vertical profile in Halifax Harbor sediments using 14C labeled glucose, glutamate, and lactate as substrates. A modified heterotrophic potential method was used to detect the amount of label that was converted to low molecular weight metabolites as well as the amounts respired to CO, and incorporated into cellular material. For all three substrates, total uptake was greatest at the transit~on from aerobic to anerobic sediments (40 cm subbottom). The percentage of the total uptake respired reflected the predominant type of heterotrophic metabolism at the various horizons with the highest values occurring above (aerobic respiration) and below (sulfate reduction) the 40 cm horizon. Incorporation of the substrates into cellular matenal was low for both glutamate and lactate (< 5 % ) but averaged approximately 30 % for glucose. For all three substrates tested, low molecular weight metabolites represented a significant portion of the substrate uptake at many horizons (approximately 50 % at 40 cm). Data are presented indicating that the observed increase in heterotroph~c activity at 40 cm sub-bottom may be due to chemoautotrophic organic carbon production at this horizon.