Biodegradation of Organic Particles by Surface and Benthic Nepheloid Layer Microbes From Lake Superior

The biodegradation of a green alga (Chlorella vulgaris) and balsam fir wood (Abies balsamea) by microbial assemblages from the epilimnion and deep hypolimnion of Lake Superior was determined. Algae and wood were radiolabeled, dried, and added to incubation bottles. Water collected from nearshore and offshore sites in the western arm of Lake Superior was used to inoculate the substrates. Near surface water (5 m) or deep hypolimnion water (1 m above the sediment) from each site was added to the bottles and incubated at 5°C or 15°C. Each bottle, including killed controls, was aerated every 48 h to recover biomineralized ¹⁴CO₂. Up to 76% of the labeled alga, but less than 40% of the labeled wood, was mineralized after 34 d. Mineralization rates were up to 4.5 times faster at 15°C than at 5°C. The presence of a benthic nepheloid layer at the nearshore site did not enhance mineralization rates. In most cases, mineralization rates of either substrate were similar at the nearshore and offshore sites. However, epilimnetic microbes from the nearshore site mineralized the wood faster than microbes from the offshore site. This may be explained by the higher density of bacteria in the nearshore inoculum. The type of substrate and temperature influenced mineralization rates more than the site or depth in Lake Superior from which the microbial inoculum was taken.