Bacterioplankton, DOM, and UVR: A complex interaction in lakes

Bacterioplankton play an integral role in the aquatic food web as they are the primary link between dissolved organic matter (DOM) and higher trophic levels. High molecular weight (HMW) DOM, which is difficult for bacteria to utilize (recalcitrant), may be a significant nutritional source for bacterioplankton through photooxidation and exoenzyme activity. The utilization of recalcitrant DOM may be significant in freshwater systems, because it provides an alternate energy source to products of primary production. In addition, DOM concentration and quality may mitigate the detrimental effects of UVR as it is strongly absorbed by DOM, especially the HMW variety. Low molecular weight (LMW) DOM provides readily utilizable substrate (labile), but it allows increased exposure to UVR because of its lower DOM-specific absorption. HMW, recalcitrant DOM provides photoprotection and may be photooxidized into labile substrate.; Bacterioplankton response to DOM quality and UVR was assessed by incubating bacterioplankton from two lakes, Giles and Lacawac, with different quality DOM (bog, terrestrial, phytoplankton and lake DOM, which varied from low to high molecular weight and exhibited a range of specific absorbance for UVR) and exposing them to varying intensities of solar irradiance. This created the potential for inhibition due to direct or indirect damage as well as stimulation via photooxidation. The incubations were carried out for one week to provide time for photooxidation to occur and for bacterioplankton to express any needed repair systems. There was evidence that UVR exposure during incubation inhibited bacterioplankton growth (bacterial abundance and thymidine and leucine uptake) and the activity of three excenzms, β-glucosidase, L-leucine-aminopeptidase and phosphatase regardless of the DOM source. Bacterioplankton growth was generally greatest when bacterioplankton were incubated with lake or phytoplankton DOM and lowest when incubated with terrestrial or bog DOM regardless of UVR exposure. These heterogenous DOM sources likely provided substrate that was more efficiently utilized than the HMW DOM commonly associated with bog or terrestrial DOM. When bacterioplankton were incubated with bog and terrestrial DOM, the exoenzyme activity declined with exposure to UVR. This is most likely the result of catabolic repression of exoenzyme production in response to photochemical production of labile substrate.