Organic carbon lability and community-level physiological profiling of bacterial populations in Lake Superior

Abstract

Our appreciation of the ecological importance of bacterioplankton in aquatic environments is increasing with comprehension of their role in decomposing organic matter and recycling nutrients. Expansion of our understanding has been hastened by the development of more effective methods for evaluating microbial processes. GARLAND & MILLS (1991) introduced a technique for characterizing and spatially and temporally differentiating bacterioplankton communities in aquatic (seawater, freshwater, and estuarine) environments. Their method, community-level physiological profiling (CLPP), examines the ability ofbacterioplankton communities to metabolize sole-carbon sources using Biolog microplates (containing 96 wells, one control, and 95 sole-carbon sources). When a carbon source is utilized, it produces a violet color and a "fingerprint" of the bacterial community (WINDING 1994). These fingerprints are further analyzed using multivariate statistics to identify samples from different habitats and along spatial gradients within habitats (GARLAND & MILLS 1994 ). Dissolved organic carbon (DOC) is usually the limiting nutrient for bacterioplankton growth in freshwater environments. The composition and amount of carbon available to bacterioplankton varies spatially and temporally. The DOC pool is partitioned into refractory and labile fractions. Refractory DOC originates primarily from watersheds and, due to its complex nature, is not immediately available to bacterioplankton. Labile dissolved organic carbon (LDOC) is produced by both active and senescing phytoplankton ( excretion) and is associated with zooplankton exudates. The LDOC typically accounts for -9-14% of the total DOC (TDOC) in freshwater lakes {TRANVIK 1988, S0NDERGAARD & MIDDELBOE 1995) and is readily available for uptake by bacterioplankton (BIDDANDA & COTNER 2003). There is no simple, direct measurement of the labile carbon content of water. Bioassays, where organic carbon utilization and/or microorganism growth serve as a reflection of bioavailability, represent the only approach suitable for detection. Here, we apply lability bioassays and CLPP to examine pattems in LDOC abundance and the association o f bacterial populations with selected sources o f DOC ( e.g., riverine inputs, autochthonous production, and diagenesis of particulate matter in the deep chlorophyll maximum) in Lake Superior.