Bacterial Extracellular Enzymatic Activity in Globally Changing Aquatic Ecosystems

Heterotrophic bacteria are key players in the processes of organic matter recycling, decomposition and mineralization in aquatic environments. Although only small and chemically simple compounds can be passively transported through bacterial membranes, substrates for bacterial utilization in aquatic environments are dominated by particulate or highmolecular-weight dissolved organic matter. Complex substrates must first be hydrolyzed outside the cell into smaller size molecules by extracellular enzymes and this process represents a limiting step in nutrient cycling. Bacterial extracellular enzymatic activity is regulated at the ecosystem level, by environmental factors and at the micro-environment level by enzyme-substrate interactions. Over the last century, changes in the atmosphere concentration of CO2 and other greenhouse gases caused changes in climate patterns that have repercussions in ecosystem function and biodiversity. Microorganisms are generally able to respond very quickly to environmental changes because of their close contact with the surrounding environment and rapid growth. As mediators in important biogeochemical processes, namely decomposition and transformation of organic matter, release of inorganic nutrients for higher trophic levels and detoxification of xenobiotics, bacterial enzyme activities have the potential to be used as descriptors of biological responses to changing environmental conditions. The present paper reviews the currently available information on environmental regulation of bacterial extracellular enzymatic activity in aquatic environments and discusses the potential implications of direct and indirect effects global changes on heterotrophic bacterial communities and on the processes of organic matter recycling.