We tested the hypothesis that algae influence the activities of extracellular enzymes involved in mineralization processes within microbial assemblages in streams. We tested the prediction that the factors that influence algal biomass and photosynthesis (i.e., diel fluctuations in photosynthetically active radiation [PAR], long-term variations in light regime, and community development stage) would have a corresponding effect on extracellular enzyme activities. We also tested the prediction that algae would influence enzyme activities on inorganic substrata and in detrital communities where they ultimately would influence plant litter decomposition rates. We allowed microbial communities to develop on inert substrata (glass-fiber filters) or on leaf litter in artificial streamside channels. For each community type, we examined the effects of long-term light manipulations, community development stage, and diel periodicity on the activities of β-glucosidase, alkaline phosphatase, leucine-aminopeptidase, and phenol oxidase. In addition, we measured the decomposition rates of the leaf litter substrata in the low- and high-light treatments. Our results support the prediction that factors that influence algal photosynthesis and biomass in the short (diel fluctuations in PAR) and long (shading, community development stage) term ultimately influence enzyme activities in microbial communities associated with both inorganic substrata and detritus. Furthermore, decomposition rates of organic detritus probably are enhanced by algal colonization and activity. Algal photosynthesis might enhance redox and pH conditions within microbial communities, and in turn, might increase the activities of oxidative and hydrolytic enzymes. As a consequence, photoautotrophic activities might stimulate heterotrophic pathways in stream ecosystems by creating conditions favorable for decomposition of both dissolved and particulate organic detritus.