Regulation of H2 utilization, as monitored by the hydrogenase-mediated(3)H2 exchange reaction, was examined among phytoplankton communitiesin situ and populations in culture. During a 2-year study in the Chowan River, North Carolina, at least 2 major groups of phytoplankton dominated(3)H2 exchange rates. They included N2 fixing cyanobacteria and NO3 (})- utilizing genera. Utilization of(3)H2 by N2 fixers was mainly dark-mediated, whereas(3)H2 utilization associated with periods of NO3 (})- abundance revealed an increasing dependence on light. Inhibitors of N2 fixation (C2H2 and NH4 (+)) negatively affected(3)H2 utilization, substantiating previous findings that close metabolic coupling of both processes exists among N2 fixing cyanobacteria. Conversely, NO3 (})- stimulated(3)H2 utilization among N2 and non-N2 fixing genera, particularly under illuminated conditions. A variety of environmental factors were shown to control(3)H2 exchange. In addition to the nitrogen sources discussed above, dissolved O2, photosynthetically available radiation (PAR), temperature, and pH changes altered(3)H2 exchange rates. It is likely that other factors not addressed here could also affect(3)H2 exchange rates. At least 2 ecological benefits from H2 utilization in natural phytoplankton can be offered. They include the simultaneous generation of adenosine triphosphate (ATP) and consumption of O2 during the oxidation of H2 via an oxyhydrogen or "Knallgas" reaction. Both processes could help sustain phytoplankton, and particularly cyanobacterial, bloom intensity under natural conditions when O2 supersaturation is common in surface waters. H2 utilization appeared to be a general feature of natural and laboratory phytoplankton populations. The magnitudes of(3)H2 utilization rates were directly related to community biomass. Although it can be shown that utilization rates are controlled by specific environmental factors, the potential relationships between H2 utilization and phytoplankton primary production remain poorly understood.