We explored a benthic community living on stones in an acidic (pH2) stream of active volcanic origin from Patagonia, Argentina, by combining in situ measurements (temperature, pH, conductivity, dissolved oxygen), photosynthesis of intact biofilms (measured with microsensors by the light–dark shift method), pure-culture experiments on isolated algae, and confocal laser scanning microscopy on the biofilms. The epilithon of the Agrio River was dominated (99% of total biomass) by one species: Gloeochrysis (Chrysophyceae). This species was observed as brown, mucilaginous, 200-m-thick films on stones, growing in clumps in a dense matrix of fungal hyphae, bacteria, and inorganic particles held together by extracellular polymeric substances. Gloeochrysis was isolated and cultivated. The photosynthetic rate measured at saturation irradiance was 120 mol oxygen (mg chlorophyll a)^–1h^–1 under laboratory conditions, and the saturation rate of photosynthesis by carbon dioxide was 90 mol oxygen (mg chlorophyll a)^–1 h^–1 for oxygen evolution. Photosynthetic activity of the biofilm was light-dependent and saturated above 200 mol photons m^–2 s^–1. In the dark, the stone surface became anoxic. Our data suggest that primary production in the Agrio River was not limited by light, carbon, or phosphorus but instead, nitrogen-limited.