The marine angiosperms, or seagrasses, constitute a small but important plant group, common to many coastal habitats. In spite of their high productivity within near-shore ecosystems, the photosynthetic mechanisms of these plants have received relatively little investigation. The exogenous inorganic carbon form utilized by seagrasses is either CO2 and HCO3- or, according to another view and/or depending on species, CO2 only. In both cases, ambient CO2 concentrations limit photosynthetic rates at saturating light. In species using HCO3-, this is owing to a rather ineffective HCO3--utilization system; although seawater contains sufficient HCO3- to saturate photosynthesis, photosynthetic rates are strongly enhanced by additional CO2. Seagrasses are characterized by relatively high 13C/12C ratios, and net photosynthetic rates do not appear to be strongly influenced by photorespiration. However, photosynthetic C4 acid metabolism is not common within this plant group; most species investigated show a more or less typical C3 incorporation pattern of inorganic carbon. Such an apparently contradictory behaviour could be explained by the photosynthetic carbon assimilation system being "enclosed" by the unstirred water layer surrounding the leaves and/or by an alternate CO2 concentrating mechanism. This would lead to efficient refixation of photorespired CO2 and alleviate the ability of ribulose bisphosphate carboxylase-oxygenase both to act as an oxygenase and to discriminate against 13C. Effects of environmental parameters such as light and temperature on biochemical pathways can presently not be evaluated; instead, these parameters are discussed only as affecting photosynthetic rates and productivity. Many species feature low light compensation and saturation levels such as are found in shade-adapted plants. Others show higher saturation levels suggestive of light limitations even at shallow depths. It seems that most seagrasses have temperature optima for both photosynthesis and growth at around 30°C. High ambient salt concentrations are reduced in the metabolically active epidermal cells by compartmentalization into underlying cell layers. In cases where light is not a limiting factor for growth, high hydrostatic pressure at increasing depths may limit growth by deviating the flow of photosynthetically derived O2 out of the lacunae rather than downwards to sustain root growth.