Photosynthesis and photorespiration of marine angiosperms

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.