Photosynthesis in Aquatic Bryophytes

Abstract

Aquatic bryophytes occupy streams, lakes, and wetlands where they face limited CO2 in solution, limited CO2 diffusion, high boundary layer resistance, and loss of light with depth, especially red light. Limitations to photosynthesis in the water are therefore somewhat different from those on land. Of primary importance is the availability of CO2 and hence, pH is important in determining the availability of this gas. There is also limited evidence that some mosses might be able to convert bicarbonates to CO2 at the moss surface or within the cell to increase access to carbon. The often one-cell-thick leaves permit light and CO2 to reach photosynthetic cells directly, but boundary-layer resistance can reduce CO2 uptake. Other nutrients can be somewhat limiting, especially phosphorus and nitrogen. Sedimentation, and overgrowth by diatoms, other algae, and detrital complex, can block light, and water decreases the light with depth. This is further complicated by the rapid attenuation of red light. The aquatic environment protects chlorophyll from UV radiation, and in areas with high light intensity, at least some bryophytes produce enhanced pigmentation to serve as a filter. In dry seasons, lack of water can limit or halt photosynthesis. Temperature also can be a problem at this time, with exposed but still hydrated mosses in some cases reaching temperatures unknown in submersed conditions, and causing elevated respiration that can exceed photosynthetic fixation. High temperatures may greatly limit the presence of many cosmopolitan species of aquatic bryophytes in tropical regions. Contrarily, many aquatic mosses have temperature optima in the 0–20 °C range, with optima depending on their usual habitats.