The evolution of aquatic embryophytes: secondary colonisers of aquatic environments

Chapter 6. Seagrasses in marine systems and freshwater plants (macrophytes) in inland waters are important primary producers that structure their local ecosystems. They comprise the embryophytes: bryophytes, pteridophytes and angiosperms. They have adapted the genetic heritage of their land plant ancestors in response to the opportunities and challenges of life underwater. This has involved a reduction in the structures and processes required to manage: water content, the low physical support in air and the high levels of UV radiation. In contrast, inorganic carbon can restrict photosynthesis underwater but can be minimised by growing in sites with high CO2 concentrations, exploiting CO2 in the air or sediment and by CO2 concentrating mechanisms that rely on bicarbonate uptake, C4 photosynthesis or Crassulacean acid metabolism. Most aquatic embryophytes are, like their terrestrial ancestors, rhizophytic, allowing nutrients to be taken up from both the sediment and water. Some, especially the bryophytes, are haptophytic and only obtain nutrients from the water column.