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.

Multicellularity among algae is not restricted to macroalgae, nor are all macroalgae multicellular. Multicellularity contrasts with colonies of identical cells by differentiation of cell types and intercellular connections; by this definition, heterocystous cyanobacteria are multicellular, as are some freshwater green algae in the streptophyte and chlorophyte lineages. Multicellular, mainly marine, macroscopic algae include Rhodophyta and Phaeophyceae. Among large marine chlorophytan green algae, Ulva has no plasmodesmata, while differentiated Ulvophyceae, such as Acetabularia, Caulerpa, Halimeda and Codium, are composed of a single cell, demonstrating that organ growth and morphogenesis are independent from cell division, so the most complex green seaweeds are both macroscopic and unicellular/acellular. Macroscopic seaweeds of the Rhodophyta, Chlorophyta and Phaeophyceae clades arose from small unicellular ancestors, with the red and green lineages dating back most likely to the Meso- or Neoproterozoic (1,600–900 Ma) and brown seaweeds to about 200 Ma. Compared to benthic microalgae, macroalgae project beyond the substrate boundary layer, but have their own diffusion boundary layer constraining nutrient acquisition, and have more self-shading of photosynthetically active radiation than do individual microalgal cells. These constraints are partly offset by wave activity.

In addition to the current uses of algae as food and as sources of pigments and polysaccharides, there is potential for the further use of algae as sources of additional specific biomolecules. In addition to this, there is the possibility of the use of alga in a wide variety of processes such as bioenergy (production of liquid fuels and algal biophotovoltaic generation of electricity), removing pollutants from wastewater, and the production of plant growth enhancers and crop protection materials. The eventual commercialisation of these requires the processes to be scalable and economical. The targeted use of algae, other than as food sources, is less than 100 years old and our knowledge of algal biology, physiology and chemistry is still growing and only a very small number of algal species have been studied in detail. One of the main limitations is the need for more reliable, lower cost and larger-scale algae production systems and their development, in turn, requires a good understanding of basic algal biology and life histories. In addition, more work is needed on increasing the efficiency of light utilisation in photosynthesis and hence in growth.

Solar radiation at the Earth’s surface contains ultraviolet (UV) radiation in the UVB (~295–315 nm) and UVA (315–400 nm) wavebands. Currently, atmospheric ozone removes shorter, more damaging UV radiation and reduces levels of UVB, but before the formation of the ozone layer, UV radiation levels would have been higher, while the recent ‘ozone hole’ increased UV radiation. UV radiation is strongly attenuated in water, but aquatic organisms can be damaged to extents that depend on the species and conditions. The targets of damage include proteins in the photosystems of photosynthesis, DNA and oxidative damage caused by the production of free radicals and reactive oxygen species. Defence against damage involves the production of new proteins, repair to the DNA and the production of antioxidants. UV stress interacts, positively and negatively, with other environmental changes such as rising temperature and CO2, ocean acidification and nutrient stress. Further research is needed to forecast responses to future environmental change.

The evolution of oxygenic photosynthesis had profound effects on the biogeochemistry of the planet. The increase in atmospheric oxygen levels brought about alterations to a range of biogeochemical processes involving changes in the availability of a host of elements, including nitrogen, sulfur and many metal ions such as iron and manganese, central to biological activities. Critically for photosynthetic organisms, the increase in oxygen levels in the atmosphere following the evolution of oxygenic photosynthesis and the Great Oxidation Event had consequences for the assimilation of inorganic carbon via the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco). Although there are a number of alternative pathways leading to autotrophic CO2 assimilation, 99% of primary productivity on the planet is carried out by processes that involve Rubisco and the Benson–Calvin–Bassham cycle. The accumulation of O2 in the atmosphere also had major repercussions for increasing the energetic yield of the catabolism of photosynthate by allowing oxidative respiration, with a much greater ATP yield than from anaerobic fermentative processes. The interaction of O2 with UVC radiation led to the production of UVC- and UVB-absorbing O3. This also significantly influenced life on Earth and facilitated the colonisation of the upper ocean and terrestrial surface.

Water is essential for life on Earth, but many organisms are subject to water loss under certain environmental conditions and this can cause biological stress. However, some cyanobacteria and algae are capable of coping with periodic exposure to potentially desiccating conditions. Thus, phototrophs in biological soil crusts can survive in desert environments, even when the only source of water is dew. Other aquatic plants and algae can be exposed to emersion following seasonal changes in water level in rivers or lakes and, importantly, during the daily emersion of intertidal species. Seaweeds living in the intertidal are poikilohydric, and each time they are emersed, they risk water loss. Dehydration can lead to inhibition of photosynthesis and respiration as well as disruption to nutrient availability and assimilation. However, intertidal seaweeds have evolved a range of adaptations/acclimations that allow them to cope with exposure to air. These include morphologies that minimise surface area:volume ratio and biochemical changes that involve, for example, enhanced capacity for detoxification of reactive oxygen species. The extent to which seaweeds can recover function following re-immersion and differences in their capacity for nutrient uptake during restricted periods of immersion appear to be correlated with the zonation of species in the intertidal.

The origin of life could have involved autotrophy, but this is most probably chemolithotrophic rather than photolithotrophic. There is evidence, from the natural abundance of carbon isotopes, of autotrophy involving Rubisco and the Benson–Calvin–Bassham cycle from about 4 Ga. However, other autotrophic CO2 fixation pathways could also have occurred. Evidence on the evolution of photosynthetic reactions suggests an early origin of the photochemical reaction centre, with the possibility of the occurrence of two photosystems in series (photosystem II plus photosystem I) and the possibility of oxygenic photosynthesis, before the origin of the single photosystem (reaction centre I or reaction centre II) photosynthesis in the multiple clades of anoxygenic photosynthetic bacteria. The origin of photosystem II and photosystem I preceded the origin of cyanobacteria and the subsequent Great Oxidation Event at about 2.4–2.3 Ga. The occurrence of oxygenic photolithotrophy is a necessary, but not sufficient, condition for the occurrence of the Great Oxidation Event and the Neoproterozoic Oxidation Event. There is no consensus on what other factors are involved in initiating the Great Oxidation Event and the Neoproterozoic Oxidation Event.