A critical reappraisal of our knowledge on the photosynthetic communities of the open oceans and large lakes, and the development of new approaches to measurement of their activity
have greatly influenced our present views on the structure and function of these communities. Based on the accumulated knowledge of the physiology and molecular biology of the photosynthetic organisms involved we can now understand some of the mechanisms underlying adaptive processes operative in Nature. The distribution pattern of the photosynthetic communities in the photic zone of aquatic ecosystems is controlled by the nature of the photosynthetic apparatus, the range of antenna pigments formed by the different organisms, and their ability to regulate quantitatively and qualitatively pigment synthesis in response to light intensity and spectral composition. An additional factor controlling distribution is the ability of many of the photosynthetic organisms to escape the oligotrophic conditions prevailing in the water column by adherence to interfaces such as the benthos or the neuston. A major factor governing the ability of organisms to adhere to these interfaces is the hydrophobicity of their cell envelope. Planktonic organisms, on the other hand, have highly hydrophilic envelopes. Benthic organisms have evolved mechanisms for dispersal, which in many cases involves the formation by the hydrophobic adherent parent cells of hydrophilic progeny cells. This has been confirmed for several hormogonia-producing cyanobacteria. Benthic photosynthetic organisms must in addition be capable of phototactic motility and have versatile metabolic patterns to adapt to the rapid fluctuations in their environment. The photosynthetic organisms of the neuston and the cyanobacteria that form surface scum share mechanisms enabling the cells to withstand conditions of photo-oxidation, lethal to most non-resistant organisms.