Structure and Trophic Relations in Hypersaline Environments

Abstract

Hypersaline waters with a salinity exceeding 35 g/L are widely distributed on the planet (hypersaline lakes and lagoons, deep-water “lakes,” and pore waters of sea ice) and are among the most extreme habitats on Earth. The present work analyzes and summarizes the results of long-term research on hypersaline lake and lagoon ecosystems in the Crimea, along with the literature data. The analysis shows that these extreme ecosystems are unique in terms of the physical and chemical parameters and processes, as well as the structure and functioning of the biota inhabiting them. In particular, salinity affects the freezing and boiling points of water. Thus, water remains liquid in a hypersaline environment, and life can exist in a wider temperature range than in fresh and sea waters, e.g., from –35 to 109°С at a salinity of 350 g/L. With a salinity increase, the species diversity sharply declines in eukaryotic organisms and grows in prokaryotes. The energy supply to ecosystems in freshwater and marine ecosystems is primarily ensured by oxygenic photosynthesis, while that in hypersaline waters comes from three phototrophic and a variety of chemosynthetic mechanisms. Thus, for example, anoxygenic photosynthesis may result in 50% or more energy (up to 85%) in an ecosystem, and its share in the total primary production increases with increases in salinity higher than 100–160 g/L. Despite the extreme nature of the environment, the majority of hypersaline waterbodies are highly productive. This paradox can be explained by two factors: the commonly high concentrations of nutrients in hypersaline waters and the high, small-scale, spatiotemporal variability of abiotic factors in such waterbodies, which allows for the interaction of oppositely directed processes to close nutrient cycles within one community. In response to high salinity and its sharp fluctuations, the osmoregulation mechanisms of primary producers include release into the environment of exopolysaccharides, the amount of which increases with salinity and can account for 50–70% of the primary production. This leads to an increased role of heterotrophic osmotrophs in food webs. Further insight into hypersaline ecosystem may expand our understanding of the organization of life in an extreme environment.