Photosynthesis is the only significant means by which new organic matter is synthesized on Earth. It is the ultimate source of biomass in almost all ecosystems, including hypersaline settings, and accounts for most organic matter buried in sediments and rocks, including biomarkers (Peters et al., 2005). Since the Archaean, almost all organic matter has been produced by photosynthetic fixation of carbon from CO2 in a process set known as primary productivity (Figure).
Photosynthesis can occur with (aerobic), or without (anaerobic) oxygen as a by-product. Respiration and other processes (such as sulphate reduction, methanogenesis and fermentation) result in the near-complete oxidation of organic matter and its recycling back to the atmosphere as CO2. Across Earth history, around 0.1% of the carbon biomass (via primary productivity) was preserved in sediments and so was then available for petroleum generation (Tissot and Welte, 1984). In the following discussion, the various biological components of the carbon cycle will now be used to discuss adaptations and niche extent in the halobiota living under hypersaline conditions.
Progressive brine concentration leads to sequential blooms of various macro and microbial species adapted to different ranges of salinity (Figure). As a surface brine is concentrated from 60‰ to around 200‰, dense eukaryotic algal and cyanobacterial populations appear, grazed by ostracodes, brine shrimp and brine fly larvae (Figure A, B). Halotolerant protists are also found feeding in this salinity range, along with yeasts and other fungi. Halotolerant microbial mats cover the bottom of many hypersaline ponds and shallow lakes at this stage. In anoxic waters in this salinity range, there are a variety of sulphur-oxidizing, sulphate-reducing, homoacetogenic, methanogenic and heterotrophic bacteria and archaea flourishing at the base of stratified brine columns or in the lower parts of mesohaline microbial mats. Increasing osmotic stress and loss of appropriate habitat as a water body shrinks with ongoing desiccation means much of the macroscopic stenohaline benthic fauna dies off or dies back to refugia about springs and seeps (Figure C).
From about 240‰ to more than 320‰, halophilic archaea and bacteria come to dominate (Ghai et al., 2011). As ever more elevated salinities are attained, most other biological activity first slows and then ceases. At the elevated salinities where halophiles and hyperhalophiles flourish, a few eukaryotes, such as brine shrimp, and a handful of algae, including various Dunaliella sp. (a naked unicellular biflagellate green alga), are the only other living forms that the halophilic microbes encounter (Figure A, B, C).
Warren (2011, 2016), discusses in species-by-species detail where and what lives in waters of varying salinity, along with a breakdown of their various salinity tolerances and geographic distributions.