What lives in saline waters?
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.
Carbon fixation and cycling. New organic matter is produced by photosynthetic fixation of carbon from CO2 (primary productivity). This can occur with (aerobic), or without (anaerobic) oxygen as a by-product of photosynthesis. Respiration and other processes (such as sulphate reduction, methanogenesis and fermentation) result in the near complete oxidation of this organic matter back to CO2.
Salinity tolerances (from Warren, 2016). A) Typical salinity ranges of the halotolerant biota where Vi is the volume of inflow to the basin and Vo is the volume of outflow (includes evaporation and reflux; after Barbé et al., 1990). B) Typical salinity ranges and biomass proportions of the biota in modern marine saltwork ponds. C) Standing crop (g/m2) of the macroscopic benthic fauna in selected athalassic (nonmarine) saline lakes of the world (after Hammer, 1986).
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.
Salinity tolerances (from Warren, 2016). A) Optimal ranges of selected halotolerant and halophilic micro-organisms. Agmenellum quadraplicatum (Aq) is a slightly halotolerant cyanobacterium, Fabrea salina (Fs) is a moderately halophilic protist, Dunaliella salina (Ds) is a halophilic green alga, Aphanothece halophytica (Ah) is an extremely halophilic filamentous cyanobacterium, and Halobacterium sp. (H) is an extremely halophilic archaea (after DasSarma and Arora, 2001). B) Decrease in variety of animal species in saline lakes of southeastern Australia (after Bayly and Williams, 1973). C) Range of observed optimum salinities for phototrophs in various microbial layers and for the sulphate-reducing bacteria and methanogenic bacteria in microbial crusts. It plots the ranges for a mat from a gypsum saltern in Eliat, Israel and compares values to those in the worldwide literature (plotted from data listed in Table 2 in Sørenson et al., 2004).
Brine flies (Ephydra hians) swarm along the brine edge of Mono Lake, California (Wikipedia)