Soda lake
[1] In spite of their apparent inhospitability, soda lakes are often highly productive ecosystems, compared to their (pH-neutral) freshwater counterparts.
Gross primary production (photosynthesis) rates above 10 g C m−2 day−1 (grams of carbon per square meter per day), over 16 times the global average for lakes and streams (0.6 g C m−2 day−1), have been measured.
Otherwise, dissolved magnesium (Mg2+) or calcium (Ca2+) will quickly remove the carbonate ions, through the precipitation of minerals such as calcite, magnesite or dolomite, effectively neutralizing the pH of the lake water.
[1] Compared to freshwater ecosystems, life in soda lakes is often completely dominated by prokaryotes, i.e. bacteria and archaea, particularly in those with more "extreme" conditions (higher alkalinity and salinity, or lower oxygen content).
Particularly in the East African Rift Valley, microorganisms in soda lakes also provide the main food source for vast flocks of the lesser flamingo (Phoeniconaias minor).
The cyanobacteria of the genus Arthrospira (formerly Spirulina) are a particularly preferred food source for these birds, owing to their large cell size and high nutritional value.
Declines in East African soda lake productivity due to rising water levels threaten this food source.
[13] In addition to their rich biodiversity, soda lakes often harbour many unique species, adapted to alkalic conditions and unable to live in environments with neutral pH.
In-depth genetic surveys also show an unusually low overlap in the microbial community present, between soda lakes with slightly different conditions such as pH and salinity.
[14] Photosynthesis provides the primary energy source for life in soda lakes and this process dominates the activity at the surface.
The most important photosynthesizers are typically cyanobacteria, but in many less "extreme" soda lakes, eukaryotes such as green algae (Chlorophyta) can also dominate.
Major genera of cyanobacteria typically found in soda lakes include Arthrospira (formerly Spirulina) (notably A. platensis), Anabaenopsis,[15] Cyanospira, Synechococcus or Chroococcus.
However, it is not clear whether this is an autotrophic process or if these require organic carbon from cyanobacterial blooms, occurring during periods of heavy rainfall that dilute the surface waters.
[1] Below the surface, anoxygenic photosynthesizers using other substances than carbon dioxide for photosynthesis also contribute to primary production in many soda lakes.
The photosynthesizing bacteria provide a food source for a vast diversity of aerobic and anaerobic organotrophic microorganisms from phyla including Pseudomonadota, Bacteroidota, Spirochaetota, Bacillota, Thermotogota, Deinococcota, Planctomycetota, Actinomycetota, Gemmatimonadota, and more.
A diversity of methanogens including the archaeal genera Methanocalculus, Methanolobus, Methanosaeta, Methanosalsus and Methanoculleus have been found in soda lake sediments.
[1][18] When the resulting methane reaches the aerobic water of a soda lake, it can be consumed by methane-oxidizing bacteria such as Methylobacter or Methylomicrobium.
[1] Ammonia, a nitrogen-containing waste product from degradation of dead cells, can be lost from soda lakes through volatilization because of the high pH.
