Abstract
Sodiomyces alkalinus is one of the very few alkalophilic fungi, adapted to grow optimally at high pH. It is widely distributed at the plant-deprived edges of extremely alkaline lakes and locally abundant. We sequenced the genome of S.alkalinus and reconstructed evolution of catabolic enzymes, using a phylogenomic comparison. We found that the genome of S.alkalinus is larger, but its predicted proteome is smaller and heavily depleted of both plant-degrading enzymes and proteinases, when compared to its closest plant-pathogenic relatives. Interestingly, despite overall losses, S.alkalinus has retained many proteinases families and acquired bacterial cell wall-degrading enzymes, some of them via horizontal gene transfer from bacteria. This fungus has very potent proteolytic activity at high pH values, but slowly induced low activity of cellulases and hemicellulases. Our experimental and in silico data suggest that plant biomass, a common food source for most fungi, is not a preferred substrate for S.alkalinus in its natural environment. We conclude that the fungus has abandoned the ancestral plant-based diet and has become specialized in a more protein-rich food, abundantly available in soda lakes in the form of prokaryotes and small crustaceans.
Highlights
Soda soils and lakes are the most alkaline natural habitats on Earth
Data obtained here from genome analyses, enzyme assays and growth experiments, in addition to our previous studies, enabled us to portray our current understanding of the ecology and life cycle of the alkalophilic ascomycetous fungus Sodiomyces alkalinus (Figure 6)
Occasional scarce plant material can potentially serve as carbon source, but the primary food appears to be protein-rich substrates, such as prokaryotes and brine shrimps with their numerous eggs
Summary
Soda (or alkaline) soils and lakes are the most alkaline natural habitats on Earth. With pH values typically ranging from 9 to 11, these environments are often very salty—with high Na+ concentrations. Some of the recovered fungi tolerate high ambient pH, but even prefer that condition for optimal growth, a physiological category called alkalophiles (Horikoshi, 2011). We hypothesized that S. alkalinus has specific adaptations to deal with those challenges and has changed the preference towards nonplant diet in this plant- deprived environment. To test this hypothesis, the genome sequence of S. alkalinus was determined and examined for footprints of adaptation to the extreme conditions of soda lakes. Results obtained here improve our understanding of how fungi evolved to thrive under extreme natural conditions of soda lakes, and provide opportunities to exploit S. alkalinus for commercial purposes, as a source of alkaline-active compounds for industrial use
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