The fungus that needs salt in order to grow

Most fungi treat salinity as a burden: they grow more slowly, but they grow. Wallemia ichthyophaga does the opposite — without salt it does not grow at all. It requires at least ten per cent sodium chloride and thrives in a saturated solution, which makes it the most halophilic fungus known. It is an obligate halophile rather than a tolerant one, and among eukaryotes such cases are rare.

In 2013 Janja Zajc's team published in BMC Genomics the genome of the species type strain together with its transcriptomic response to conditions near both ends of its salinity range. The genome proved unusually compact: 9.6 megabases, 1.67% repetitive sequence and 4,884 predicted protein-coding genes covering almost three quarters of it. Phylogenomic analysis placed the Wallemiomycetes as a sister group of the Agaricomycotina, separated from it about 250 million years ago.

The most interesting finding is a negative one. In the usual salt-tolerance mechanisms — inorganic ion transport and the synthesis of compatible solutes — nothing unusual was found. The sodium/hydrogen exchanger family is not expanded, and the transcription of nearly every cation transporter is independent of salinity. What is clearly expanded is the hydrophobin family, the cell-wall proteins, and half of them are differentially expressed.

The response to salt therefore lies in the cell wall rather than in pumping ions, and that is the real content of the paper. The species also turns out to be a narrow specialist with little adaptive range which, unlike its close relative Wallemia sebi, appears to have lost sexual reproduction. Specialisation has a price: it allows life where others die, and it closes the way back.

What the evidence actually shows

Wallemia ichthyophaga is an obligate halophile, not merely a fungus that tolerates salt. Its type strain does not grow on ordinary medium without reduced water activity; in vitro it requires about 10% NaCl (at least 1.5 M, or an osmotically equivalent solute environment), grows optimally near 15–20%, and remains active even close to saturated brine. It has been isolated from solar salterns, salt-production bitterns, and salted meat. Its genome is unusually compact: about 9.6 Mb with 4,884 predicted protein-coding genes. Comparing transcriptomes at 10% and 30% NaCl did not reveal one “super-salt gene.” Adaptation involves glycerol balance, cell-wall changes, hydrophobins, and ion transport. These thresholds describe a studied strain under culture conditions; other salts, temperature, and water activity also shape its natural limits.