Osmotic adaptation of the halophilic fungus Hortaea werneckii: role of osmolytes and melanization

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Abstract
This study was intended to determine the osmoadaptation strategy of Hortaea werneckii, an extremely salt-tolerant melanized ascomycetous fungus that can grow at 0–5.1 M NaCl. It has been shown previously that glycerol is the major compatible solute in actively growing H. werneckii. This study showed that the exponentially growing cells also contained erythritol, arabitol and mannitol at optimal growth salinities, but only glycerol and erythritol at maximal salinities. The latter two were both demonstrated to be major compatible solutes in H. werneckii, as their decrease correlated with the severity of hypoosmotic shock. Besides higher amounts of erythritol and lower amounts of glycerol, stationary-phase cells also contained mycosporine-glutaminol-glucoside, which might act as a complementary compatible solute. H. werneckii is constitutively melanized under various salinity conditions. Ultrastructural study showed localization of melanin in the outer parts of the cell wall as a distinct layer at optimal salinity (0.86 M NaCl), whereas cell-wall melanization diminished at higher salinities. The role of melanized cell wall in the effective retention of glycerol is already known, and was also demonstrated in H. werneckii by lower retention of glycerol in cells with blocked melanization compared to melanized cells. However, these non-melanized cells compensated for the lower amounts of glycerol with higher amounts of erythritol and arabitol. We hypothesize that H. werneckii melanization is effective in reducing the permeability of its cell wall to its major compatible solute glycerol, which might be one of the features that helps it tolerate a wider range of salt concentrations than most organisms.