# Metabolic Potential of Halophilic Filamentous Fungi--Current Perspective
## 5 Biomolecules Produced by Halophilic Fungi
### Osmoprotectants  
As mentioned earlier, halophiles produce organic osmolytes to counteract the damaging effects of high salt concentrations. Fungi mostly accumulate polyols (for example, glycerol, erythritol, arabinitol, xylitol, and mannitol), free amino acids and their derivatives, nitrogen-containing compounds such as glycine betaine, mycosporines, and mycosporine-like amino acids (MAAs) [34; 46; 105]. The compatible solutes produced by halophilic fungi are summarized in Table 4.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Halophilic Fungi** & **Hydrohobins Class** & **Salinity Growth Conditions** & **References** \\ \hline _Roussoellaceae_ sp. MUT 4859 & I & 2.0 mM NaCl \\ \hline _Penicillium rosepurpureum_ MUT 4892 & I & 2.0 mM NaCl \\ \hline _Acernonium sclerotium_ MUT 4872 & I & 0.5 M NaCl \\ \hline _Penicillium chrysogenum_ MUT 5039 & II & 2.0 mM NaCl \\ \hline _Mycielophthora verrucose_ MUT 4878 & II & 0.25 M NaCl \\ \hline _Arthopyroxenia salicis_ MUT 4879 & II & 0.5 M NaCl \\ \hline _Wallemia ichthyophaga_ EXF-994 & Unclassified & 1.7 and 5.1 M NaCl & [95] \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._Glycerol is the major compatible solute, with great importance in maintaining positive turgor pressure at high salinity [34]. It is the simplest organic osmotic solute, has the smallest size, and its synthesis is the least complex, requiring less energy than the biosynthesis of any other compatible solutes detected in halophilic fungi [22]. Since the production of organic compatible solutes is energetically expensive, halophilic organisms attempt to reduce the energy generated for osmotic adaptation by producing smaller and simpler solutes [106]. In halophilic fungi, glycerol is produced from the glycolytic intermediate dihydroxyacetone phosphate by the NAD-dependent glycerol 3-phosphate dehydrogenase (Gpd) and glycerol 3-phosphatase (Gpp) as in _Debaryomyces hansenii_[107] or via the high-osmolarity glycerol (HOG) signaling pathway that is related to the adaptation of fungi cells to hyperosmotic stress detected in _Hortaae werneckii_ and _Wallemia ichthyophaga_[28,108]. As reported, the amount of glycerol correlates with increase in salinity in the medium but also could be connected with the possible presence of other compatible solutes or ions [109].  
Other compounds that could act as osmoprotectants are mycosporines and mycosporine-like amino acids (MAAs). They are small water-soluble molecules containing an aminocytoleisenimine or aminocytoleisenome unit substituted with amino acid residues [105]. MAAs play the role of sunscreen compounds protecting cells against damage dealt by harmful levels of UV radiation absorbing the wavelength range 310-365 nm, with maximum at 310-320 nm for fungi [52,110]. They also have other features, serving as antioxidant molecules scavenging toxic oxygen radicals or accumulating as compatible solutes in response to osmotic stress. In fungal species exposed to hypersaline conditions, MAAs act as supplementary osmotic solutes, supporting the cells to better tolerate the osmotic challenge [52].