## 3 Results and Discussion
### Transcriptional Profiling of Genes Coding for Osmolyte Synthesis Enzymes
While trehalose, mannitol, and glycerol are directly under the influence of the regulatory frame of glycolysis, erythroid and arabitol are produced from pentose phosphate pathway intermediates and are therefore produced preferentially in conditions where the energy and redox balance in the cell favors anabolic reactions. Arabitol is produced from L-arabinose by the L-arabinose reductase (LAR) or from L-xylulose by a reversible reaction catalyzed by the L-arabitol dehydrogenase (ARDH). A key enzyme controlling the flux to xylulose is the transketolase which is encoded by the gene _tkt_. In _A. sydowii_ we found two homologs of this gene, named _tktA_ and _tktB_. The former was downregulated in 2.0 M NaCl, while the latter had an expression that increases with increasing salinity (Figure 3C). Neither of these expression patterns correlated with erythiritol or arabitol accumulation. On the other hand, erythroid is produced from erythrose by the erythrose reductase (ER), which in _A. niger_ was identified as an aldehyde reductase with a broader specificity for other five-carbon aldehydes [94]. The first step in the shunt from glycolytic intermediates to erythrose-4-P is carried out by the enzyme trans-aldolase (TAD) and it is, therefore, a possible point of regulation of this pathway. The _tad_ gene in _A. sydowii_ did not show a high differential expression in the salinity conditions tested, and therefore was not analyzed further. Compared to glycerol, erythroid has lower hygroscopicity and antioxidant properties [73]. Both low-molecular-weight polyols are more effective in osmotic protection than mannitol or arabitol, which have higher molecular size. Some studies have reported that osmolytes with higher-molecular-weight can even inhibit enzymatic activity as compared with the same concentration of glycerol [67; 95; 96]. Besides, there is an added \"carbon cost\" in the synthesis of larger osmolytes, which the cell may not afford if it is under stress. Consistent with our results, _Hortaae werneckii_, _Penicillium chrysogenum_, and some Aspergillus allo accumulate mannitol, arabitol, and erythroid at the optimal growth condition. However, a contrasting observation with our results was obtained for _A. oryzae_, _A. fischeri_, and _A. niger_, where trehalose was the most abundant osmolyte [3; 97]. As indicated, mannitol was the most abundant polyol in _A. sydowii_ BMH004, found in different days of growth, both in absence of salt and under optimal salinity (Figure 1B). Consistently, in _P. chrysogenum_ and _A. niger_ in the absence of salt, mannitol is accumulated in greater proportion, and arabitol is only detected in minimal quantities [97]. It is known that this polyol can protect against the inactivation of enzymes by heat [98], but due to its limited solubility and tendency to crystallize, mannitol provides little protection against osmotic stress and freezing [99; 100; 101]. In contrast, at higher salinity _H. werneckii_ (4.28 M NaCl) and _W. ichthyophaga_ (4.25 M NaCl) accumulate mainly glycerol and lower amounts of erythiritol [9; 11]. Our results show that _A. sydowii_ accumulated both polyols in similar concentrations (82.9 and 105.27 umol/g at day 7) when the fungus is growing at 2.0 M of NaCl (Figure 1). In several reports of different yeasts and fungi, glycerol was the main solute accumulated in response to hyperosmotic stress [102; 103]. For instance, _Yarrowia lypoliticta_ also accumulates erythroid in higher concentrations than mannitol when exposed to high osmotic pressure [104].