## 3 Results and Discussion
### HPLC Analysis of Organic Acids  
The HPLC chromatogram peaks suggest the presence of oxalic, citric, tartaric, succinic, glutararic, acetic, propionic, and butyric acids in the wood samples of _A. saligna_ inoculated with _A. niger_, _A. flavus_, _A. tenuissima_, _F. culmorum_, and _T. harzianum_ (shown in Figure 1). The changes in organic acids from the HPLC analysis of the _A. saligna_ wood after inoculation with fungi compared with the control treatment (without fungi) are shown in Table 2. Oxalic acid was present in the wood samples of _A. saligna_ (293.34 \(\upmu\)g/g o.d. ); decreased in the wood incubated with _A. tenuissima_ and _T. harzianum_; and increased in wood incubated with _A. flavus_, _A. niger_, and _F. culmorum_. The wood samples inoculated with _A. flavus_ and _A. niger_ showed the highest amounts of citric acid. Tartaric acid was found in high amounts (1150.98 \(\upmu\)g/g o.d. sample), while acetic and propionic acids were observed in low amounts, only in the wood samples inoculated with _A. niger_. Succinic acid was found only after the wood was inoculated with _A. niger_ and _A. tenuissima_. Glutaric acid was found in the wood samples that were inoculated with _A. flavus_, _A. niger_, _F. culmorum_, and _T. harzianum_, but not detected in the wood inoculated with _A. tenuissima_ or in the control treatment. Butyric acid was found in small amounts in the control treatment, but the values increased in the wood inoculated with _A. flavus_, _A. niger_, and _T. harzianum_, and decreased in the wood sample inoculated with _A. tenuissima_. The wood sample inoculated with _A. niger_ showed the presence of all of the organic acids, with high amounts of oxalic, citric, tartaric, succinic, and glutararic acids. Oxalic acid was found, in either high or low amounts, in all of the samples inoculated with fungi, suggesting that this acid is dominant in _A. saligna_ wood. During the alkaline pulping of _Acacia_, a significant amount of oxalate was formed and released from the wood, where a higher hydroxide concentration led to a higher oxalate content in the black liquor during the early stages of _Acacia_ wood cooking [45]. During fermentation, fungi produce not only oxalic acid, but also other organic acids, such as citric, gluconic, glucuronic, and malic, depending on the operating conditions [28; 46; 47; 48]. Oxalic acid \begin{table}
\begin{tabular}{c c} \hline \hline
**Treatment** & **pH Value** \\ \hline Control & 6.06 \\ \hline _A. flavus_ & 5.60 \\ \hline _A. niger_ & 4.39 \\ \hline _A. tenuissima_ & 5.24 \\ \hline _F. culmorum_ & 5.81 \\ \hline _T. harzianum_ & 5.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Effect of fungi on the pH values of powdered wood samples of _A. saligna_. is involved in wood-rotting fungi (brown- and white-rot fungi) mediated processes, including lignocellulosic degradation and the breakdown of xenobiotic pollutants [18; 49]. Oxaloacetase and glyoxylate oxidase are two oxalate enzymes produced from wood-rotting fungi [50]. Brown-rot fungi produce large amounts of oxalic acid [49]. The synthesis of oxalate is an energy efficient process, and the excreted oxalic acid, as a by-product of sugar oxidation, proves that oxaloacetic acid can be formed through the carboxylation of pyruvate in the cytoplasm [51]. After two weeks of exposure to _A. availanttii_, a significant reduction in the pH was observed [44]. The reason for this drop may be from the oxalic acid produced by brown-rot fungi [52]. Oxalic acid has an important role in the first, pre-visual, non-enzymatic stages of decay [53; 54]. At a pH higher than 6, _A. niger_ effectively secretes oxalic acid, while if the fermentation is carried out under acidic conditions (pH 2 to 3), the main product is usually citric acid [46; 47]. It was reported that _A. niger_ is an established organism for the industrial production of citric and oxalic acids, as well as the enzymes gluconic acid and ethanol [55; 56; 57]. In low concentrations, _A.