732 Figure 47.B). At pH 11, the tolerance index indicated moderate inhibition, which was statistically significant for all pH conditions except pH 7. Moreover, this strain performed better at pH levels above 10, demonstrating growth up to pH 12, where A. niger growth was completely inhibited. Under variable salinity conditions, A. niger exhibited high tolerance at 5% w/v, moderate tolerance at 10% w/v, and slight tolerance at 15% w/v ( Figure 48.A). This strain showed better halotolerance than other A. niger strains isolated from plant tissues, which displayed a growth diameter of 1.6 cm at 15%, whilst our analysis recorded 1.3 cm after 10 days of incubation (Chauhan, Singh, Sharma, Chadha, & Kaur, 2025). P. oxalicum showed significant inhibition at all salinity conditions, with moderate and strong inhibition at sodium chloride concentrations above 5% w/v and complete inhibition at 20% w/v NaCl ( Figure 48.B). The halotolerance exhibited by P. oxalicum in this study is consistent with the behaviour previously reported for P. oxalicum isolates obtained from bauxite residue. However, this pattern contrasts with the growth observed under highly alkaline conditions, where the site‑isolated strain demonstrated enhanced growth at pH 10-12. These findings highlights the potential of employing site‑isolated organisms with superior tolerance (Zhang et al., 2024). Increasing the initial pH prolonged the lag phase and promoted organic acid synthesis ( Figure 49). At pH 12, P. oxalicum exhibited a 6-day lag phase and a markedly reduced tolerance index, indicating severe growth inhibition; nevertheless, the surface pH decreased by 4.3 units. A similar pattern was observed for A. niger at pH 10, where the lag phase extended to 3 days and the surface pH decreased by 6.4 units, representing the largest pH reduction recorded. Despite impaired growth, both strains produced the greatest decline in surface pH under highly alkaline conditions. In contrast, elevated salinity substantially reduced organic acid production, as indicated by minimal pH reduction ( Figure 49). This suppression of acid synthesis under salt stress has been reported previously in filamentous fungi, where increasing salinity redirects metabolism toward biomass preservation rather than organic acid secretion, thereby diminishing leaching and neutralization capacity (Prusky & Yakoby, 2003; Thangavelu, Tang, Ryan, & Valix, 2006). This suggests that alkalinity may enhance organic acid production and neutralization capacity, while high salinity likely triggers a stress response that prioritizes biomass maintenance over acid secretion. This metabolic shift is consistent with activation of the high‑osmolarity glycerol pathway, which responds to osmotic stress and has been previously reported to play a role in mediating these metabolic changes under hyperosmotic conditions, leading to reductions in pyruvate and tricarboxylic acid cycle intermediates (Kim, Oh, & Sung, 2016). A) B)
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