Table 9- Tolerance index ranges and definition. Tolerance Index Definition TI > 1 Overtolerance or stimulation TI = 1 Neutral 0.8 < TI < 0.99 High tolerance 0.5 < TI < 0.79 Moderate inhibition TI < 0.5 Severe inhibition TI = 0 Complete inhibition 3. RESULTS AND DISCUSSION 3.1 Effect of pH and salinity on the growth of the selected fungal strains At pH 7, A. niger showed no inhibition, and its mean linear growth was statistically similar to the control (Figure 23.A). A slight but significant inhibitory effect occurred at pH 8 and pH 9 compared to the control, with tolerance indices of 0.98, and 0.98, respectively. At pH 10, the tolerance index dropped to 0.89, indicating high tolerance but significantly reduced growth. However, growth was completely inhibited under more alkaline conditions. These findings align with previous reports showing that this strain can grow within a pH range of 1.4-9.8 (Upton, McQueen-Mason, & Wood, 2017). P. oxalicum maintained high tolerance from pH 7 to 10, with tolerance indices ranging from 0.99 to 0.88, and optimal growth at pH 7 (Figure 23.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 24.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 24.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 25). 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 (
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