Physiochemical and micro-morphological response for four bloom-forming algal strains under phosphite (P^+III) stress...

Phosphite (P⁺ᴵᴵᴵ), an emerging reduced phosphorus (P) species of environmental significance, exerts complex but poorly characterized effects on algal communities. In this study, we systematically evaluated interspecific variability in P⁺ᴵᴵᴵ tolerance and recovery potential among four typical bloom-forming taxa by exposing each to a concentration gradient of P⁺ᴵᴵᴵ (0.07-14 mg L⁻¹), followed by phosphate (P⁺ⱽ) recovery. Results revealed species-specific responses to P⁺ᴵᴵᴵ were pronounced. During phosphite stress periods of 15-27 days (strain-dependent, see Methods), Chlamydomonas reinhardtii (C. reinhardtii) demonstrated exceptional P⁺ᴵᴵᴵ tolerance, with culture density gain inhibited by only 21.3% (final A₆₈₀ relative increase: 798% in phosphate control vs. 628% in 14 mg·L⁻¹ P⁺ᴵᴵᴵ) and intact cellular ultrastructure even at the highest concentration. In contrast, two strains of Chlorella pyrenoidosa (C. pyrenoidosa) exhibited severe growth inhibition-83.3% for C. pyrenoidosa-9 (final A₆₈₀ relative increase: 448% vs. 75%) and 84.6% for C. pyrenoidosa-10 (final A₆₈₀ relative increase: 344% vs. 53%)-along with extensive membrane rupture. Chlorella vulgaris (C. vulgaris) displayed transient adaptation at low P⁺ᴵᴵᴵ levels (0.07-0.7 mg·L⁻¹) but suffered serious oxidative damage at concentrations exceeding 0.7 mg·L⁻¹ , with density gain inhibited by 69.9% at 14 mg·L⁻¹ (final A₆₈₀ relative increase: 851% vs. 256%). Notably, P⁺ᴵᴵᴵ proved to be an ineffective as a bioavailable P source, as the intracellular P accumulation remained below 10% of that observed in P⁺ⱽ controls. During the subsequent P⁺ⱽ recovery phase, C. vulgaris achieved a rapid rebound, with cell densities increasing by 167%. Conversely, recovery in C. reinhardtii was delayed, attributable to its comparatively low P⁺ⱽ metabolic efficiency. Scanning electron microscopy (SEM) confirmed that P⁺ᴵᴵᴵ-induced cellular damage is partially reversible. However, exposure to 14 mg·L⁻¹ P⁺ᴵᴵᴵ caused reversible growth inhibition in sensitive species. Collectively, these results underscores the dual role of P^+III as both a stressor and a potential modulator of algal blooms in eutrophic systems, capable of restructuring community structures. The findings enhance our understanding of reduced P bioavailability mechanisms and offer critical insights for ecological risk assessment and eutrophication management.