Extremophile Algae Threatened by Coastal Environmental Changes: Halophilic Microalgal Communities Are Resilient But Not Resistant

Abstract

We examined the responses of extremophilic microalgal communities to environmental disturbance. Salt flat microbial mats are halophilic communities that thrive in extreme environments characterized by high salinity and frequent desiccation. However, they may be vulnerable to lower, mesophilic salinity levels resulting from accelerated hydrologic cycles and sea level rise caused by climate change. Therefore, we simulated these disturbance scenarios: one of a short-term extreme rainfall event and another of a long-term sea level rise. We measured the ecological and physiological effects of these scenarios by total and relative cell density, chlorophyll -a concentrations, diversity, equitability, richness, relative contribution per taxon, and taxonomic composition of the microalgal mats. We found that lower salinity levels significantly altered these parameters of extremophile microalgal communities and compromised their original functional traits. Short-term salinity stress, simulating extreme rainfall events, demonstrated the resilience of the microalgal mat community. After the disturbance, parameters returned to the original values, albeit with a minor taxonomic turnover. Long-term salinity stress, simulating sea-level rise, demonstrated the halophile community’s limited resistance. Community parameters quickly deviated from their original values and underwent significant changes. In both treatments, mesophilic salinities resulted in a loss of diversity and lower equitability. Maintaining high salinity levels emerged as pivotal for the stability of salt flat ecosystems. Restoration and management efforts should prioritize restoring field site salinities to previous baseline levels to facilitate the recovery of original functionality within these ecosystems. The lack of endemism in halophilic microalgal communities and the cosmopolitan distribution of our sampled taxa support the generality and applicability of our findings to salt flat microalgal communities worldwide. This study’s broader implications highlight the fragility and stenotolerance of apparently invulnerable extremophilic communities, underscoring the need for global conservation and management strategies to protect these delicate ecosystems in the face of a changing climate.