Structural and functional dynamics of coralline algal systems under warming scenarios: Insights into vulnerability, and resilience

Globally, climate change affects reef ecosystems, causing community reconfigurations and ecological impacts. Coralline rhodophyta are one of the reef groups most vulnerable to synergistic warmer temperatures, and ocean acidification. Yet, mineralogic, microstructural, and chemical studies of their thalli are necessary to understand their vulnerability and resilience. Here, we studied mineralogy, microstructure, and chemistry of adjacent coralline rhodophyta systems in La Paz Bay: Neogoniolithon trichotomum-dominated tidal pools, and rhodoliths from a shallow rhodolith bed at San Lorenzo channel. These systems depicted mineral diversity, including different phases of high-magnesium calcites in rhodoliths. The carbonates in N. trichotomum included Mg-calcite, aragonite, ankerite, and Fe-carbonates. Compositional X-ray maps of calcium and magnesium in rhodoliths reproduce very well the concentric growth band-structure. In a complementary way, the bands richest in calcium are the poorest in magnesium, and vice versa. They are most likely related to the high-magnesium phases detected by bulk XRD-Rietveld, as indicated by their similar EPMA-WDS Mg:Ca ratios. Chemical imaging at the microscale revealed iron was distributed in the primary mineral structure, in contrast with detritic elements, such as Al and Si, located in the algal surface (perithallus). Because algal elemental content is sensitive to environmental conditions, La Paz Bay coralline rhodophyta systems emerge as interesting models to monitor high-magnesium carbonate, in a scenario of tropicalization.