Symbiodinium biogeography tracks environmental patterns rather than host genetics in a key Caribbean reef-builder, Orbicella annularis

Abstract

<jats:p> The physiological performance of a reef-building coral is a combined outcome of both the coral host and its algal endosymbionts, <jats:italic>Symbiodinium</jats:italic> . While <jats:italic>Orbicella annularis</jats:italic> —a dominant reef-building coral in the Wider Caribbean—is known to be a flexible host in terms of the diversity of <jats:italic>Symbiodinium</jats:italic> types it can associate with, it is uncertain how this diversity varies across the Caribbean, and whether spatial variability in the symbiont community is related to either <jats:italic>O. annularis</jats:italic> genotype or environment. Here, we target the <jats:italic>Symbiodinium</jats:italic> -ITS2 gene to characterize and map dominant <jats:italic>Symbiodinium</jats:italic> hosted by <jats:italic>O. annularis</jats:italic> at an unprecedented spatial scale. We reveal northwest–southeast partitioning across the Caribbean, both in terms of the dominant symbiont taxa hosted and in assemblage diversity. Multivariate regression analyses incorporating a suite of environmental and genetic factors reveal that observed spatial patterns are predominantly explained by chronic thermal stress (summer temperatures) and are unrelated to host genotype. Furthermore, we were able to associate the presence of specific <jats:italic>Symbiodinium</jats:italic> types with local environmental drivers (for example, <jats:italic>Symbiodinium</jats:italic> C7 with areas experiencing cooler summers, B1j with nutrient loading and B17 with turbidity), associations that have not previously been described. </jats:p>