Formal recognition of host‐generalist species of dinoflagellate (Cladocopium, Symbiodiniaceae) mutualistic with Indo‐Pacific reef corals

Abstract

The existence of widespread species with the capacity to endure diverse, or variable, environments are of importance to ecological and genetic research, and conservation. Such “ecological generalists” are more likely to have key adaptations that allow them to better tolerate the physiological challenges of rapid climate change. Reef‐building corals are dependent on endosymbiotic dinoflagellates (Family: Symbiodiniaceae) for their survival and growth. While these symbionts are biologically diverse, certain genetic types appear to have broad geographic distributions and are mutualistic with various host species from multiple genera and families in the order Scleractinia that must acquire their symbionts through horizontal transmission. Despite the considerable ecological importance of putative host‐generalist symbionts, they lack formal species descriptions. In this study, we used molecular, ecological, and morphological evidence to verify the existence of five new host‐generalist species in the symbiodiniacean genus Cladocopium. Their geographic distribution and prevalence among host communities corresponds to prevailing environmental conditions at both regional and local scales. The influence that each species has on host physiology may partially explain regional differences in thermal sensitivities among coral communities. The potential increased prevalence of a generalist species that endures environmental instability is a consequential ecological response to warming oceans. Large‐scale shifts in symbiont dominance could ensure reef coral persistence and productivity in the near term. Ultimately, these formal designations should advance scientific communication and generate informed research questions on the physiology and ecology of coral‐dinoflagellate mutualisms.

Gene duplication is the primary driver of intraspecific genomic divergence in coral algal symbionts

“…Here we defined a TD block as a block comprising two or more consecutive genes with high sequence identity on a genome scaffold. In our independent survey of TD genes in all 19 available Suessiales genomes, we found the largest number and proportion of TD genes in the freeliving lineages of P. glacialis (7.8% in CCMP1383, 9.2% in CCMP2088) and S. natans (7.1%), followed by the symbiotic S. tridacnidorum CCMP2592 (6.5%) and C. proliferum SCF055 (6.0%; this taxon was formerly described as Cladocopium goreaui SCF055 [29]), with smaller proportions observed in the freeliving E. voratum (3.9% in rt-383, 4.4% in RCC1521), and the smallest in S. microadriaticum (1.0-2.2%) (table 1). Some of the largest TD blocks consisted of 13-16 genes, found in genomes of free-living lineages (S. natans, and the P. glacialis CCMP1383 and CCMP2088).…”

Flexibility in coral–algal symbiosis is positively correlated with the host geographic range

“…Our study did not focus on the role of symbionts, even though Symbiodiniaceae also includes generalists and specialists (Baker, 2003). For example, a coral host harbouring a generalist symbiont may show higher niche acclimatization due to the symbiont's greater physiological tolerances (Butler et al, 2023;Ziegler et al, 2015) and therefore can occupy wider ranges and large habitat gradients, but remains a symbiont specialist (Osvatic et al, 2021). Future systematic assessments of Symbiodiniaceae flexibility and its impact on coral ecology are essential.…”

Heat‐evolved algal symbionts enhance bleaching tolerance of adult corals without trade‐off against growth

“…Experimental evolution has successfully enhanced the thermotolerance of multiple marine microalgal species in vitro (Chan et al, 2021), including Cladocopium proliferum (Butler et al, 2023;Chakravarti et al, 2017). Recent studies have shown that some heatevolved C. proliferum strains can enhance coral bleaching tolerance in larvae and juveniles (Table 1).…”