Specificity is rarely absolute in coral–algal symbiosis: implications for coral response to climate change

Abstract

Some reef-building corals have been shown to respond to environmental change by shifting the composition of their algal symbiont (genus Symbiodinium) communities. These shifts have been proposed as a potential mechanism by which corals might survive climate stressors, such as increased temperatures. Conventional molecular methods suggest this adaptive capacity may not be widespread because few ($25%) coral species have been found to associate with multiple Symbiodinium clades. However, these methods can fail to detect low abundance symbionts (typically less than 10-20% of the total algal symbiont community). To determine whether additional Symbiodinium clades are present, but are not detected using conventional techniques, we applied a high-resolution, real-time PCR assay to survey Symbiodinium (in clades A-D) from 39 species of phylogenetically and geographically diverse scleractinian corals. This survey included 26 coral species thought to be restricted to hosting a single Symbiodinium clade ('symbiotic specialists'). We detected at least two Symbiodinium clades (C and D) in at least one sample of all 39 coral species tested; all four Symbiodinium clades were detected in over half (54%) of the 26 symbiotic specialist coral species. Furthermore, on average, 68 per cent of all sampled colonies within a given coral species hosted two or more symbiont clades. We conclude that the ability to associate with multiple symbiont clades is common in scleractinian (stony) corals, and that, in coral-algal symbiosis, 'specificity' and 'flexibility' are relative terms: specificity is rarely absolute. The potential for reef corals to adapt or acclimatize to environmental change via symbiont community shifts may therefore be more phylogenetically widespread than has previously been assumed.

Biogeography and molecular diversity of coral symbionts in the genus Symbiodinium around the Arabian Peninsula

“…Coral colonies of Porites were found in association with a large diversity of other Symbiodinium types in addition to the two main types (C3 and C15), and the genus Porites roughly contained half of all recorded Symbiodinium OTUs in each of the three regions, but this may potentially be confounded by sampling of different (cryptic) species in this genus. In this regard, our observations contradict the notion of Porites as a symbiont specialist genus (Silverstein et al ., 2012) and support the latest observations suggesting a large symbiont flexibility in Porites (Ziegler et al ., 2015). Overall, corals from the PAG hosted the least diverse Symbiodinium communities compared to the RS and the SO, reflecting patterns observed in species diversity of coral hosts, fishes, and other reef‐associated fauna (Sheppard et al ., 1992; Burt et al ., 2011; Bauman et al ., 2013).…”

Long-term monitoring of algal symbiont communities in corals reveals stability is taxon dependent and driven by site-specific thermal regime

“…These findings support the hypothesis that coral specificity contracts niche space for Symbiodinium clade D among different hosts, although the niche may never disappear entirely (as indicated by the single colony of Porites dominated by clade D, see also Silverstein et al 2012). We can rank different coral hosts in their propensity to host clade D at high temperatures as follows: Pocillopora = Galaxea > Pavona > Acropora > Porites.…”

“…Symbionts that represent < 5 to 10% of the community are typically not detected by DGGE, although this depends on the particular ITS-2 variants involved (Thornhill et al 2006, LaJeunesse et al 2008. For this reason, we categorized samples in which only clade C or D symbionts were detected as 'C dominant' or 'D dominant', respectively, and recognize that additional diversity is likely to be found in these samples using techniques with lower detection limits, such as quantitative PCR (Baker & Romanski 2007, Cunning & Baker 2012, Silverstein et al 2012). Samples in which both clades were detected using DGGE were classified as 'C + D mixed communities'.…”

Mesophotic coral depth acclimatization is a function of host-specific symbiont physiology

“…Symbiodinium type C15 is the most common type in the genus Porites in the Red Sea and Western Indian Ocean (LaJeunesse, 2005;Barshis et al, 2010), and our study extends the codistribution of Porites with C15 into the mesophotics (LaJeunesse, 2005). Further, these findings underline the relative specificity of Porites species with clade C in the Indo-Pacific (LaJeunesse et al, 2008;Silverstein et al, 2012). In the Persian Gulf, Porites is associated with C3 (Hume et al, 2013).…”