Photosynthetic, endosymbiotic dinoflagellates of the genus Symbiodinium are vital energy-providers in the coral-microbial community known as the coral holobiont. In common with the bacterial, archaeal, fungal, and viral members of coral microbiota, Symbiodinium contributes to the resilience/susceptibility of the coral animal to suboptimal environmental conditions. As sea surface temperatures (SSTs) rise alongside instances of coral bleaching (i.e. the potentially fatal loss of Symbiodinium and thus pigmentation from corals), evidence that some Symbiodinium clades/subclades have a high temperature tolerance has led to speculation into their potential influence on coral health during climate change.
Stat & Gates (2011) recently reviewed the literature regarding the thermotolerant and relatively rare clade D Symbiodinium, which are known to inhabit various types of tropical coral species (including branching, massive and encrusting species), and have been particularly linked to reef environments with high SSTs or local stressors (e.g. sedimentation) and/or a history of bleaching. Whether corals acquire clade D Symbiodinium vertically (from the parent) or horizontally (from the environment) is dependent on the host species; interestingly, whether clade D Symbiodinium dominates (over clade B or C) may vary within a coral species according to location, however as the authors discuss, the mechanisms governing for these differences are not clearly understood.
Collectively, the studies cited by Stat & Gates (2011) appear to provide some good evidence for the dominance of clade D Symbiodinium on stressed coral reefs, with multiple examples of clade D dominance on warmer, highly saline, or coastal (turbid) reefs, compared to dominance of alternative Symbiodinium clades on (in some cases neighbouring) reefs that experience less stress. However, some other studies reported no such clade D dominance in corals which experience high temperatures on a regular basis. The authors attribute such inconsistencies to the following: a) The possibility that only corals with previously attained clade D Symbiodinium can display the ‘shuffling’ of clades with elevated temperatures which produces Clade D dominance (Clade D Symbiodinium having been initially attained under certain environmental conditions during ontogeny); b) Complications in the detection of Clade D Symbiodinium, due to the reversion of some recovering, previously bleached corals to the Symbiodinium clades which were dominant before bleaching (i.e. not clade D).
Evidence which implies that clade D Symbiodinium confers resistance to high SSTs and bleaching in host corals is discussed by Stat & Gates (2011). However, seemingly contradictory evidence, that of clade D Symbiodinium having had less photochemical efficiency at an elevated temperature than Clade C Symbiodinium (Littman et al., 2010) was not mentioned in this review. It seems plausible that the lack of information available regarding any variation in the thermal tolerances of the ten identified clade D Symbiodinium subclades (as discussed by Stat & Gates, 2011), may be at least partially responsible for this discrepancy. Furthermore, lack of clade D Symbiodinium dominance in some thermally stressed corals could possibly be attributable to which different subclades are present, although the authors do not make this connection themselves.
Stat & Gates (2011) also discuss the drawbacks of Clade D Symbiodinium dominance for the coral host: i.e. slower growth rates compared to corals dominated by Clade C Symbiodinium. However, the lack research into the long-term effects on survival and reproductive fitness for clade D dominated corals is also highlighted. As such, and together with the unknown thermal tolerances of clade D Symbiodinium subclades, the authors’ final suggestion to use Symbiodinium clade D as an indicator of coral health seems a little premature. Nevertheless, the combined reports of Clade D Symbiodinium dominance in many thermally and locally stressed corals appears to be important to our understanding of how coral reefs will react to such pressures, and therefore addressing these gaps in research appears to be a worthy priority.
Stat M and Gates R D (2011) Clade D Symbiodinium in Scleractinian Corals: A “Nugget” of Hope, a Selfish Opportunist, an Ominous Sign, or All of the Above? Journal of Marine Biology 2011 p. 1-9
Hi Jo,
ReplyDeleteIt looks like it might be really interesting to combine a study looking at this with the paper I've just reviewed (Correa et al. 2013)!
Correa et al. (2013) showed that thermally stressed M. cavernosa with clade C dinoflagellates contained viruses with high sequence similarity to viruses found in toxic algal bloom dinoflagellates. Perhaps the presence of viruses in some clade C Symbiodinium is facilitating the switch to clade D?
Robyn
Correa, A., Welsh, R. & Thurber, R. (2013) Unique nucleocytoplasmic dsDNA and +ssRNA viruses are associated with the dinoflagellate endosymbionts of corals. The ISME Journal. 7, 13-27
Hi Robin,
ReplyDeleteFirstly, thanks for the succinct review of the Correa et al. (2012) paper; as you say, there is a lot going on in there, but this new strong evidence for Symbiodinium viruses is really interesting. I reckon that viruses of each type of coral associate are probably important to overall function, but as Symbiodinium are so very vital to the way tropical corals make their living it seems logical that Symbiodinium viruses are especially important to coral health, and could well be linked with coral bleaching, or indeed, the shift in Symbiodinium clades observed in some stressed corals.
Considering these two papers together makes me wonder if:
a) Increased temperatures could induce viral action of viruses that only infect certain Symbiodinium clades, allowing other clades to dominate
b) Whether the lack of clade shift/bleaching in some thermally stressed corals could sometimes be linked to Symbiodinium resistance to such viruses in those areas
I’m not sure how these ideas work with the observed reversion to pre-bleaching Symbiodinium clades some period after bleaching though: perhaps the original Symbiodinium could become virus-resistant over time and then outcompete Clade D to become dominant once again?
Of course this is only one possible facet of coral bleaching, but still, very interesting stuff!