Background thermo-tolerant symbionts may be the secret to future coral success

It is well established by now within the microbial world, that the relationship between coral hosts and their symbiodinium spp is complexly delicate. Differences in function and performance of Symbiodinium can affect the growth, disease susceptibility, and thermal tolerance of hosts. This paper aims to assess the relative contribution to bleaching avoidance/survival that certain symbionts have.  Visual bleaching and mortality of coral hosts, along with the relative abundances of C- and D-type Symbiodinuium cells observed in transplanted corals, with analyses revealing ‘dynamic changes’ in the symbiont associations was observed over time.  They examined the absolute and relative changes in C and D symbionts in Acopora millepora corals, from 3 populations when exposed to warm summer temperatures (temperatures exceeding their upper long-term temperature regimes by 2-4°C for extended lengths). This is a common reef-building coral within the GBR.

Temporal changes in C and D Symbiodinium cell abundance were examined in adult colonies (20-30cm diameter) of A.millepora at 6 sites, which encompassed a summer season. 32 colonies were collected between 3-7m (depth) at North Keppel Island along the GBR, 33 colonies from Davies Reef, and 17 from Magnetic Island. All colonies were placed on wire mesh racks (4m’s deep) in Geoffrey Bay, Magnetic Island. Single branches were collected and preserved in 100% ethanol for symbiont genotyping; this was carried out 5 more times through the following 13 months. Partial and whole colony mortality and visual colour score (out of 1-5 on a coral chart) were also estimated for colonies. Although 377 samples were collected for this purpose, only 345 were molecularly analysed, as 32 samples were lost (nobody’s perfect)!

DNA was then extracted from the 345 samples using a MoBio kit (commonly used for soil DNA extraction) using a denaturing gradient of 30-50%. Direct sequencing of DGGE bands distinguished C1, C3 and C131 but not D, D1 or D1a types. A single-clade C type was evident from the DGGE analysis in all samples .qPCR of the Actin locus was used to quantify coral host, Symbiodinium clade C and clade D on the samples following methods by Mieog et al (2009). Symbiont C or D host ratios were calculated with a unique formula which was created solely for this research, resulting in ratios of overall symbiont: host (S: H) and ratios of symbiodinium clade (D: C).

Throughout the experiment, the transplant site temperature remained within the 2°C local long-term average, apart from an elevation of 4°C for extended periods between September and December 2005. In this elevated temperature period, bleaching thresholds (population specific) were exceeded November 8^th and 14^th for Keppels and Davies Reef, and mortality thresholds exceeded on December 6^th for Keppels, but November 14^th for Davies Reef. Magnetic Island native corals did not exceed their bleaching thresholds during this study and showed no observed bleaching; remaining D-dominant throughout the experiment. The Keppels population was 93%C C3-dominant at the time of transplanting, with the remaining percentile made up of D. A substantial shift in the dominant symbiont type occurred between September and December 2005, with 64% becoming D-dominant. 3 weeks later, 12/14 of the surviving colonies were D-dominant, and the two that weren’t displayed low symbiont densities(less than 0.006 symbionts per host cell). Overall- the results from the experiment confirmed that all but one D-dominated colony, resisted bleaching regardless of their source population. Interestingly, 37.5% of Keppels corals increased their abundance of D over time, despite the absence of measurable D symbiodinium at the beginning of the experiment. Unfortunately, the authors were not able to find out whether this D symbiodinium came from an exogenous or environmental source. Their data highlights the importance of the shuffling mechanism in corals, especially in such uncertain times, were the rapid shuffling of symbiodinium may just be sufficient to adapt to our increasingly warming oceans.

This paper had a great research aim and hypothesis, and the results from this paper corroborate with findings of many other papers; strengthening the idea that symbiodinium D is a  more thermo-tolerant symbiont, and that it has the potential to alleviate host stresses in certain conditions. The authors also highlight that defining and measuring predictors of shuffling and survival from bleaching stress in wild populations of corals should be a research priority. The methods for the paper were clear and concise, but the results and conclusions were jumpy, lengthy, and hard to follow at times. I have tried to be as concise as possible, without leaving out any crucial information, but please ask questions if you are confused by anything!

Reviewed paper: Bay, L.K., Doyle, J., Logan, M. and Berkelmans, R., 2016. Recovery from bleaching is mediated by threshold densities of background thermo-tolerant symbiont types in a reef-building coral. Royal Society Open Science, 3(6), p.160322.

http://rsos.royalsocietypublishing.org/content/3/6/160322