Corals and their associates

Corals have associated eukaryota, bacteria, archaea, viruses and algal symbionts. These can be mutualistic, pathogenic or provide functions. The associated bacterial communities are known to be related to coral health, nutrition and disease such as nutrient fixing and antibiotic production. Previous studies have shown that corals have a high diversity of bacteria, and that microbial communities were species specific and geographically consistent. On the contraire, more recent studies have shown that communities are highly dynamic and dissimilar between colonies of the same species, host specificity is insignificant and diversity differs with location and not always species.

In this study in the South China sea, 16S rRNA sequencing was used to compare the bacterial community of coral tissue, mucus and surrounding seawater and the relationship between coral associated bacterial communities and environmental variables. The sample collection and environmental variables analyses was conducted using the same methods as Huang et al. 2003. There were four sample sites with triplicate samples. DNA extraction, PCR amplification, cloning, sequencing and phylogenetic analysis were carried out. The sequencing was outsourced to Huada Gene Biotechnology Co Ltd.

It was found that the associated bacteria of both the tissue and mucus were different to the bacterial communities of the surrounding seawater suggesting they have specific associations. There was a high abundance of Gammaproteobacteria in the tissue and mucus of the corals and a high abundance of Cyanobacteria in the seawater. Different populations were found in the tissue and mucus of the same fragment of coral; there were different abundances of species. Coral tissue had a higher microbial diversity and OTUs than mucus. Similar bacteria species were found in samples of the same coral species from geographically separate locations meaning that bacteria could have stable, species specific associations with coral. Certain interactions with bacteria could link to holobiont functioning and potentially maximise the health of the holobiont (Rohwer et al. 2002; Reshef et al. 2006). Another view is that corals potentially adapt to new environment conditions by altering their specific symbiotic bacterial partners (Guppy & Bythell, 2006; Littman et al. 2009). Coral samples from sights with more human activity had a higher diversity and evenness than other sites – they were found to have a different community structure. The study also recorded that composition varied with seasons, geography and pollution. It was found that corals from sites of high activity were observed to have unique bacteria related to chemical oxygen demand which is likely to be linked to pollution tolerance. 

This paper has explored some useful insights into understanding more about corals and their interactions with their bacterial communities and what affects their structure. This could lead to a better understanding about the drivers behind coral disease and possibly help to control or prevent disease spreading in the future which would be beneficial to conservation efforts. However, it is limited by the fact that there are still discrepancies regarding the cause of coral disease and the functions of the microbes that make up the associated microbial assemblages of corals. Whether it is coral specific bacteria leading to disease or corals adjusting to form new bacteria function structure will require some further research.

References

Blog paper:

Zhang, Y., Ling, J., Yang, Q., Wang, Y., Sun, C., & Sun, H. et al. (2015). The diversity of coral associated bacteria and the environmental factors affect their community variation. Ecotoxicology, 24(7-8), 1467-1477. http://dx.doi.org/10.1007/s10646-015-1454-4

Other mentioned papers:

Huang LM, Tan YH, Song XY, Huang XP, Wang HK, Dong JD (2003) The Status of ecological environment and a proposed protection strategy in Sanya Bay, Hainan Island, China. Mar Pollut Bull 47:180–186

Rohwer F, Seguritan V, Azam F, Knowlton N (2002) Diversity and distribution of coral-associated bacteria. Mar Ecol Prog Ser 243:1–10

Reshef L, Koren O, Loya Y, Zilber-Rosenberg I, Rosenberg E (2006) The coral probiotic hypothesis. Environ Microbiol 8(12):2068–2073

Guppy R, Bythell JC (2006) Environmental effects on bacterial diversity in the surface mucus layer of the reef coral Montastraea faveolata. Mar Ecol Prog Ser 328:133–142

Littman RA, Willis BL, Pfeffer C, Bourne D (2009) Diversities of coral-associated bacteria differs with location, but not species, for three Acroporid corals on the Great Barrier Reef. FEMS Microbiol Ecol 68:152–163