Metagenomic analyses finds 38% fungi in a Caribbean coral species!

A review of:
Linda Wegley, Robert Edwards, Beltran Rodriguez-Brito, Hong Liu & Forest Rowher. (2007). Metagenomic analyses of the microbial community associated with the coral Porites astreoides. Environmental Microbiology. 9(11), 2707-2719.

Coral bleaching occurs due to stresses on corals and it causes expulsion of the symbiotic dinoflagellate algae, Zooxanthellae which is detrimental to the health of the coral species. Although zooxanthellae are the keystone symbiosis with corals it is important not to forget that the coral holobiont is a dynamic assemblage, not just the coral species and zooxanthellae but also includes endolithic algae, fungi, Bacteria, Archaea and Viruses. All of which play a vital role in the health and survival of the coral species. The metabolic contributions of these microbes to the function of the coral holobiont are essentially unknown. It has been hypothesised that the coral species can adapt to differing ecological niches by ‘switching’ its microbial associates.

In this paper the authors use metagenomic approaches to describe the microbial metagenome of the Caribbean coral Porites astreoids (P.astreoides).
Sampling occurred from Bocas del Toro, panama using a punch hammer, the samples were placed in Ziploc bags and rinsed with sterile seawater at the surface. There were ten samples collected from different colonies and were subjected to a fractionation procedure to separate out the microbes, mitochondria and viruses from the coral tissue and the zooxanthellae. This fractionation procedure involved dilutions of PBSE (1x Phosphate buffered saline plus EDTA)(EDTA = ethylenediaminetetraacetic acid) centrifugation and filtration. The pellets were re-suspended in PBSE and fixed with 2% paraformaldehyde for microscopy. The supernatant was homogenised and centrifuged again and the supernatant again poured off then fixed using 2% paraformaldehyde for microscopy. DNA was also extracted using Ultraclean Power Soil DNA extraction kit (Mo bio). PCR was carried out using the bacteria specific 16s rDNA gene sequences and the euaryote-specific 18s rDNA gene sequence (both of which are printed in the paper). The metagenome sequences were compared with the SEED non-redundant database using Blastx. (I must stress this fractionation procedure is very complex as is the whole method, it has been summed up for the purpose of the review).

The authors found that 79% of the sequencing returned unknown and of the 21% of the known sequences 49% were mitochondria, 38% were fungi, 7% were Bacteria, 3% phage, 2% eukaryotic viruses and 1% Archaea. They also found that overall the bacterial community was heterotrophic and acquires carbon, sulphur and nitrogen from the coral species. The authors do talk in more detail about proposed cycling methods of sulphur, nitrogen fixation and cycling of the P.astreoides microbial communities including some useful images. They did sum up this method well by saying: while initially unanticipated, the co-purification of coral mitochondria with the other microbes was a useful way of obtaining taxonomic information on the animal host at the same time as the microbial constituents.

This paper was an interesting read with the results showing 38% fungi. I could not for the life of me work out how to include some of the images which I did try to include, this would have helped a lot with the interpretation of the proposed models. Further research would be useful on how these fungi contribute to these coral holobionts.