The extended holobiont hypothesis

A phase shift to algal dominance on coral reefs has occurred over the last three decades (Hughes, 1994). Algae are able to outcompete corals, as they are fast growing, and the top down control that evolutionary has kept them in check, have been fished out by humans. It has previously been documented that algae can impact negatively on corals, not just as competitors for space. Rasher and Hay (2010) showed that some macroalgae species caused tissue necrosis when in direct contact with a coral, and Slattery & Lesser et al (2014) reported bleaching. Both these studies suggest that it is a physiological response of coral to chemical compounds associated with the macroalgae. However a recent publication by Barott et al (2011) suggests microbes could be to blame.

In this study, Barott et al (2011) was interested in the microbial communities found associated with common macroalgae species from the reef.  Here they looked at the microbial consortium of four algal growth forms, upright calcifying, encrusting calcifying, fleshy and turf, using 454 pyrosequencing and compared these to the community associated with a coral species (Monteastrea annularis).

What they found was that the microbial community found on macroalgae was more diverse than that of coral, and that the same species of algae harboured similar microbial communities. Thus, suggesting the concept of the holobiont may also be true for algae.

At a closer look the differences between coral and algae were quite pronounced. They were metabolically very different, with all four algal communities having a larger proportion of autotrophic bacteria, such as cyanobacteria whereas the coral communities were mainly heterotrophic. I found this particularly interesting, because a study by Dinsdale et al (2008) showed that the concentration of heterotrophic bacteria increased as macroalgae cover on the reef increased, although this is likely to be as a result of increases dissolved organic carbon availability, and not necessarily colonising algal surfaces.

Another interesting result from this study suggests that macroalgae could be a reservoir for coral disease. As all of the algal species harboured cyanobacteria that have previously been isolated from black band disease, a prolific coral disease. Additionally, there were also blast hits with the only known coral pathogen to be previously linked to algae, Autantimonus coralicida, and a higher abundance of coral disease pathogens associated with the macroalgae, except for crustose coralline algae (CCA).

If these common macroalgae are harbouring coral pathogens, it is possible the prevalence of macroalgae now seen on the reef may be contributing to the number of disease outbreaks. It also provides and alternative explanation to why corals in contact with macroalgae show bleaching and tissue necrosis; disease rather than a response to secondary metabolites.

The idea in this paper that algae harbour species-specific communities could have benefitted from using algal samples from different locations, as here they use only one location. It’s possible that environmental conditions may have supported these communities, although the similarities between communities associated with algae of the same species and differences between inter-species does suggest algae may indeed harbour specific communities. I think a water sample taken from the surrounding water near to where the algae and coral were collected would have been interesting. I know that water communities are often distinct from coral communities, but is this the case for algae? 

Primary: Barott, K. L., Rodriguez‐Brito, B., Janouškovec, J., Marhaver, K. L., Smith, J. E., Keeling, P., & Rohwer, F. L. (2011). Microbial diversity associated with four functional groups of benthic reef algae and the reef‐building coral Montastraea annularis. Environmental microbiology, 13(5), 1192-1204.

Link: http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2010.02419.x/full 

Additional references:

Rasher, D. B., & Hay, M. E. (2010). Chemically rich seaweeds poison corals when not controlled by herbivores. Proceedings of the National Academy of Sciences, 107(21), 9683-9688.

Slattery, M., & Lesser, M. P. (2014). Allelopathy in the tropical alga Lobophora variegata (Phaeophyceae): mechanistic basis for a phase shift on mesophotic coral reefs?. Journal of Phycology.

Hughes, T. P. (1994). Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. SCIENCE-NEW YORK THEN WASHINGTON-, 1547-1547.