You need me, man, I don't need you; what causes a macro-algae to change its microbes?

The model system that was used in this paper was Phyllospora comosa and its microbial assemblages due to their disappearance off the coast of Sydney, Australia. Campbell et al. (2014) focussed on the effect that this spatial change can have on the microbial biofilms that can be key to the health and development of the macroalgae. Disruption to their normal habitat may have detrimental effects to allowing the macroalgae to thrive if the microbe communities cannot survive in different conditions. The decreasing number of this species was linked to sewage pollution in the metropolitan area of Sydney.

The authors discuss how functional redundancy can occur in large colonies with high species diversity. This is the process where the phylotypes in the biofilm can perform core functions that the host may need. This is particularly beneficial for the host because it doesn’t have to be picky with its microorganisms. Any microorganisms that can perform a certain function like, for example, nitrogen-fixing, can be utilised by the algae which means that disruption to the biofilm is not necessarily an issue. This is a far cry from specific host-bacterial symbioses which, if disturbed, can be detrimental to both parties. With massive amounts of ocean and coastal degradation more prevalent in the recent years than ever before such as warming oceans, lower pH and increasing pollution, the macroalgal populations can be severely devastated. The authors referred to a paper that showed evidence of variability within the biofilm on the surface of Ulva australis. However, the assemblage show conservation of functional gene profiles within these communities, among different samples. It was suggested that the green alga required its microbe association in providing key functions. This would mean that any disruption would have negative impacts on the macroalgae if those interactions weren’t re-established quickly.

Samples of two extant populations of P. comosa on the outer limits of Sydney (donor habitats) were transplanted into reef habitats that exhibited physical similarity closer to Sydney where the macroalgae wasn’t found (recipient habitats). Forty adults were collected randomly at the same depth (1-2m) from 28^th February to 9^th May, 2011. Individuals were randomly allocated to one of three treatments; transplanted individuals (moved to a recipient site from their original donor site), disturbed individuals (removed from donor site and treated the same as transplanted individuals but placed back in the same donor site) and translocated individuals (same treatment as the other two but placed in the alternate donor site). The controls were randomly selected individuals but were not handled or disturbed. After two months, a blade was taken from each sample and their surface biofilms were sampled. A second experiment was started on 9^th August, 2011. They altered it slightly by taking algae from both donor populations and transplanting them to all recipient sites to allow comparison of algae from different sources at the same place. DNA fingerprint analysis was done on bacterial assemblages and PCR amplified the 16S rRNA genes of each sample using the community DNA as a template. 

The bacterial TRFLP fingerprints were different in both structure and composition but these features differed in accordance with the origin of the sample in the first experiment.  In the second experiment, the TRFLP differed across all the treatments from the samples from both different donor populations. Although where these differences occurred could not be determined through pairwise comparisons. The results showed that the biofilms were more affected by local conditions than the type of habitat they occurred in. They suggested that simply moving the algae to a different habitat caused a change in their microbial communities. However they also found evidence for host-specificity where algae moved from one site to another did not change its assemblages to the same found in undisturbed individuals at the same site. They also report how communities on transplanted individuals that were placed in the same location but collected from different sample sites, still had different communities after five months (NB. The phrasing used in this sentence is difficult to understand so this is what I inferred from what had been written).

Overall, there is a lack of evidence concerning consistency in algae and their microbial communities with regards to its environment. The authors referenced a paper that showed consistent results regarding species-specificity in the context that their method was not sufficient to provide a clear enough result. The authors concluded that the system implemented by this algal species was a type of ‘competitive lottery model’ and that Phyllospora may just require a set of functions rather than species. That could be why individuals may keep their original microbial assemblages as they have the necessary functions accounted for. Individuals that may change their assemblages may be subject to microbial competition from the microbes present in the environment that the algae has been moved to. They theorised that the environment in Sydney may be causing the failure of regrowth of Phyllospora but no evidence has been found to suggest that this might be the case. With lowering rates of pollution in the area than before, they suggested that the algae might be able to recolonize. This is supported by another study referenced in this paper that Phyllospora is sensitive to high levels of nutrients brought about by the dumping of high concentrations of sewage in that area.

This paper is key to understanding the environmental impact human activity can have on the population levels of different species. By understanding the effects these activities have, safer alternatives can be implemented to prevent ecological impacts like this from occurring. There would even be methods to reverse such effects such as with the lowering sewage concentrations which might allow the algae to recolonize. The triggers that cause individuals from a population to alter their microorganisms is an interesting facet of not just macroalgal biology, but also provides a greater comprehension of symbioses between a host and its symbiont.

Ref: Campbell, A. H., Marzinelli, E. M., Gelber, J. & Steinberg, P. D. (2014) Spatial variability of microbial assemblages associated with a dominant habitat-forming seaweed. Front. Microbiol. (5) 737.

doi:10.3389/fmicb.2014.00737