Characterising Microbial Communities of Corals

This paper details surveys of microbial communities conducted on four atolls in the Northern Line Islands in the Central Pacific. The atolls are located along a north-south transect, and are progressively larger and more populated moving south. The atolls are separated by approximately 750km. They span different oceanographic conditions and the amount of human impact differs for each one.

Kingman, the smallest and most northern atoll, is uninhabited. Palmyra has around 20 persons present at any one time; Tabuaeran is inhabited by approx. 2500 people, and Kiritimati approx. 5500 people.

A number of techniques were used to survey the sites. Direct counts using epifluorescent microscopy were used to quantify bacteria, Archaea, virus-like particles (VLPs) and protists, and results showed that the abundance of microbes and VLPs increased along the transect, from Kingman to Kiritimati.

Metagenomic analyses were also performed. Sequence comparisons with the 16s rDNA database revealed that Kingman had a roughly equal proportion of autotrophs to heterotrophs, whereas Kiritimati was dominated by heterotrophs. This corresponds with water chemistry data collected, which revealed that concentrations of total dissolved inorganic nitrogen and inorganic phosphate increased from Kingman to Kiritimati. Dissolved organic carbon (DOC) concentrations were highest at Palmyra and Tabuaeran, however they were lowest at Kiritimati, presumably because the carbon was being utilized at Kiritimati.

Sequences were categorized into metabolic subsystems (group of genes that form a metabolic function/pathway). Sequences associated with photosynthesis were common at Kingman, Palmyra and Tabuaeran, but not at Kiritimati. However a subsystem associated with the consumption of fixed carbon comprised 8.2% of identifiable subsystems on Kiritimati yet less than 1% on the other atolls.

Coral cover was assessed and was found to decrease from Kingman through to Kiritimati, with disease prevalence highest on Kiritimati and Tabuaeran. The authors identified that coral losses were due to human impacts but questioned whether local or global effects were to blame. Microbial shifts can lead to an increase in pathogenic microbes; this was supported by data which found that culturable Vibrio spp. were only found at Kiritimati, and a higher number of pathogens were identified here also.

The authors proposed two hypotheses for the microbial shifts observed when moving from Kingman to Kiritimati; the first being the hydrology/regional hypothesis, which suggests that the larger atolls could induce upwelling of nutrients, and sustain larger seabird populations. The authors pointed to guano (bird excrement) as a possible factor influencing nutrient levels, although they did not explore this further, but ruled out sewage as a reason for elevated nutrients. The nutrient rich waters encourage growth of algae, which support growth of heterotrophic bacteria through their production of DOC. The second hypothesis is the human-driven food web shift hypothesis, which suggests that overfishing of herbivorous fish leads to more algae and therefore microbial growth. This was the favoured hypothesis although the authors suggest that the two are not mutually exclusive and both processes have an important role in influencing communities.

This was a very extensive paper which outlines a lot of important results, and gives a real insight into the mechanisms behind community shifts leading to coral death and disease.

A review of: Dinsdale, E.A. et al (2008) Microbial Ecology of Four Coral Atolls in the Northern Line Islands, PLos ONE 3(2)