Coral reefs: ship-wrecked?

A review of Kelly, L.W et al (2011) Black reefs: iron induced phase shifts on coral reefs, The ISME Journal.

Several pristine coral reefs, isolated from pollution and human activity, lie in the central Pacific surrounded by the Line Islands (LI). The reefs have flourished due to the rich supply of calcium carbonate and also the lack of iron. However, black reefs, characterised by high abundance of algae, cyanobacteria and corallimorphs, a significant loss of corals and crustose coralline algae (CCA) have increased around the islands and researchers have linked these to shipwrecks reported in the area, which release iron-rich debris, polluting the reefs. Similar phase shifts, where conditions favour algal growth in normally coral-dominated ecosystems, have been documented on the Phoenix Islands and were also found offshore of Sumatra, where wildfires caused iron-rich ash deposits on the surrounding reefs and killed nearly 100% of corals. However, few studies have measured the iron availability to benthic organisms in reefs and therefore are unable to directly link wreck debris to the formation and maintenance of black reefs.

This study aims to show the detrimental impact of iron enrichment on reefs in normally iron-depleted oceans such as the LI. Research was focused on 3 black reefs, Millenium atoll, Tabuaeran and Kingman. Chemical analysis and iron concentrations from the reef algae and rubble were measured, the effects on coral, algae and associated microbes were documented and the microbial communities were identified to the lowest taxonomic level possible using metagenomics and counted using epifluorescence microscopy.

Looking at the natural history of the black reefs, researchers concluded that they (1) are always congruent with sources of iron, such as wrecks; (2) develop quickly, such as at Kingman, which harboured the second highest coral diversity in the central Pacific and in less than 3 years the CCA cover alone had declined to <10% and D.teniussima, a green algae had increased to 80%; (3) persist for decades, such as at Tabuaeran, where effects are persistent after 40years and there is little coral recovery, and (4) have similar biotic composition, in terms of turf algae, cyanobacterial mats and corallimorphs. Millennium atoll, for example, was characterised by a high abundance corals, CCA and low abundance of benthic algae prior to the 1993 shipwreck. After, turf algae and cyanobacterial mats formed and benthic coral cover was 16.7% compared to 65.9% on surrounding reference sites.

Iron concentrations also increased at the sites, specifically 6 times higher at the millennium site (~663μmol) in comparison to reference site samples (~105μmol), showing that iron is being recycled and accumulating in the algae, allowing it to spread kilometres away from the wrecks, carrying iron with them.

These findings are also supported by Martin et al (1994) who determined that low primary production despite the low-chlorophyll and high nutrient, phosphorous and nitrogen in the LI, is due to the limited iron levels, as after iron addition, there is rapid increase in primary production.

Furthermore, P.meandrina coral collected from a reference site was incubated in the presence or absence of ferric chloride and algal-covered rubble from the black reef. Results showed that coral incubated in both iron and rubble had the largest mortality (50% dead, 20%dying) and when using antibiotics to modulate the microbes, the mortality rate decreased, determining the detrimental effect of reef-associated microbes on coral.

Microbial communities also differed significantly between the black reefs and healthy reference sites. The black reef microbial abundance was three times higher and 10-fold more heterotrophic microbes as well as 2- to 10-fold more opportunistic coral pathogens and iron-associated virulence factor genes were present, suggesting that iron promotes subsets of virulent microbes which import iron to flourish.

Due to the results and background support, the researchers have concluded that large-scale algal overgrowth on at black reefs is due to iron supplementation.

This study has helped to confirm the links between iron-enrichment and the destruction of coral reefs. They also extended the experiment by looking at other variables such as algal mats and how they damages, spread over large areas and prevent coral recovery even without the presence of iron. This emphasises the major impact that abandoned wrecks are having on marine ecosystems and that simply attempting to remove the iron is just not enough. Much emphasis has gone onto global warming and its effect on reefs , but perhaps more focus needs to be put on other contributory factors, like these wrecks and other man-made debris, that are left abandoned in the sea, damaging the decreasing, sensitive, pristine coral reefs.

Other references: Martin, J.H et al (1994), Testing the iron hypothesis in ecosystems of the equatorial Pacific-Ocean, Nature 371: 123-129