The "Black Reef" Phenomenon

black reef

Author: Eleanor Carrano


Coral reefs are arguably the most iconic representation of global marine health and their continued demise haunts claims to advances in environmental conservation efforts. An extensive Australian study from 1985-2012, indicating a devastating coral cover loss of more than 50% in the Great Barrier Reef alone, is one of countless alarming reports. As a result of multi-disciplinary attempts to tackle the problem, global warming, ocean acidification, and invasive species have emerged as the central targets of study and intervention. During my summer research, I was introduced to iron fertilization as a compelling but comparatively unexplored explanation for coral reef decline. Increased iron input to normally oligotrophic coral environments results in a well-observed phenomenon known as black reef, in which coral reefs degrade and drastically change in composition. Iron may be introduced via shipwrecks, ash deposition from onshore wildfires, or geoengineering; increased levels result in murkiness and the replacement of normal coral with large cyanobacterial mats, macroalgae, and turf algae.

Collaborative research efforts by the biochemistry and biology laboratories of San Diego State University (SDSU) are on the cutting edge of elucidating the role of iron in the black reef phenomenon. Scientists aim to understand the mechanism of iron bio-availability in black reef-associated algal blooms. In response to an iron-deplete environment, many marine bacteria secrete siderophores, which are small, iron-binding organic compounds. Marine siderophores contain a chelating group which makes the iron complex photolabile. In the presence of sunlight, complexed Fe(III) is reduced to Fe(II), which rapidly oxidizes under the aerobic conditions of the marine environment. It is thought that the resulting, soluble Fe(III) becomes bioavailable to algae.

Studying photoactive marine siderophores has proved a challenge to researchers, however, in part because such siderophores have very short lifetimes and low concentrations in the trophic zones in which they are studied. The collaboration on black reefs at SDSU has begun to explore quantitative polymerase chain reaction (qPCR) as an alternative means of characterizing these siderophores. Although searching for genes associated with the production of photoactive siderophores does not allow for the direct quantification of in situ siderophore production, it does provide a good indicator, as it has been ascertained that transcription rates are high in iron-deplete marine environments. Using qPCR techniques, the researchers at SDSU plan to quantitatively compare specific siderophore production genes from proximate black reef and pristine coral reef environments in the Pacific Line Islands. Iron content in water samples from both environments will also be tested via cathodic stripping voltammetry. This research will hopefully allow for correlations between siderophore production and iron content to be established.

Although it is unknown what insights such comparisons will yield, several hypotheses have already been proposed. It might be reasonably expected for photoactive siderophore production to decrease in black reef environments, since increased iron levels could result in selection for bacterial species which produce non-photoactive siderophores that are metabolically more economic. Another hypothesized result is that increased iron input will stimulate photoactive siderophore production so greatly that less efficient iron-using algae will dominate. Limited data obtained from previous samples collected at the same location in 2005 and 2010 did indicate the lack of specific siderophore production genes in black reef DNA samples. A return cruise scheduled for 2013 is expected to provide researchers with the opportunity for further verification of changes in siderophore production.

The collaborative efforts being undertaken by researchers at SDSU are particularly exciting because of their combined scope. An understanding of how iron becomes available to black reef communities, using qPCR techniques, can be complemented by broader-scaled metagenomic studies. According to a 2011 publication by Dr. Forest Rohwers team at SDSU, metagenomic analysis suggested that increased iron levels in black reefs actually selected for pathogenic, iron-efficient microbes, providing a possible explanation for observed coral death. The proposed iron study is a particularly fascinating example of how searching for a solution to a problem affecting a particular organism can increase our knowledge of another interacting organism. Researchers believe that the iron made available by siderophore-producing bacteria in healthy reefs may figure more prominently in the nutritional needs of coral than in the turf algae that dominates black reef environments.

While much investigation remains, existing understanding suggests that the black reef phenomenon may actually be a bright spot in the often gloomy prognosis for global coral health. Black reefs present comparatively ideal situations for coral recovery efforts. Many are located within swaths of otherwise pristine reef, far from human habitation and the compromising effects of pollution and over-fishing. Additionally, heavy surrounding growth of healthy coral may facilitate rapid re-growth in affected areas. Interventions as simple as removing shipwreck debris from affected environments are a luxury almost unknown to modern conservation efforts, which often contend with a host of complex, intangible forces. As environmental concerns continue to weigh on the global consciousness, it becomes increasingly crucial that solutions to these problems advance in intimate coordination with the discoveries of politically neutral, academically excellent scientific research. Better understanding the role of iron in coral reef interactions has the potential to illumine a host of academic disciplines, from biochemistry to marine virology, while powerfully informing global environmental policy as a whole. Recently, the urgency for an enhanced understanding of marine iron interactions has increased with the proposal of intentional iron fertilization as a means of carbon sequestration. It has been conjectured that iron, as a limiting resource for carbon sequestering algal growth, might reduce atmospheric carbon dioxide levels when introduced on a supplemental basis to the marine environment. Knowledge gleaned from black reef studies may ultimately be the difference between the approval of this mitigation method and the prevention of a flawed plan ultimately detrimental to reef health.

1. Death, G., K. E. Fabricius, H. Sweatman, and M. Puotinen. 2012. The 27-year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences. 109:17995-17999.

2. Barbeau, K., Rue, E. L., Bruland, K. W. and Butler, A. 2001.Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands. Nature 413: 409-413.

3. Kelly, L. W., K. L. Barott, E. Dinsdale, A. M. Friedlander, B. Nosrat, D. Obura, E. Sala, S. A. Sandin, J. E. Smith, M. J. A. Vermeij, G. J. Williams, D. Willner, and F. Rohwer. 2011. Black reefs: iron-induced phase shifts on coral reefs. The ISME Journal. 6:638-649.

4. Powell, H. 2008. Fertilizing the ocean with iron. Woods Hole Oceanographic Institution. Oceanus Vol. 46, No. 1 [Online]


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