The "Black Reef" Phenomenon
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
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.
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of coral cover on the Great Barrier Reef and its causes. Proceedings of the National
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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
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
4. Powell, H. 2008. Fertilizing the ocean with iron. Woods Hole Oceanographic Institution.
Oceanus Vol. 46, No. 1 [Online]