As the
first slide of our last lecture on coral diseases describes, disease epizootics
do not occur just by pathogens infecting the host and producing the disease.
However, it is the result of complex set of interactions between various
factors such as immune status of the host, pollution, virulence of pathogens
and many others. Nutrient enrichment and subsequent eutrophication of the
ecosystem is one of the factors that may induce the occurrence and severity of
diseases in a variety of organisms. Caribbean Sea has been witnessing
frequently occurring epizootics of numerous coral diseases and mass bleaching
events, both of which are reducing the coral cover of Caribbean. Many previous
studies have shown significance of nutrient enrichment in coral health,
diseases and its susceptibility to bleaching. Nevertheless, there is yet no
experimental evidence that nutrient loading encourages frequency of coral
bleaching and disease in the field.
This in situ
experiment investigated effects of chronic (long-term) exposure of nutrient
loading on a coral reef in Florida Keys, USA, as an assessment of impacts of anthropogenic
nutrient pollution on the reef. The experiment was conducted by enrichment of
phosphorous and nitrogen for a period of three years.
The results
revealed that nutrient loading caused disease outbreaks on the coral reef, with
the dark spot syndrome (DSS) being the dominant disease. Nutrient enrichment
caused more than hundred percent increase in the occurrence of DSS in a common Caribbean
coral Siderastrea siderea. It
increased not only occurrence of a disease but also its severity. This study
confirms that nutrient enrichment is one of the etiological factors for this
disease and probably other diseases as well. Though such a link between
nutrient enrichment and coral diseases has previously been suggested;
separating it from other stressors and experimentally prove it as one of the
key etiological factors for diseases and bleaching is challenging and the
authors have discussed these challenges.
This
study reinforces this link and proposes “nutrient loading” as a critical etiological
factor for disease progression in many corals, even in the absence of other
etiological factors. Surprisingly, nutrients also increased the prevalence of
bleaching in a coral Agaricia spp.
under normal temperature conditions. Nutrients may lower temperature threshold
at which bleaching usually occurs in this species of coral. Explanation of this
nutrient enrichment-induced bleaching probably lies in the abrupt elevation of
zooxanthellae density within the host, which is prone to oxidative stress by
zooxanthellae; making the host more susceptible to bleaching. Interestingly,
this effect of disease and bleaching prevalence was no longer visible after
nutrient loading was stopped.
At
present, DSS in S. siderea is a
mystery. It is caused by whether a pathogen/s or by physiological disorder, is
still unknown. Hence, even though, knowing nutrient enrichment increases
prevalence and severity of DSS, it is difficult to answer how does it do that;
as the disease causing agent is unknown. The authors hypothesized few
mechanisms of how nutrient loading might increase the severity of DSS, based on
possible scenarios of origin of DSS. In my opinion, the explanation that
excessive nutrients may alter coral holobiont’s homeostasis and at the same
time may also impair its ability to resist the pathogen (whether a pathogen
causes DSS is not known but it is possible that an opportunistic pathogen,
already present in the holobiont may become virulent under such conditions)
seems likely.
In
future, finding the DSS causing agent will be very important. The authors
suggested comparative metagenomics amplicon sequencing of bacterial, fungal and
archaeal markers and continuous analysis of microbiomes and physiology of
healthy individuals under nutrient stress until healthy individuals show signs
of DSS. I think DSS causing agent would probably be a pathogen or a consortium
of pathogens; whatever it would be, it would certainly be unable to produce DSS
in the coral host, unless coral holobiont’s physiological status is compromised.
This physiological status would include health of the coral animal, its
bleaching status, its associated microbiota and any taxonomic and/or functional
alterations in the associated microbiota.
Vega Thurber, R. L., Burkepile, D. E., Fuchs, C., Shantz, A.
A., McMinds, R., & Zaneveld, J. R. (2013). Chronic nutrient enrichment
increases prevalence and severity of coral disease and bleaching. Global
change biology. doi: 10.1111/gcb.12450.
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