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.