Limited phosphorus availability is the Achilles heel of tropical reef corals in a warming ocean.
Ezzat, L. et al. Limited phosphorus availability is the Achilles heel of tropical reef corals in a warming ocean. Sci. Rep. 6, 31768; doi: 10.1038/srep31768 (2016).
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This is one of the best articles I’ve posted!!! Some selected quotes….
Furthermore, we show that H. fuscescens exhibited higher DIN uptake rates during its pulsating activity, as previously demonstrated for the jellyfish Cassiopeia sp34. Therefore, rhythmic pulsations not only improve H. fuscescens photosynthesis by preventing seawater re-filtration31, but also enhanced its acquisition of essential nutrients.
Although it might only be due to a temperature dependency of the phosphorus transporters at the cell membranes36, our results tend to confirm the fact that phosphorus is actively needed in corals to offset the negative effect of thermal stress26 and that corals underpin an active control mechanism for phosphorus uptake and allocation, as observed in plants37.
Wiedenmann et al.26 also demonstrated that limited phosphorus availability induces a shift from phospholipids to sulpholipids in symbiont membranes, and increases the susceptibility of corals to temperature-induced bleaching38. In addition to being a structural membrane component, phosphorus takes part in the synthesis of nucleic acids and other energy generation processes39. It is also involved in cellular signalling via thylakoid protein phosphorylation40. It has to be noticed that although corals are able to increase their rates of phosphorus acquisition under warming conditions, a minimal concentration of inorganic phosphorus, which remains to be determined, is required to avoid bleaching.
A decrease in nitrogen uptake rates in the scleractinian coral species may lead to nitrogen limitation, which in turn may impair protein repair23, weaken the photosynthetic capacities, and decrease carbon fixation, as already noted in mulberry leafs46.
Overall, the decrease in the N:P uptake ratios during thermal stress, which is consistent for most coral species, may imply that the corals internally experience an increase in their N:P ratio, maybe due to reallocation of nitrogen that was stored in tissue reserves10. The relevance of the N:P ratio for terrestrial and marine symbiotic associations has recently been discussed through a meta-analysis50which showed that a decoupling in mutualism performance occurs whenever phototrophs benefit from nutrient enrichment at the expense of their heterotrophic partners.
The benefits that the symbiotic association retrieves from a low N:P ratio condition are highlighted in our second experiment, in which the supplementation of seawater with 2 μM phosphate, and the subsequent decrease in the N:P ratio, prevented bleaching, increased the photosynthetic capacities, the carbon translocation, as well as the carbon retained into animal biomass during thermal stress.
Coral colonies were maintained in aquaria for several months at 25 °C ± 0.5. The total daily light received by our corals in these conditions (13 E m−2 d−1 which corresponds to 150 μmol photons m−2 s−1) was equivalent to the one received on reefs at 3–5 m depth (a typical ideal day with no cloud generates a total of 14 E m−2d−1 day at 3 m depth, while a cloudy day generated 6.2 E m−2d−1 54.
Polyp pulsation by H. fuscescens under non stressful conditions was shown to improve photosynthesis, by enhancing oxygen and inorganic carbon exchange between seawater and polyp tissue31 and was suggested to favor uptake of inorganic nutrients32.
This is deep, but a couple of quick take-aways.
One is that lowering tank temp toward 25ºC (77ºF) could be a legit way to improve coral colors by removing or reducing excessive browns and yellows from zoxanthellae and chlorophyl.
For those with nutrient accumulation problems, the other take-away is that raising the tank temp toward 30ºC (86ºF) can cause an increase in nitrate and phosphate consumption.