DNP-Enhanced NMR of metamorphosing young corals shows that skeleton construction entails modulation of organic material

The ability of corals to maintain homeostasis and mineralize has been compromised by ocean acidification and temperature rise causing reefs to recede and even vanish. Therefore, corals have been serving lately as important proxies of environmental impact on marine life. Early in their life, corals undergo transformation from a swimming organism (planula) to an benthic immobile one (polyp), that lives in colonies and forms exoskeleton for protection. During this metamorphosis process, they are vulnerable leaving them exposed to detrimental external changes. It is therefore very important to carefully characterize the two developmental states – planula and polyp- and to be able to assist corals withstand ongoing hazardous variations in their surroundings.

Recently, we have shown using ¹³C MAS NMR on whole ¹³C-labeled young Stylophora pistillata corals that mineralization starts before the coral settles. We also found using 2D ¹³C DARR that Glu-rich proteins bind soft amorphous mineral in the planula and Asp-rich proteins bind aragonite crystals in the polyp. Regulation of the two mineral states by the disparate proteins suited well the needs of the coral changing from motile to sessile.

Here, we expand investigations of the metamorphosis using DNP-enhanced MAS NMR measurements, obtaining favorable signal enhancements of about x 16 – x 48. Using 2D ¹³C DQ-SQ and PDSD measurements on the intact corals, labeled either via ¹³C carbonate/glycine or via ¹³C₆ glucose/glycine, we follow the changes in organic level production related to the transformation and the onset of aragonite precipitation. We monitor contribution of symbiotic dynoflagellates via carbonate metabolism by using the latter as a food source.

The carbon fingerprint changes observed relate to increased carbohydrate production and certain proteins which were not detected without enhancement. These changes are indicative of the transitions entailing onset of colonization and expedited mineralization effort initiated after settling on the bottom of the ocean.