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The geochemical and biological study of corals
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Unraveling nutrient assimilation into the coral skeletal organic matrix has implications for historical interpretations
P-G3-02-S
Jonathan D Cybulski* , Ph.D Candiate, main author
Inga Conti-Jerpe, colleague, Ph.D. candidate
David M. baker, Supervisor
Presenter Email: cybulski.j@gmail.com |
| The skeletal organic matrix (SOM) (the organic portion of the skeleton comprised of proteins, lipids, and polysaccharides) of scleractinian corals is significant for both biogeochemical and paleoceanographic research because it records elements over geologic timescales. The nitrogen bound inside the SOM reflects the nutrient sources of the coral-algal holobiont at the time of carbonate nucleation. However, research to date has yet to identify the leading source of nutrients corals preferentially assimilate, undermining accurate interpretation of stable isotope records preserved in their skeleton. We still do not know what the delta values from the SOM are actually telling us. Furthermore, the low nitrogen content of SOM, a limiting nutrient in coral ecosystems, has made it difficult to measure and understand nitrogen fluxes during calcification. To fill this gap, we aimed to answer the question: What form of nitrogen is incorporated in the skeletal secretion process of calcifying corals? We first tested a new skeleton cleaning methodology that ensures no coral tissue or other organic contamination was included in analysis, and that we were only measuring the SOM signal. Preliminary experiments demonstrated that the use of enriched stable isotope tracer allowed for the detection of nitrogen signals on an isotope ratio mass spectrometer. We therefore exposed coral nubbins to 3 different forms of nutrients: 1) inorganics nutrients (nitrate) that can be assimilated by algal symbionts, 2) organic nutrients (urea) accessible by the entire holobiont, and 3) phytoplankton (Isochrysis galbana) or zooplankton (Artemia salina) for host heterotrophic feeding. In each treatment, one food source was enriched in nitrogen allowing us to identify which was preferentially ingested, and subsequently which are used for calcification. Our results demonstrate that Porites sp. does not feed on Isochrysis sp. but does assimilate nutrients from all other sources. The important implications of our results extend beyond simply identifying a corals preferred food, but can give insights into potential nitrogen assimilation changes with anthropogenic impacts to the nitrogen cycle. Most immediately, this study's implications extend to fossil coral in the paleo record, which can only be interpreted correctly by knowing the source of the incorporated nitrogen at that time. |
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