Abstract
Compound-specific stable isotopes of amino acids (CSI-AA) from proteinaceous deep-sea coral skeletons have the potential to improve paleoreconstructions of plankton community composition, and our understanding of the trophic dynamics and biogeochemical cycling of sinking organic matter in the Ocean. However, the assumption that the molecular isotopic values preserved in protein skeletal material reflect those of the living coral polyps has never been directly investigated in proteinaceous deep-sea corals. We examined CSI-AA from three genera of proteinaceous deep-sea corals from three oceanographically distinct regions of the North Pacific: Primnoa from the Gulf of Alaska, Isidella from the Central California Margin, and Kulamanamana from the North Pacific Subtropical Gyre. We found minimal offsets in the δ13C values of both essential and non-essential AAs, and in the δ15N values of source AAs, between paired samples of polyp tissue and protein skeleton. Using an essential AA δ13C fingerprinting approach, we show that estimates of the relative contribution of eukaryotic microalgae and prokaryotic cyanobacteria to the sinking organic matter supporting deep-sea corals are the same when calculated from polyp tissue or recently deposited skeletal tissue. The δ15N values of trophic AAs in skeletal tissue, on the other hand, were consistently 3–4‰ lower than polyp tissue for all three genera. We hypothesize that this offset reflects a partitioning of nitrogen flux through isotopic branch points in the synthesis of polyp (fast turnover tissue) and skeleton (slow, unidirectional incorporation). This offset indicates an underestimation, albeit correctable, of approximately half a trophic position from gorgonin protein-based deep-sea coral skeleton. Together, our observations open the door for applying many of the rapidly evolving CSI-AA based tools developed for metabolically active tissues in modern systems to archival coral tissues in a paleoceanographic context.
Highlights
A diverse array of analytical tools is used to examine ocean ecosystem and biogeochemistry cycling responses to changing climatic conditions (Gordon and Morel 1983; Henderson 2002; Rothwell and Rack 2006; Katz et al 2010)
We found minimal offsets in the δ13C values of both essential and non-essential AAs, as well as the δ15N values of source AAs between polyp tissue and protein skeleton
The δ15N values of trophic AAs in skeletal material were consistently 3-4‰ less than polyp tissue for all three genera. These observations suggest that these patterns of δ13C and δ15N offset between coral polyp tissue and proteinaceous skeleton are likely robust for gorgonin-based proteinaceous corals, linked to fundamental aspects of central metabolism and tissue synthesis
Summary
A diverse array of analytical tools is used to examine ocean ecosystem and biogeochemistry cycling responses to changing climatic conditions (Gordon and Morel 1983; Henderson 2002; Rothwell and Rack 2006; Katz et al 2010). The geochemical composition of well preserved, accretionary biogenic tissues (hereafter bioarchives) has the potential to close this gap, shedding light on the structure and function of past ocean ecosystems and their responses to changing climatic and oceanographic conditions on the scale of decades to millennia (Druffel 1997; Barker et al 2005; Ehrlich 2010; Robinson et al 2014). Deep-sea (azooxanthellate) corals were discovered over two hundred years ago (Roberts and Hirshfield 2004), yet their potential as bioarchives of past ocean conditions is just starting to be fully appreciated (Robinson et al 2014). They are found on hard substrates in every ocean from near the surface to over 6000 m water depth (Cairns 2007). Proteinaceous deep-sea corals can be long-term (millennial), high-resolution (annual to decadal) bioarchives of past ocean conditions
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