Nitrogen isotopic relationship between diatom-bound and bulk organic matter of cultured polar diatoms

Authors

Matthew G. Horn, University of Rhode Island
Rebecca S. Robinson, University of Rhode Island
Tatiana A. Rynearson, University of Rhode Island
Daniel M. Sigman, Princeton University

Document Type

Article

Date of Original Version

9-1-2011

Abstract

Using batch cultures, the ¹⁵N/¹⁴N (hereafter δ¹⁵N) of diatom-bound organic matter was measured and compared to the δ¹⁵N of total diatom biomass during the progressive consumption of a nitrate pool in four polar diatom species (Fragilariopsis cylindrus, Fragilariopsis kerguelensis, Pseudo-nitzschia seriata, and Thalassiosira nordenskioeldii) and one temperate species (Thalassiosira aestivalis). In general, the δ¹⁵N of the dissolved nitrate in seawater was greater than that of the biomass, which was greater than that of the diatom-bound N. Rayleigh-type relationships were observed, allowing for estimation of the isotope effect (ε) for each species, with a range from 1.0‰ to 14.0‰ across all species. For all cultured strains, the δ¹⁵N values of the diatom-bound (δ¹⁵N DB) fraction was lower than those of the total diatom biomass (δ¹⁵Nbiomass). The isotopic offset between the biomass and diatom-bound N (δ¹⁵NDBoffset = δ¹⁵Nbiomass-δ¹⁵NDB) was relatively constant along the growth curve for each individual species but varied among species, with a range of 1.9‰-11.2‰. Weak relationships were determined when ε and the δ¹⁵N DBoffset were compared to cellular size and surface area:volume ratio. More significantly, with the exception of Pseudo-nitzschia seriata, a strong positive relationship was found between ε and δ ¹⁵NDBoffset. While the culture data indicate a positive δ¹⁵NDBoffset across all studied diatom species, surface sediment data suggest a negative δ¹⁵N DBoffset for sedimentary assemblages. This indicates that either (1) the growth conditions of our cultures had some effect on δ ¹⁵NDBoffset or (2) a low-δ¹⁵N component of the N that we measure as diatom frustule-bound is lost during early diagenesis. Given documented assemblage changes, our culture data for relevant species do not suggest that the higher δ¹⁵NDB observed in the Antarctic during ice ages can be explained by species related changes in the sedimentary bulk-to-diatom-bound isotopic difference. Future work on the diatom-bound material in cultured diatoms grown under in situ nutrient conditions, analysis of sediment trap and net tow material, and frustule dissolution experiments will more completely assess this paleoproxy. Copyright 2011 by the American Geophysical Union.

Publication Title, e.g., Journal

Paleoceanography

Volume

26

Issue

3

Citation/Publisher Attribution

Horn, Matthew G., Rebecca S. Robinson, Tatiana A. Rynearson, and Daniel M. Sigman. "Nitrogen isotopic relationship between diatom-bound and bulk organic matter of cultured polar diatoms." Paleoceanography 26, 3 (2011). doi: 10.1029/2010PA002080.