Amino acid nitrogen isotopic fractionation patterns as indicators of heterotrophy in plankton, particulate, and dissolved organic matter

Bulk nitrogen (N) isotope signatures have long been used to investigate organic N source and food web structure in aquatic ecosystems. This paper explores the use of compound-specific dN patterns of amino acids (dN-AA) as a new tool to examine source and processing history in non-living marine organic matter. We measured dN-AA distributions in plankton tows, sinking particulate organic matter (POM), and ultrafiltered dissolved organic matter (UDOM) in the central Pacific Ocean. dN-AA patterns in eukaryotic algae and mixed plankton tows closely resemble those previously reported in culture. dN differences between individual amino acids (AA) strongly suggest that the sharply divergent dN enrichment for different AA with trophic transfer, as first reported by [McClelland, J.W. and Montoya, J.P. (2002) Trophic relationships and the nitrogen isotopic composition of amino acids. Ecology 83, 2173–2180], is a general phenomenon. In addition, differences in dN of individual AA indicative of trophic transfers are clearly preserved in sinking POM, along with additional changes that may indicate subsequent microbial reworking after incorporation into particles. We propose two internally normalized dN proxies that track heterotrophic processes in detrital organic matter. Both are based on isotopic signatures in multiple AA, chosen to minimize potential problems associated with any single compound in degraded materials. A trophic level indicator (DTr) is derived from the dN difference between selected groups of AA based on their relative enrichment with trophic transfer. We propose that a corresponding measure of the variance within a subgroup of AA (designated RV) may indicate total AA resynthesis, and be strongly tied to heterotrophic microbial reworking in detrital materials. Together, we hypothesize that DTr and RV define a two dimensional trophic ‘‘space’’, which may simultaneously express relative extent of eukaryotic and bacterial heterotrophic processing. In the equatorial Pacific, DTr indicates an average of 1.5–2 trophic transfers between phytoplankton and sinking POM at all depths and locations. The RV parameter suggests that substantial variation may exist in bacterial heterotrophic processing between differing regions and time periods. In dissolved material dN-AA patterns appear unrelated to those in POM. In contrast to POM, dN-AA signatures in UDOM show no clear changes with depth, and suggest that dissolved AA preserved throughout the oceanic water column have undergone few, if any, trophic transfers. Together these data suggest a sharp divide between processing histories, and possibly sources, of particulate vs. dissolved AA. 2007 Elsevier Ltd. All rights reserved. 0016-7037/$ see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.06.061 * Corresponding author. E-mail address: [email protected] (M.D. McCarthy).