SIPing in a sea of new production

2018-12-05The new production model attempts to quantify the amount of organic material exported from surfaces waters based of the form of nitrogen (N) being metabolized. Organic N itself is rarely assessed during these investigations, which can impact total production and export efficiency. Even less is understood about the organisms involved in these different transformations with this complex cycle. Stable isotope probing and uptake activity measurements were combined using multiple substrates involved in new and regenerated production over several light levels to better characterize these organisms across all three domains of life. Prokaryotic growth was overwhelming fueled by NH₄⁺ assimilation with transitions from Flavobacteria to Rhodobacteraceae and Marine Group II to Marine Group I with increasing depth for bacterial and archaeal clades respectively. Several reoccurring members closely related to the eukaryotic diatoms Chaetoceros socialis, Chaetoceros simplex, and Eucampia antarctica, dominated assimilation of all substrates investigated through the water column, contributing greatly to the overall primary production measured at this site. Diatoms roles with high nutrient areas are well known but less is understood about their role in DON cycling in highly stratified regimes. Their high sinking rates likely increase the removal of DON from surfaces waters. Our results provide unique biogeochemical and ecological insight into the dynamics and diversity of N cycling and the organisms involved within the surface waters of the Southern California Bight