SIP profiles for April SPOT

The new production model attempts to quantify the amount of organic material exported from surfaces waters based on the form of nitrogen (N) being metabolized. Dissolved organic N (DON) itself is rarely assessed during these investigations and even less is understood about the organisms involved in these different transformations within this complex cycle. This can greatly impact estimates of total production and export efficiency. Stable isotope probing (SIP) and uptake activity measurements were combined to investigate the dynamics of both new and regenerated production during the spring within the Southern California Bight (SCB). Multiple substrates were assessed over several light levels to characterize these processes and identify the active communities across all three domains of life that are driving each transformation. Several reoccurring members closely related to the eukaryotic diatom Chaetoceros, dominated assimilation of all substrates investigated through the water column, contributing greatly to the overall primary production measured at this site. Prokaryotic growth was overwhelming fueled by NH4+ assimilation with transitions from Flavobacteria to Rhodobacteraceae and Marine Group II Euryarchaeota to Marine Group I Thaumarchaeota with increasing depth for bacterial and archaeal clades respectively. Only urea uptake rates and SIP activity, driven primarily by diatoms, correlated, likely demonstrating that cellular transport and incorporation of this substrate were coupled. SIP was therefore potentially able to delineate the organisms regulating urea cycling at each depth during this investigation. Diatom's roles within high nutrient areas are well defined but their part in DON cycling in highly stratified regimes less is understood. Here we demonstrate their ability to scavenged urea quickly and efficiently in situ, allowing them to outcompete the rest of the community. This diversion of DOM away from the inefficient microbial loop directly back into the larger, particle forming populations would alter the current view of microbial food webs. This proposed 'phytoplankton shunt' of organic material could potentially enhance the biological pump by mitigating losses due to trophic transfers while increasing DON flux due to ballasting. 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 SCB.