Temporal Dynamics of the Diazotroph UCYN-A and its Known and Possible Hosts over 10 Years at the San Pedro Ocean Time-series (SPOT)

Marine nitrogen fixation was traditionally thought to occur only in strongly nutrient-limited regions where nitrogen is scarce, typically tropical waters. Despite expectations, diazotrophs have been detected in mesotrophic and temperate environments with seasonal access to bio-available nitrogen, such as SPOT in Southern California. Hamersley et al. found surprisingly high abundances of the diazotroph UCYN-A at SPOT through qPCR of the nifH gene; Needham et al. found that UCYN-A was a significant fraction of phytoplankton abundance via 16S rRNA tag sequencing of a nearby daily time series. This nitrogen-fixing cyanobacterium has an aberrant metabolism, lacking TCA, PSII, and other key metabolic pathways, suggestive of a symbiotic lifestyle. Two types of UCYN-A have been shown to live in association with hosts closely related to Braarudosphaea bigelowii. Other UCYN-A variants exist, but their hosts have not yet been identified. Here we show abundance patterns of UCYN-A and putative hosts over ten years’ worth of tag sequencing data collected from SPOT. We used “universal” primers to amplify 16S rRNA sequences of prokaryotes and chloroplasts and 18S rRNA sequences of eukaryotes on a monthly basis over 10 years. Three amplicon sequence variants (ASVs) of UCYN-A occur sporadically at this location, but the abundance pattern of only one parallels that of B. bigelowii chloroplast sequences. To identify other possible hosts, we are examining 16S and 18S rRNA sequences in microbial association networks to determine if any other organisms at SPOT correlate with UCYN-A ASVs. In addition, abundance patterns of other diazotrophs identified via Stable Isotope Probing (SIP) may be traced over the time series. Diazotroph abundances will also be correlated with environmental parameters. These findings have the potential to enhance our understanding of nitrogen cycling dynamics, how these dynamics change over time, and how microbial communities access this limiting nutrient in a mesotrophic environment.