Investigating the microbial ecology of coastal hotspots of marine nitrogen fixation in the western North Atlantic

Abstract

Variation in the microbial cycling of nutrients and carbon in the ocean is an emergent property of complex planktonic communities. While recent findings have considerably expanded our understanding of the diversity and distribution of nitrogen (N2) fixing marine diazotrophs, knowledge gaps remain regarding ecological interactions between diazotrophs and other community members. Using quantitative 16S and 18S V4 rDNA amplicon sequencing, we surveyed eukaryotic and prokaryotic microbial communities from samples collected in August 2016 and 2017 across the Western North Atlantic. Leveraging and significantly expanding an earlier published 2015 molecular dataset, we examined microbial community structure and ecological co-occurrence relationships associated with intense hotspots of N2 fixation previously reported at sites off the Southern New England Shelf and Mid-Atlantic Bight. Overall, we observed a negative relationship between eukaryotic diversity and both N2 fixation and net community production (NCP). Maximum N2 fixation rates occurred at sites with high abundances of mixotrophic stramenopiles, notably Chrysophyceae. Network analysis revealed such stramenopiles to be keystone taxa alongside the haptophyte diazotroph host Braarudosphaera bigelowii and chlorophytes. Our findings highlight an intriguing relationship between marine stramenopiles and high N2 fixation coastal sites.

UCYN-A/haptophyte symbioses dominate N2 fixation in the Southern California Current System

“…N 2 fixation by UCYN-A1 was more important in offshore waters, while UCYN-A2 contributed a higher share of the total N 2 fixation in nearshore waters, suggesting that even when these sublineages co-occur, their activity may be influenced by different environmental factors. More measurements in other coastal ecosystems are needed to better understand the variability and environmental controls on UCYN-A N 2 fixation, however, these findings lend support to speculation about its importance to N 2 fixation in other coastal systems [ 6 , 7 , 10 , 89 , 90 ]. In addition, although whole community N 2 fixation does not account for a large amount of new production in this system, this work establishes that N 2 fixation is a widespread process along the Baja California peninsula, during both upwelling and oceanic seasons and throughout the euphotic zone, with the highest volumetric rates in euphotic waters overlying the continental shelf.…”

Microevolutionary patterns in ecotypes of the symbiotic cyanobacterium UCYN-A revealed from a Northwest Atlantic coastal time series

“…Not only does our network analysis provide evidence of the symbiotic UCYN-A/host consortium within the Bedford Basin, but presumably other nonsymbiotic microbial interactions involving UCYN-A are embedded within our nifH versus 16 S rRNA datasets. Such interactions may include phytoplankton growth connected to the input of new nitrogen from UCYN-A ( 69 ) or potential predator-prey interactions ( 70 – 72 ). Although a full investigation into these topics was outside the scope of our study, we anticipate that the fine-scale in situ measurements of 16 S and nifH offered by the Bedford Basin time series will provide a rigorous baseline from which to formulate and test new hypotheses regarding the interactions between the UCYN-A/host consortium and other diazotrophs and phytoplankton.…”

Unusual Hemiaulus Bloom Influences Ocean Productivity in Northeast U.S. Shelf Waters

“…cyanobacteria. This symbiosis, known as a diatom-diazotroph association has been observed around the globe, mostly in oligotrophic regions (Foster and Zehr, 2019), but also in temperate continental shelf waters (Wang et al, 2021). Furthermore, some diatom-diazotroph association have the capability to grow very quickly, forming localized blooms (Villareal et al, 2011).…”