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Pseudo-nitzschia harmful algal blooms have recently caused elevated domoic acid in coastal environments of the Northeast United States. In 2017, the toxigenic species P. australis was observed in Narragansett Bay, Rhode Island, a temperate estuarine ecosystem, for the first time since 2009 when DNA monitoring for Pseudo-nitzschia species began. This highly toxic species likely contributed to toxin-related shellfish harvest closures and is hypothesized to have been introduced by an offshore source. Little is known about offshore Pseudo-nitzschia spp. populations in the Northeast Continental Shelf marine ecosystem or how often toxigenic species enter Narragansett Bay through physical processes. Here, we collected filtered biomass samples from multiple time series sites within Narragansett Bay and along the Northeast U.S. Shelf Long-Term Ecological Research transect in winter and summer to investigate the frequency and seasonality of potential Pseudo-nitzschia spp. inflow from the continental shelf to the estuary. Species were taxonomically identified using DNA sequencing of the ITS1 region and domoic acid concentrations were quantified by liquid chromatography with tandem mass spectrometry and multiple reaction monitoring. During six years of sampling, Pseudo-nitzschia species assemblages were more similar between Narragansett Bay and the Northeast shelf in winter than summer, suggesting greater ecosystem connectivity in winter. These winter assemblages were often accompanied by higher domoic acid. Several Pseudo-nitzschia species co-occurred most often with domoic acid and were likely responsible for toxin production in this region, including P. pungens var. pungens, P. multiseries, P. calliantha, P. plurisecta, P. australis, and P. fraudulenta. Domoic acid was detected during periods of relatively low macronutrient concentrations in both seasons, warmer sea surface temperatures in winter, and colder temperatures in summer within this dataset. This study represents some of the first domoic acid measurements on the offshore Northeast U.S. Continental Shelf, a region that supplies water to other coastal environments and could seed future harmful algal blooms. The elevated domoic acid and frequency of hypothesized inflow of toxigenic Pseudo-nitzschia spp. from the Northeast continental shelf to Narragansett Bay in winter indicate the need to monitor coastal and offshore environments for toxins and harmful algal bloom taxa during colder months. 

The growth of diatoms in the Southern Ocean, especially the region surrounding the West Antarctic Peninsula, is frequently constrained by low dissolved iron and other trace metal concentrations. This challenge may be overcome by mutualisms between diatoms and co-occurring associated bacteria, in which diatoms produce organic carbon as a substrate for bacterial growth, and bacteria produce siderophores, metal-binding ligands that can supply diatoms with metals upon uptake as well as other useful secondary compounds for diatom growth like vitamins. To examine the relationships between diatoms and bacteria in the plankton (diatom) size class (> 3 µm), we sampled both bacterial and diatom community composition with accompanying environmental metadata across a naturally occurring concentration gradient of macronutrients, trace metals and siderophores at 21 stations near the West Antarctic Peninsula (WAP). Offshore Drake Passage stations had low dissolved iron (0.33 ± 0.15 nM), while the stations closer to the continental margin had higher dissolved iron (5.05 ± 1.83 nM). A similar geographic pattern was observed for macronutrients and most other trace metals measured, but there was not a clear inshore-offshore gradient in siderophore concentrations. The diatom and bacteria assemblages, determined using 18S and 16S rDNA sequencing respectively, were similar by location sampled, and variance in both assemblages was driven in part by concentrations of soluble reactive phosphorous, dissolved manganese, and dissolved copper, which were all higher near the continent. Some of the most common diatom sequence types observed were Thalassiosira and Fragilariopsis , and bacteria in the plankton size fraction were most commonly Bacteroidetes and Gammaproteobacteria. Network analysis showed positive associations between diatoms and bacteria, indicating possible in situ mutualisms through strategies such as siderophore and vitamin biosynthesis and exchange. This work furthers the understanding of how naturally occurring gradients of metals and nutrients influence diatom-bacteria interactions. Our data suggest that distinct groups of diatoms and associated bacteria are interacting under different trace metal regimes in the WAP, and that diatoms with different bacterial partners may have different modes of biologically supplied trace metals. 

In 2016-17, shellfish harvesting closed for the first time in Narragansett Bay, Rhode Island, USA, from domoic acid (DA), a neurotoxin produced by diatoms of the Pseudo-nitzschia genus. Pseudo-nitzschia have occurred frequently for over 60 years in Narragansett Bay’s Long-Term Plankton Time Series (NBPTS), therefore it is surprising that the first closure only recently occurred. Pseudo-nitzschia species are known to vary in their toxin production, thus species identification is critical for understanding the underlying ecological causes of these harmful algal blooms (HABs). DNA in plankton biomass can be preserved for many years, so molecular barcoding of archived samples is useful for delineation of taxa over time. This study used amplification of the Pseudo-nitzschia -specific 18S-5.8S rDNA internal transcribed spacer region 1 (ITS1) in plankton samples and high throughput sequencing to characterize Pseudo-nitzschia species composition over a decade in Narragansett Bay, including eight years before the 2016-17 closures and two years following. This metabarcoding method can discriminate nearly all known Pseudo-nitzschia species. Several species recur as year-round residents in Narragansett Bay ( P. pungens var. pungens, P. americana, P. multiseries , and P. calliantha ). Various other species increased in frequency after 2015, and some appeared for the first time during the closure period. Notably, P. australis , a species prevalent in US West Coast HABs and known for high DA production, was not observed in Narragansett Bay until the 2017 closure but has been present in several years after the closures. Annual differences in Pseudo-nitzschia composition were correlated with physical and chemical conditions, predominantly water temperature. The long-term composition trends of Pseudo-nitzschia in Narragansett Bay serve as a baseline for identifying the introduction of new species, understanding shifting assemblages that contributed to the 2016-17 closures, and monitoring species that may be cause for future concern. 

ABSTRACT Salegentibacter sp. strain BDJ18 was isolated from a plankton-associated seawater sample from the northeast Atlantic Ocean. We report its draft genome assembly, which includes genes potentially important for microbial interactions in the marine environment.