Development, calibration, and evaluation of a model of Pseudo-nitzschia and domoic acid production for regional ocean modeling studies

Molecular Forecasting of Domoic Acid during a Pervasive Toxic Diatom Bloom

In 2015, the largest recorded harmful algal bloom (HAB) occurred in the Northeast Pacific, causing nearly 100 million dollars in damages to fisheries and killing many protected marine mammals. Dominated by the toxic diatom Pseudo-nitzschia australis , this bloom produced high levels of the neurotoxin domoic acid (DA). Through molecular and transcriptional characterization of 52 near-weekly phytoplankton net-tow samples collected at a bloom hotspot in Monterey Bay, California, we identified active transcription of known DA biosynthesis ( dab ) genes from the three identified toxigenic species, including P. australis as the primary origin of toxicity. Elevated expression of silicon transporters ( sit1 ) during the bloom supports the previously hypothesized role of dissolved silica (Si) exhaustion in contributing to bloom physiology and toxicity. We find that co-expression of the dabA and sit1 genes serves as a robust predictor of DA one week in advance, potentially enabling the forecasting of DA-producing HABs. We additionally present evidence that low levels of iron could have co-limited the diatom population along with low Si. Iron limitation represents a previously unrecognized driver of both toxin production and ecological success of the low iron adapted Pseudo-nitzschia genus during the 2015 bloom, and increasing pervasiveness of iron limitation may fuel the escalating magnitude and frequency of toxic Pseudo-nitzschia blooms globally. Our results advance understanding of bloom physiology underlying toxin production, bloom prediction, and the impact of global change on toxic blooms.

Significance

Pseudo-nitzschia diatoms form oceanic harmful algal blooms that threaten human health through production of the neurotoxin domoic acid (DA). DA biosynthetic gene expression is hypothesized to control DA production in the environment, yet what regulates expression of these genes is yet to be discovered. In this study, we uncovered expression of DA biosynthesis genes by multiple toxigenic Pseudo-nitzschia species during an economically impactful bloom along the North American West Coast, and identified genes that predict DA in advance of its production. We discovered that iron and silica co-limitation restrained the bloom and likely promoted toxin production. This work suggests that increasing iron limitation due to global change may play a previously unrecognized role in driving bloom frequency and toxicity.

The Pacific Northwest Harmful Algal Blooms Bulletin

A bulletin communicating risk of toxic Pseudo-nitzschia blooms to shellfish harvest along the open coast of the Pacific Northwest region of the United States (the northeast Pacific Ocean spanning Washington and Oregon) is discussed. This Pacific Northwest Harmful Algal Blooms (PNW HAB) Bulletin is designed for shellfish managers with a focus on the razor clam fishery, but may also be informative to managers of the Dungeness crab fishery since domoic acid accumulation in crabs tends to lag accumulation in razor clams by a couple of weeks. The Bulletin complements beach phytoplankton monitoring programs by alerting coastal shellfish managers about adverse environmental conditions that could be conducive to a toxic Pseudo-nitzschia bloom. Beach monitoring programs are effective at determining when toxins have arrived at shellfish beaches, but a risk forecast based on near real-time biophysical information can provide managers with additional forewarning about potential future toxin outbreaks. Here, the approaches taken in constructing the risk forecasts, along with the reasoning and research behind them are presented. Updates to a historical PNW HAB Bulletin are described, as are the current workflow and the individual components of the updated Bulletin. Some successes and failures realized throughout the process are also pointed out for the benefit of the broader community. A self-assessment suggests that when the necessary data sources are available, the PNW HAB Bulletin provides an accurate forecast of risk associated with toxic Pseudo-nitzschia blooms. The Bulletin has proven beneficial to coastal shellfish managers by better informing decisions on sample collection, and harvest limits, openings, extensions, and closures.

Annual variation in domoic acid in phytoplankton and shellfish samples from Daya Bay of the South China Sea

Domoic acid (DA) is a well-known phycotoxin that causes amnesic shellfish poisoning (ASP) and is mainly produced by diatom species belonging to the genus Pseudo-nitzschia. An annual survey was conducted monthly over the period of September 2020 to August 2021 in Daya Bay of the South China Sea to investigate the dynamics of particulate and shellfish DA and their relationships with the abundance of Pseudo-nitzschia spp. and environmental parameters. Pseudo-nitzschia spp. was one of the most dominant phytoplankton taxa, and a Pseudo-nitzschia bloom occurred during the survey with the highest abundance of 1.91 × 106 cells L-1. DA was detected in almost all plankton samples with the highest value of 120.7 ng L-1, and high DA concentrations coincided with the abundant presence of Pseudo-nitzschia. DA is prevalent in Daya Bay throughout the year, with detection rates of 98.3%, 82.6%, and 82.6% in plankton samples, in-situ and purchased shellfish, respectively. Higher DA concentrations were detected in the scallop (Chamys nobilis), with the highest concentration of 5.34 µg g-1. High water temperature and low DSi:DIN ratio promoted the growth of Pseudo-nitzschia and DA production. The results suggest that the increasing nitrogen loading and silicate limitation during Pseudo-nitzschia blooms together with the increase in water temperature may increase the risk of DA contamination in Daya Bay.

Identification of Pseudo-nitzschia Cryptic Species Collected in the Gulf of Naples Using Whole-Cell Fluorescent In Situ Hybridization: From Cultured Sample to Field Test

The planktonic diatom genus Pseudo-nitzschia contains several genetically closely related species that can produce domoic acid, a potent neurotoxin known to cause amnesic shellfish poisoning (ASP). An early identification and an adequate monitoring of the potential toxic Pseudo-nitzschia spp. are necessary. However, effective monitoring programs are time consuming due, in some cases, to the cell morphology similarities among species, determined with light microscopy, that can result in insufficient data to give a definitive species and toxins attribution. In this paper, Whole-Cell Fluorescent In Situ Hybridization (WC-FISH) has been evaluated as a powerful tool to detect and enumerate harmful cryptic and/or pseudo-cryptic Pseudo-nitzschia spp. collected in the Gulf of Naples. Fluorescently labelled probes directed against the ribosomal RNA (rRNA) of the 28S large subunit (LSU) were used. In particular, five probes detecting four cryptic species of Pseudo-nitzschia delicatissima complex and one specific for Pseudo-nitzschia multistriata gave good results for the molecular identification of potentially toxic target species in natural samples. Finally, we can state that the WC-FISH method, to identify Pseudo-nitzschia species, is faster and more cost-effective if compared with other rDNA-based methods.