Linking source to bloom: nitrogen stable isotopes

Stable Isotopes - Nitrogen M.S. Candidate, Taylor, is running batch-culture experiments to determine isotope fragmentation on various chemical forms of nitrogen. Photo by J. Smith

Sieve Away Large volumes of seawater are concentrated using a plankton net and sieve. We will determine the δ15N of the particulate organic matter, > 0.7µm, and the purified saxitoxin, and compare them to source N. Photo by J. Smith

Cell Concentrate After pumping large volumes of seawater at depth, cells are concentrated using a plankton net and sieve to produce high biomass samples. Saxitoxin will be purified and the δ15N (ratio of 15N to 14N) determined by EA-IRMS. Photo by J. Smith

Multi-system Sampling We are applying the species-specific tracer, δ15N-saxitoxin, in three systems. Here Theresa Hattenrath-Lehmann, a postdoc in Chris Gobler's lab at Stony Brook University, prepares concentrates of Alexandrium from the Northport-Huntington Bay Complex, NY for enumeration. Cell abundance data will be paired with toxicity data to acquire toxin content - or toxin/cell. Photo courtesy of T. Hattenrath-Lehmann

Closures in the NMS Red chalk warns of areas closed to shellfishing in the Nauset Marsh Estuary System. Photo by J. Smith

Title: Identification of nitrogen sources for toxic Alexandrium blooms using a novel species-specific tracer, δ¹⁵N-saxitoxin

Investigators: Juliette Smith (VIMS), Don Anderson (WHOI), Chris Gobler (Stony Brook), Deana Ernder (UTX-MSI), and Jim McClelland (UTX-MSI)

Gradute Student: Taylor Armstrong (VIMS)

Funding Source: NSF Chemical Oceanography

Abstract: The US and other countries throughout the world are affected by harmful algal blooms (HABs) that negatively impact human health, marine ecosystems, fisheries resources, and local economies. Anthropogenic nutrient loadings have been linked to expanding HAB incidence, but the relationship is site- and organism-specific, and is still poorly understood. The main challenge in this regard is to determine the relative importance of natural versus anthropogenic nutrient sources in the development of an individual HAB species. Given the diverse nature of the planktonic assemblage in which HABs occur, and the lack of appropriate measurement techniques, this is exceedingly difficult to accomplish.

Here we propose a novel approach to this challenge – to use the isotopic signature of a “species-specific” HAB toxin to identify the nitrogen source and chemical form that promotes cell growth and toxin production. The bloom-forming dinoflagellate Alexandrium fundyense and its class of bioactive compounds, saxitoxins (STXs) are an ideal model system, as STXs are nitrogen-rich and are typically only produced by a single species in mixed plankton assemblages. While the use of the isotopic ratio, δ¹⁵N, of bulk biomass to identify nitrogen sources to coastal waters is a widely accepted practice, our use of a toxin as a species-specific tracer or marker is new and will provide details on the explicit source, chemical form, and processing of nitrogen that results in blooms of a particular HAB species.