Determining the potential for the proliferation of the harmful cyanobacterium Cylindrospermopsis raciborskii in Currituck Sound, North Carolina

Abstract

Cylindrospermopsis raciborskii is a tropical invasive, toxin-producing, filamentous-heterocystous, N 2-fixing cyanobacterium that has recently expanded its range in temperate waterways. Because it is capable of exploiting low salinity (oligohaline) waters experiencing nutrient enrichment, it opens up brackish eutrophying systems to potential invasion. We examined the susceptibility of oligohaline (historically 0–3.5 salinity) Currituck Sound (CS), North Carolina to C. raciborskii proliferation during 2007–2008. This component of the Albemarle-Pamlico Sound estuarine system is experiencing incipient eutrophication. We addressed the following questions: (1) Is C. raciborskii currently present in CS, and (2) what conditions favor its growth and expansion? In 2007, C. raciborskii was confirmed by microscopy, diagnostic photopigment and molecular analyses, which further revealed its genetic potential to produce the cyanotoxin cylindrospermopsin. In 2008, CS salinity had risen due to a persistent drought, and C. raciborskii was no longer microscopically observed. The potential for C. raciborskii to grow in CS was assessed using in situ nutrient addition bioassays. Primary productivity, nitrogen fixation (nitrogenase activity) rates and chlorophyll a measurements demonstrated that C. raciborskii could grow in CS water, and dissolved inorganic nitrogen (nitrate and ammonium) additions increased its growth potential. Salinity was a key factor influencing C. raciborskii growth, with elevated salinity (8.4) significantly limiting biomass accumulation. Interestingly, nitrogen enrichment enabled C. raciborskii to better withstand elevated salinities and increased its competitive success in the CS phytoplankton community. Emphasis should be placed on controlling nutrient, particularly nitrogen, enrichment in order to prevent the expansion of C. raciborskii in this and other nutrient-sensitive oligohaline ecosystems.