Toxigenicity and biogeography of the diatom Pseudo-nitzschia across distinct environmental regimes in the South Atlantic Ocean

The community composition and toxigenicity of the diatom Pseudo-nitzschia in the northern Benguela Upwelling Zone and the open South Atlantic Ocean were characterized as part of a transoceanic survey conducted during the austral spring of 2007. Multiple morphological types of Pseudo-nitzschia were detected by light microscopy in coastal waters. Automated ribosomal intergenic spacer analysis (ARISA), a DNA-fingerprinting technique used to assess Pseudo-nitzschia community composition, detected 37 ARISA types distributed among 17 stations in both coastal and open-ocean regions. Through statistical analysis of abiotic factors, we identified 6 distinct environmental regimes across which Pseudo-nitzschia community composition varied. Pseudo-nitzschia were detected in open-ocean waters, where community composition differed between surface and deep chlorophyll maxima. The toxin produced by Pseudo-nitzschia, domoic acid (DA), was present in coastal waters both inside and outside the northern Benguela Upwelling Zone at potentially ecologically harmful levels, up to 184 ng DA l−1 and 4.6 pg DA cell−1. Partial internal transcribed spacer 1 (ITS1) clone libraries putatively identified at least 10 species in the South Atlantic, including P. inflatula, P. subpacifica, P. heimii, and P. galaxiae. Previously, these species were reported to produce DA at levels several orders of magnitude lower than our field measurements. Simple correlations were not able to identify obvious environmental triggers of DA production. Our findings suggest that species commonly believed to be weakly toxigenic could pose harm to humans and marine organisms, including those inhabiting southwestern African coastal regions.