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Near-surface waters ranging from the Pacific subarctic (58°N) to the Southern Ocean (66°S) contain the neurotoxin
domoic acid (DA), associated with the diatom Pseudo-nitzschia. Of the 35 stations sampled, including ones from historic
iron fertilization experiments (SOFeX, IronEx II), we found Pseudo-nitzschia at 34 stations and DA measurable at 14 of the
26 stations analyzed for DA. Toxin ranged from 0.3 fg·cell−1 to 2 pg·cell−1, comparable with levels found in similar-sized
cells from coastal waters. In the western subarctic, descent of intact Pseudo-nitzschia likely delivered significant amounts
of toxin (up to 4 μg of DA·m−2·d−1) to underlying mesopelagic waters (150-500 m). By reexamining phytoplankton
samples from SOFeX and IronEx II, we found substantial amounts of DA associated with Pseudo-nitzschia. Indeed, at
SOFeX in the Antarctic Pacific, DA reached 220 ng·L−1, levels at which animal mortalities have occurred on continental
shelves. Iron ocean fertilization also occurs naturally and may have promoted blooms of these ubiquitous algae over
previous glacial cycles during deposition of iron-rich aerosols. Thus, the neurotoxin DA occurs both in coastal and oceanic
waters, and its concentration, associated with changes in Pseudo-nitzschia abundance, likely varies naturally with climate
cycles, as well as with artificial iron fertilization. Given that iron fertilization in iron-depleted regions of the sea has been
proposed to enhance phytoplankton growth and, thereby, both reduce atmospheric CO2 and moderate ocean acidification
in surface waters, consideration of the potentially serious ecosystem impacts associated with DA is prudent.
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