Effects of Iron Fertilization on Diatom Populations and Domoic Acid Concentrations

Iron fertilization has been proposed as a solution to increased levels of CO2 in the atmosphere and the resulting acidification of the world’s oceans.  Iron fertilization is a process in which iron is added to areas of open ocean to induce phytoplankton blooms; the species of phytoplankton targeted sequester CO2 in their cells removing it from the atmosphere and upper levels of the oceans.  The idea I that as these cells die they will sink to the bottom of the ocean with the acquired carbon where it will remain.  It is important, however, to weigh the benefits of this procedure against the possible consequences.  Silver et al. (2010) have shown that iron fertilization leads to increased biomass of diatom species from the genus Pseudo-nitzschia, which are known to produce the neurotoxin domoic acid (DA).  This increase is directly correlated with increased levels of DA in the environment.  This neurotoxin has been shown to have negative effects in both coastal and oceanic ecosystems, as well as at depth below fertilization sites; and the extent of this increases still is not fully understood.—Anna Fiastro

Silver, M., Bargu, S., Coale, S., Benitez-Nelson, C., Garcia, A., Roberts, K., Sekula-Wood, E., Bruland, K., Coale, K.,2010.Toxic diatoms and domoic acid in natural and iron enriched waters of the oceanic Pacific. Proceedings of the National Academy of Science 107, 20762.doi:10.1073/pnas.1006968107.

Silver and colleagues surveyed 34 stations ranging from the Pacific subarctic to the Southern Ocean, some of which were historic iron fertilization sites.  They collected and analyzed near surface water samples and sediment samples.  They quantified and identified eleven Pseudo-nitzschia species from these samples and measured DA concentrations in the cells and water of each sample.
A correlation was found between increased Pseudo-nitzschia cell abundance and increased DA concentrations.  Eleven species from the genus were identified in the samples, some containing just one species and some up to 4 different species.  Due to the method of data collection toxin levels could not be assigned to specific species.  Variability in toxin levels was attributed to different combinations of species as well as between variability among individuals or cells within a species.  This variability is due to varying physical and chemical conditions as well as different strains within the species.  This, however, does not affect the clear correlation between cell abundance and DA toxin levels.
It was also found that increased levels of DA toxin and Pseudo-nitzschia abundance was linked to areas of historic iron enrichment experimentation.  While it has not yet been shown that these elevated toxin levels impact higher trophic levels in oceanic ecosystems, as seen in coastal regions, it has been proven that DA has reached levels that pose a threat to the oceanic ecosystems.
The data from this study also suggest that DA neurotoxins are delivered to deeper depths in the intact cells of these Pseudo-nitzschia species and at higher rates with iron fertilization.  In areas where prior studies using iron fertilization took place, intact diatom cells containing DA were found in sediment samples that correlated with blooms occurring after fertilization.

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