Toxic Pseudonitzschia on the Rise!

A review of Trick, C.G., Bill, B.D., Cochlan, W.P., Wells, M.L., Trainer, V.L & Pickell, L.D., (2010), Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas, PNAS, 107(13): 5887-5892

Many Pseudonitzschia species are known to produce domoic acid (DA), a neurotoxin detrimental to numerous marine mammals, birds and humans. Toxic species tend to be coastal, however oceanic Pseudonitzschia species are considered nontoxic but still capable of DA production under certain conditions.
Recent efforts to help reduce atmospheric CO2 and to mitigate the effects of climate change have resulted in the targeting of ocean high-nitrate, low-chlorophyll (HNLC) environements with large-scale iron fertilisation. It is still unclear how beneficial these initiatives are, both on carbon removal and also on its impacts on ecosystems. This study aims to show that the sparse community of oceanic Pseudonitzschia at a specific HNLC site produce DA and become toxic in response to iron additions and that the DA alters phytoplankton community structure to benefit the Pseudonitzschia.

Samples were collected from the treatment site (Ocean Station PAP, in the eastern Pacific) and phytoplankton biomass, species abundance and DA concentrations were measured using cELISA.

Only two species, P. turgidula and P. cf granii were observed at the site. A vertical profile of DA concentration showed its presence throughout the phonic zone, with concentrations of >30pg.L-1 with an estimated minimum cell toxin quota of 15 fg DA.cell-1, assuming all cells were producing equal amounts. These findings confirm that oceanic Pseudonitzschia species produce DA in situ and that DA accumulation is constitutive with their presence in HNLC waters.
Samples placed in bottle-type growth experiments for 6 days showed increased Pseudonitzschia abundance and DA concentrations. Moreover DA concentrations were further increased (by 160%) by a small addition of iron (1nM) and copper (10nM), in comparison to control bottles with no additions. Two isolates of P. turgidula were returned to the lab and continued to produce DA in laboratory cultures, although these concentrations were lower compared to those measured in the natural population.

Iron increase from mesoscale enrichment is short-lived as the majority aggregates and sinks, with the remaining iron dissolving out quickly. To better evaluate the effects of elevated concentrations of iron, continuous cultures were used with filtered seawater and iron supplied at a dilution rate. This doubled the abundance of Pseudonitzschia after 9 days compared with controls (25,000 vs. 12,400 cells/mL). Although dissolved DA was not detectable in this experiment due to the high daily medium dilution.

Another point raised is that DA is involved in iron and copper acquisition and increased uptake by Pseudonitzschia. This ability can be a concern, especially when DA levels are increased, as it gives Pseudonitzschia a competitive advantage over other diatoms in iron-limited waters. The effects were tested in continuous cultures, finding that diatoms in both the control, iron and DA treatments were dominated by Pseudonitzschia species, whilst the DA caused an twofold increase in chlorophyll a biomass by increasing availability of dissolved iron.

The study acquired results from Pseudonitzschia both from lab cultures as well as in their natural environment, showing clear elevated production of DA with iron addition. However, the samples were taken in spring so perhaps further samples should be taken seasonally, before and after blooms to fully determine Pseudonitzschia activity and DA production. Moreover, although toxicity levels were in the range measured in coastal water, it needs to be determined whether they are high enough to cause ecosystem damage.

Overall, considering how much damage coastal Pseudonitzschia causes to marine life, as shown in previous blogs, these results are essential in highlighting potential issues of large-scale iron fertilisation and possible detrimental effects on ecosystems.