Paralyzing people, but what about the shellfish?
So we know the economic implications of paralytic shellfish poisoning (PSP) caused by harmful algal blooms (HABs) as well as the effects in human health (i.e. paralysis) but what effects does the toxin have on one of it's phycotoxin vectors, the Pacific oyster?
Mello et al., (2013) set out to compare the effects of a saxitoxin (STX) producing algae, Alexandrium minutum and the STX itself on in vitro haemocytes of Crassostrea gigas. Haemocytes are the primary component of molluscan immunity so any changes in their behaviour could have catastrophic effects on whole populations. In vitro also provides a much more controlled environment with less inter-individual variability then using whole organisms, especially where pooled haemolymph samples are used as was in this case. A. minutum and STX both caused significant reductions in phagocytic ability of haemocytes though only A. minutum alone caused swelling of haemocytes. This is indicative of the oyster mounting an immune response with the cells increasing in size ready for phagocytosis, however there seems to be a mechanism relating to the algae and toxin that reduces their ability to do this. Furthermore both treatments had negative impacts on the oxidative burst, a mechanism by which the haemocytes produces reactive oxygen species (ROS) to damage invading pathogens. This was most pronounced in the STX only treatment with between 72 and 80% reductions. As well a looking at functional biomarkers the authors used the quantitative polymerase reaction (qPCR) to determine expression of immune and stress related genes was modulated. Of the genes studied, four showed some level of up or down regulation relating to STX and one was up-regulated by the presence of A. minutum. This gene was an interleukin (IL-17), which are usually related to inflammation, so this could signify the preparation of an immune response in relation to algal infection. In contrast, the STX treatment caused a reduction in expression of IL-17. STX seems to inhibit the interleukin's production and the difference is likely due to differences in the bioavailability of the toxin between treatments, as A. minutum cultures generally have a low extracellular toxin level until they are 30 days old (6 days old in this experiment). However the mechanism for reduction is unclear. The heat shock protein (HSP) gene HSP70 was up-regulated by STX implying that the toxin has a denaturing affect as this HSP is a chaperone that acts against misfolded proteins. Similarly defensin, an antimicrobial peptide (AMP), has increased expression under STX treatment. AMPs are poorly understood relating to xenobiotics but it could be due to the oyster recognising STX as a pathogenic product and preparing for infection, this is however wild speculation on my part and would need to be studied further. The only other change in expression was relating to biotransformation. The down regulation of CYP356A1 by STX could leave oysters vulnerable to other harmful chemicals though further research into the synergistic effects would need to be carried out.
So from this research it appears that the immune system of the Pacific oyster is indeed compromised by STX and Alexandrium. The implication of this is potential infection by other pathogens which would be devastating for fisheries already impacted by HABs. However the immune response is very complicated and just looking at 11 genes is not enough. There are many isoforms of CYPs and HSPs in oysters and these may be activated/deactivated as part of the response. Ideally transcriptome wide expression would be carried out for the two treatments and then all differences could be observed. This is unlikely at the current price but if HABs continue to increase in frequency it might be an avenue worth investigating in the future.
Mello, D. F., Silva, P. M. da, Barracco, M. A., Soudant, P., & Hégaret, H. (2013). Effects of the dinoflagellate Alexandrium minutum and its toxin (saxitoxin) on the functional activity and gene expression of Crassostrea gigas hemocytes. Harmful Algae, 26, 45–51.