Tuesday, 28 January 2014

Effects of the dinoflagellate Alexandrium minutum and its toxin (saxitoxin) on the functional activity and gene expression of Crassostrea gigas hemocytes

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.


  1. I always wondered what effects these toxins must have on the algae bloom feeding shellfish; this study sounds very interesting. As you mentioned, the immune response is dramatically restricted by the toxin if introduce directly, but it still appears that the bivalve is able to cope. Since the dinoflagellates are ingested by the oysters, it could be possible that the animals may have associated gut bacteria that can degrade the toxin/dinoflagellates of some kind and or may be aid the host by the eliminating other pathogenic bacteria/viruses that could harm the oysters even more. Do you know where the authors have introduced the stx to the oysters in this study? May be the response of the host is more extreme depending on the tissues where they are introduced to?

  2. The STX was added to pools of haemolymph maintained in vitro i.e. in a test tube or similar vessel. I did think that perhaps in the time it takes to produce significant levels of toxin (similar to that which has the negative effect) the oysters may have already rid themselves of the dino's or maybe are constantly removing the alga meaning an effective STX concentration is never reached.

  3. By suppressing bivalve immune systems, could harmful dinoflagellates be setting the scene for pathogens such as Vibrio vulnificus to persist in these molluscs, increasing the chance of other types of shellfish poisoning?

  4. Seems a reasonable assumption, depending on the parameters that are suppressed. On this case where phagocytosis is reduced this applies to any invading pathogen so the build up of other species could well follow though interspecific competition between pathogens would have to be considered, perhaps the saxitoxin would have some effect on, for example Vibrio vulnificans. Either way you would hope that the presence of STX would be detected and the stock removed from the supply chain before consumption so hopefully no pathogens would reach the consumer. You'd hope.