The impact of toxins produced by dinoflagellates on bottlenose dolphins

In parts of the Gulf of Mexico red tides occur regularly and it is proved that they have negative impacts on shellfish, coastal wildlife and also human health. These red tides are caused by the dinoflagellate Karenia brevis that produces brevetoxins. Brevetoxins are neurotoxin compounds and cause neurotoxic shellfish poisoning (NSP) that results in mortality events of birds, fish, shellfish and turtles. Also Tursiops truncates, bottlenose dolphins, are influenced by these dinoflagellate blooms. Previous studies showed that brevetoxins occur in dolphin tissues and in prey fish during red tides and after the bloom. The background levels of brevetoxins are unknown, so are chronic exposure or repeated exposure on the same population of bottlenose dolphins. Fire et al. (2007) investigated the toxin exposure on bottlenose dolphins in Sarasota Bay in Florida, a location that is regularly effected by red tides. Furthermore they quantified the brevetoxin levels when associated with the blooms and established baseline exposure levels. In addition Fire et al. compared the brevetoxin levels in exposed and non-exposed bottlenose dolphins.

The samples were taken from a dolphin tissue bank and database. There were three groups of samples. Firstly, 19 exposed dolphins that were recovered during red tides in 2001/2002 and 2003. 70 samples from liver, lung, kidney, muscle, urine, feces, stomach content and blubber where examined. Secondly, 16 non-exposed dolphins which mean that they stranded at least three months after a red tide. 56 samples were investigated. The third group was the negative control recovered in a location where K. brevis is very rare in 2004. Samples of lung, liver, kidney and blubber where examined (11 samples of each). The brevetoxins where extracted and analysed from the samples by using enzyme-linked immunosorbent assay (ELISA) to quantify the total brevetoxin meolecules (including metabolites) and liquid chromatography-mass spectrometry (LCMS) to precise the detection of toxin molecules.

In the exposed group 43 of the 70 samples contained brevetoxins with concentrations from 7 to 2,896 ng PbTx-ea g^-1 (PbTx-3 equivalents) and the highest concentrations in urine, feces and stomach contents. The non-exposed group showed brevetoxin concentrations from 6 to 44 ng PbTx-eq g^-1 in 13 of 56 samples. No brevetoxins where detected in the negative control. A significant difference between exposed and non-exposed dolphins was detected.

The results show that in the exposed dolphins moderate to high levels of brevetoxins were present and Fire et al. suggest that it led to death of the exposed dolphins. The presence of brevetoxins in the non-exposed group might be a result of accumulation and/or remaining in the body. It is not known how long the toxin remains in the body of the bottlenose dolphins after the bloom neither is the metabolisation rate of the toxin. Previous studies showed that even months after red tide events brevetoxins were found in finfish, shellfish, etc. The researchers suggest that the non-exposed dolphins ingested contaminated prey fish or ingested the toxin by other pathways that are still to be investigated. Further research on accumulation and transfer of brevetoxins is needed.

A problem with this study is that it is not well known how the toxins are degrading in the frozen dolphin tissue samples or in stranded dolphin carcasses. All in all this study established baseline toxin levels that can be used as a point of reference for studying the accumulation of brevetoxins. It would be interesting to see how the brevetoxins and also other toxins are ingested by the dolphins and if there are pathways like dissolved toxins in the water column.

In my point of view it is important to do long-term studies on the influence of exposure to toxins produced by algal blooms also in live dolphins and to search for other toxins influencing negatively the health of bottlenose dolphins and other marine mammals (see Twiner et al., 2011). Additionally further research on effects of algal toxins on commercially used fish and shellfish has to be conducted to get a better insight on NSP in human.

Reviewed paper:

Fire, S. E., Fauquier, D., Flewelling, L. J., Henry, M., Naar, J., Pierce, R., & Wells, R. S. (2007). Brevetoxin exposure in bottlenose dolphins (Tursiops truncatus) associated with Karenia brevis blooms in Sarasota Bay, Florida. Marine Biology, 152(4), 827-834

Further reading:

Twiner, M. J., Fire, S., Schwacke, L., Davidson, L., Wang, Z., Morton, S., ... & Wells, R. S. (2011). Concurrent exposure of bottlenose dolphins (Tursiops truncatus) to multiple algal toxins in Sarasota Bay, Florida, USA. PLoS One, 6(3), e17394.