Karenia mikimotoi

estimated to be 0.1 to 0.5 that of a Karenia brevis cell (Bridgers et al., 2004). It was
suggested by these workers that blooms of raphidophytes would have to be of
correspondingly higher densities to have the same effect as that of a K. brevis bloom.
Also, as discussed by Smayda (2006), there is a higher probability that blooms will
occur in chemically modified habitats and at fish-farming sites.
It is interesting to note that two of the fatty acids from Fibrocapsa reported by Fu et al.
(2004) as having strong haemolytic activity are widely promoted as human dietary
supplements. Eicosapentaenoic acid (EPA) is a component of ‘omega-3’-rich fish oils,
and non-toxic microalgae such as Porphyridium cruentum have been investigated as a
possible source of arachidonic acid for use in infant feeding formulations. This
highlights the fact that the toxic effect of such compounds probably operates at a
microsopic level where there may be microzones of very high concentrations around
individual cells (see sect. 3.4.3).
3.9 Heterosigma akashiwo
This alga is cosmopolitan and is widely distributed in the Atlantic and Pacific oceans
and is found in European waters from Norway in the north to Spain and Portugal in the
south (Smayda, 2006). Its toxic properties have been reviewed by Landsberg, (2002)
and Smayda (2006). To quote the latter: “Heterosigma akashiwo is probably the most
versatile harmful algal species known: it is antagonistic to organisms ranging in size
from bacteria to invertebrates to fish. Its multiple modes of antagonism range from
nutritional inadequacy, to suppression of feeding and growth, and to toxicity causing
mortality.” These antagonistic properties towards various organisms have been
tabulated in detail by Landsberg (2002).
The exact nature of the toxins and mechanisms of toxicity in H. akashiwo and other
raphidophytes remains unclear. Some mechanisms of ichthyotoxicity, which may also
be involved with allelopathy, that have been postulated are production of reactive
oxygen species and the production of toxins that have yet to be fully characterized
(Twiner et al., 2004; Smayda, 2006; Kempton et al., 2008).
Khan et al. (1997) reported that they had isolated from H. akashiwo four toxic
components which were, on the basis of the analytical methods used, tentatively
identified as brevetoxins, neurotoxins that have been well characterized and are also
produced by the dinoflagellate Karenia brevis (see sect. 5.).
Uncharacterised extracellular compounds excreted by H. akashiwo were found to have
an irreversible effect on respiratory activity in human and mammalian cells in vitro
(Twiner et al., 2004). These compounds were also found to affect cytosolic calcium
concentrations in cultured sf19 insect cells leading to apoptosis (cell death). The effect
was related to the concentration of extracellular Ca 2+ . It was concluded that the
extracellular compounds were inhibiting the plasma membrane Ca 2+ -ATPase
transporter (Twiner et al., 2005). It was suggested that the compounds may play a role
in the ichthyotoxicity and allelopathy of the alga.
In laboratory studies Keppler et al. (2005) demonstrated sublethal effects (significantly
increased oyster hepatopancreas lysosomal destabilization rates) of cells of H.
akashiwo on the southeastern oyster, Crassostrea virginica.
The most notable impact of H. akashiwo has been on aquaculture where it has caused
massive kills of farmed fish at various locations round the world and has also affected
shellfish (Landsberg, 2002; Smayda, 2006; Kempton et al., 2008).
24 A Literature review of the potential health effects of marine microalgae and macroalgae