Not all members of the genus Pseudo-nitzschia
produce domoic acid. At the present time only a handful of Pseudo-nitzschia
produce the toxin.
For managing the risk of domoic acid, monitoring the water column
for domoic acid content is beyond the scope of current
Overlapping occurs at the ends (dark bands) of the
Therefore, monitoring for the specific domoic acid producing diatoms
provides the only proactive method that permits some early warning
that shellfish might become toxic.
Unfortunately, P. multiseries
which produces the toxin and P. pungens (which does not produce
significant amounts of the toxin) are virtually identical under the
standard light microscope. Therefore, a current means to identify the
toxic species from non-toxic is by the scanning electron microscope
(SEM), a method that magnifies cells about 20,000 times.
An important step in managing toxic blooms is to be able to identify
species of algae.How can you tell which is which? With the Scanning
Electron Microscope (SEM), fine detail in the structures in these organisms
can be seen (we are looking at only one-half of the silicate shell of
the diatom. Both parts fit together like a box with a lid). Under high
magnification, more detail becomes apparent as we look down between
the "ribs" or striae, in the photomicrograph.
The SEM below show three species of Pseudo-nitzschia: (1) pseudodelicatissima,
found in a recent record-setting toxic Pacific Northwest bloom; (2)
multiseries; and (3) pungens. Notice the small pores between
the striae. In order to identify and differentiate between some species
of this diatom, for example between P. pungens and P. multiseries,
examination of these pores is necessary. Note that pseudodelicatissima
has just 1 row of square pores between its striae (rib-like structures);
multiseries has 3 to 4 rows; and pungens has 2 to 3 rows,
with larger pores than multiseries. A magnification bar equivalent
to 2 microns is shown in the first photo. One thousand microns equal
(Micrographs courtesy of Carla Stehr - Northwest Fisheries Science Center)