Northwest Fisheries Science Center


Staff Directory


Pseudo-nitzschia detail.
Pseudo-nitzschia detail.
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 technologies.

Chains of Pseudo-nitzschia Overlapping occurs at the ends (dark bands) of the Pseudo-nitzschia.
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.

Identifying Pseudo-nitzschia

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 one millimeter.

SEM image of Pseudo-nitzschia delicatissima    SEM image of Pseudo-nitzshia multiseries    SEM image of Pseudo-nitzschia pungens
(Micrographs courtesy of Carla Stehr - Northwest Fisheries Science Center)