Claudio Marangoni
School of Animal, Plant and Environmental Sciences
University of the Witwatersrand

The diatom genus Pseudo-nitzschia was initially placed within the larger genus Nitzschia, but is now recognised as a separate genus. It includes some 20 marine planktonic species that have pointed valves that unite into step-like colonies of overlapping cells. In previous surveys of South African waters, Pseudo-nitzschia species were more than likely recorded as Nitzschia species with some of these having been dominant in the late summer blooms along the west coast of South Africa.

Pseudo-nitzschia has been in the spotlight over the last 15 years because of its role in the human syndrome, Amnesic Shellfish Poisoning (ASP). Mild symptoms of ASP involve gastrointestinal disorders with amnesia, seizures and death occurring in extreme cases. The responsible toxin is an excitatory amino acid, domoic acid, which is produced by 9 species of Pseudo-nitzschia. The toxin is transferred and enhanced via the food chain from various shellfish and fish (e.g. anchovies, pilchards and sardines) to humans, sea birds and marine mammals (mass mortalities of seals, dolphins and whales have been reported along the Californian coast).

There have been no recordings of ASP in South Africa, even in the last couple of years, where Pseudo-nitzschia concentrations have exceeded 2 million cells.L^-1. At Lambert’s Bay, on the west coast of South Africa, it was a dominant component of a persistent inshore phytoplankton bloom in cold waters of about 13 0C from the 18th to the 25th of February 2000. A Pseudo-nitzschia species was isolated from Lambert’s Bay at the end of March 2001, when it was once again a major component of a bloom in the area.

Figs. 1 & 2. Pseudo-nitzschia australis viewed with LM (1) and SEM (2) (scale bar = 10mm).

The taxonomy of Pseudo-nitzschia is expanding rapidly (7 new species having been described in 2002), so the isolate needed to be

carefully characterised and identified. The isolated cells are lanceolate in valve and girdle view and contain 2 chloroplasts, one at each end of the cell, and a central nuclear area (Fig. 1). Cell dimensions are 79-90mm in length and 6.7-7.7mm in width. The cells overlap by 0.3 to 0.2 of the total cell length when forming colonies. The details of acid cleaned frustules were observed with SEM and TEM. The valves lack a large central nodule (Fig. 2). There are 14-17 striae in 10mm, 12-15 fibulae in 10mm and 20-21 band striae in 10mm. The poroids are reasonably large and simple and are arranged in 2 rows, one row along each side of the interstriae (Figs. 3 & 4). There are 4-5 poroids in 1mm. From the observed features and measurements, the Lambert’s Bay isolate was identified as Pseudo-nitzschia australis.

Pseudo-nitzschia australis is one of the Pseudo-nitzschia species that is capable of producing domoic acid and has been linked to the disastrous incidents involving this species along the Californian coast. Preliminary tests for domoic acid production by the South African isolate of P. australis have been negative. This is surprising, as it represents the first record of a P. australis culture that does not produce domoic acid. However, considering the large concentrations of Pseudo-nitzschia that have occurred along the west coast, there is still great concern about the possible threat of ASP occurring in our waters.

Figs. 3 & 4. Detail of frustule viewed with SEM (3) and TEM (4) (scale bar = 1mm).

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