Quick Dips

Detecting Toxic Phytoplankton

New Zealand has an internationally accepted shellfish biotoxin monitoring programme, administered by the New Zealand Marine Biotoxin Management Board. Part of that programme involves the detection of toxic micro-algae in water samples to both predict and confirm toxins for New Zealand's regulatory authorities.

Micro-algae are usually identified by light microscopy -- in some cases by staining the cell walls (for example cellulose thecae or silica frustules) with fluorescent dyes. Cells are then observed under ultraviolet light and identified by the form of the cell walls, or the presence of specific pores, or appendages such as spines.

Most of the toxic micro-algae are easy to identify to genus level, but it can be difficult to discriminate between toxic and non-toxic species, and some cells do not have obvious differences. This is where lectins come in.

Lectins are glycoproteins obtained from a variety of plant and animal sources -- as diverse as peanuts and snails -- and different lectins bind to specific sugars. They can thus be used to discriminate between organisms which look the same, but which have different surface sugars.

New Zealand has several species of Gymnodinium which can be difficult to tell apart. One species produces brevetoxins, which cause neurotoxic shellfish poisoning (NSP), as occurred when people in Orewa suffered respiratory distress during the much publicised toxic algal bloom of 1993.

Another species produces a compound called gymnodimine, which is apparently non-toxic to humans, but which can kill fish; and a third species is non-toxic. These three Gymnodinium species, G. cf breve, G. mikimotoi and G. pulchellum, can be told apart by their different surface sugars and therefore different binding of lectins. The lectins can be bought "off the shelf" with fluorescent "tags", and the binding can be observed as a fluorescent halo around the cells under an epifluorescent microscope.

Not all species can be so easily distinguished in this way, and fluorescently "tagged" genetic probes are being developed that will only attach to the "right" cells.

Already such genetic probes have been tested in New Zealand against micro-algae that cause amnesic shellfish poisoning (ASP), the Pseudonitzschias. These algae are difficult to separate into toxic and non-toxic species without the aid of electron microscopy. Soon the species will be able to be detected in the field with the latest in molecular genetic technology.

Dr Lesley Rhodes, Cawthron Institute, Nelson