Feature

Tiny Toxic Killers

Last summer's toxic algal bloom wasn't the first, and probably won't be the last.

By Vivienne Cassie Cooper

Everyone knows about Moses striking the waters of the River Nile with his rod, and the waters turning red and becoming undrinkable. Moses would have been amazed if he could have examined the water microscopically, and seen the 50,000 cells per litre or more that are common in an algal bloom.

There is nothing new in the occurrence of red tides where a huge build-up of phytoplankton occurs -- there are indications that such blooms may have occurred as far back as the Palaeozoic Era, which began around 590 million years ago. More recently, in the last two decades, there has been a dramatic increase in the frequency and intensity of harmful algal blooms, due chiefly to:

• increased scientific awareness of toxic species
• increased eutrophication and aquaculture in coastal waters, and hence stimulation of plankton blooms
• unusual climatic conditions
• transportation of spores and cysts in ship's ballast waters or in shellfish stocks moved from one area to another. It has been estimated that in one ballast tank alone, 300 million cysts could germinate into toxic planktonic stages.

While last summer's scare gained national attention, with the first recorded instances of poisoning, previous blooms have also seen major damage. In 1989, the dinoflagellate Heterosigma akashiwo caused the death of about $12 million worth of cage-reared salmon in Big Glory Bay, Stewart Island.

There is a great diversity of species involved in algal blooms, but less than one per cent of all microscopic marine algae produce toxins. These tiny single-celled organisms include dinoflagellates, which spin through the water using whip-like flagella; diatoms, which have silica-containing cellular walls making them look like intricate boxes; and blue-green algae or cyanobacteria, which often cluster together in long filamentous chains.

Range of Poisons

Toxins differ from species to species and strain to strain, and they are not always produced in each species or strain. It is extremely difficult to identify toxic organisms accurately, but some microscopic phytoplankton are associated with specific toxins and with specific types of shellfish poisoning which these cause. Mussels and clams apparently accumulate toxins more quickly than scallops and oysters, and people eating contaminated shellfish suffer a variety of symptoms.

Overseas research has shown that the dinoflagellate Gymnodinium breve produces neurotoxins which can cause respiratory problems in people and kill fish and dolphins. Neurotoxic shellfish poisoning was the main cause of the 200 or so reported poisonings from last summer's toxic bloom.

This was the first time that a microalga like G. breve had been found in New Zealand waters. The phytoplankton is best known in Atlantic waters around the southern United States, and there has been speculation that it arrived in New Zealand via ballast waters from overseas ships. Cell counts of up to 100,000 per litre were made; elsewhere, fisheries are closed when cell counts of this particular dinoflagellate reach 5,000 cells per litre.

Paralytic poisoning is caused by gonyautoxins and saxitoxins, released by various species of Alexandrium dinoflagellates. Diatoms of the Nitzschia group can lead to amnesic shellfish poisoning, so named for the loss of short-term memory which occurs. It also involves loss of balance, mental confusion, nausea, vomiting and diarrhoea.

These toxins and others have been implicated in the various harmful algal blooms that have occurred in New Zealand waters. To the surprise of researchers, some of the toxins involved differ from their counterparts overseas, and have yet to be characterised adequately.

Origin of Toxins

Some dinoflagellates form cysts which provide either short-term breaks between activity or longer term resting stages in the form of deepwater benthic cysts. There is evidence that such benthic cysts can be up to ten times more toxic than temporary planktonic cysts, but at present few workers can identify benthic cysts and relate them to their corresponding planktonic stages. An enormous amount of exacting work, particularly with cultures, is required.

The role of bacteria which live symbiotically within phytoplankton remains to be elucidated. There are suggestions that toxins may come from such bacteria, while other bacteria work to inhibit dinoflagellates. Overseas, saxitoxin has been identified from bacteria growing inside five toxic strains of Alexandrium tamarense, but further careful research is necessary to prove this fact.

Bacteria may play a role in flagellate blooms in that they can produce more vitamin B12 than they can use. Blooms of flagellates like Heterosigma and Fibrocapsa may be enhanced by this bacterial vitamin over-production. Such may well be the case where bacteria accumulate on the sea floor beneath fish and shellfish farms.

Blooming Well

What are the factors which lead to a bloom of a toxic species?

With regard to trace element requirements, dinoflagellates have a high requirement for iron, which can definitely influence the growth of Alexandrium species. One such is A. ostenfeldii, a possible toxin producer, which occurs off the North Island's West Coast where iron sand is loaded for Japan. Similarly, dinoflagellates flourish in run-off water from forested and deforested areas where humus abounds, such as off the West Coast of the South Island.

Most marine planktonic microalgae require a source of vitamin B12 and other members of the vitamin B group such as biotin and thiamine. Some researchers believe that Gymnodinium may liberate a substance which temporarily binds vitamin B12. This makes it unavailable for other species and frees the way for a population boom of this harmful dinoflagellate.

The metabolites that algae release can contribute to suitable conditions for a bloom. Algae excrete from 2% to 30% of their carbon acquired through photosynthesis. In addition, amino acids, sugars, polypeptides, enzymes, inhibitors and poisons are all excreted. When deep upwelling waters, rich in nitrogen, phosphates and trace metals, are mixed with surface waters, they provide suitable conditions for rich plankton growth.

Relatively little is known about the life-cycle of New Zealand toxic species, and research can take a long time. It took Danish researchers twelve years to work out the life-cycle of the silicoflagellate Dictyocha speculum, some stages of which are potentially toxic.

Obviously, there is a need for in-depth study of the toxic organisms concerned, both in laboratory culture and in "natural" or "unnatural" environments. A recent scientific meeting to discuss the issue agreed that more monitoring of phytoplankton in "at risk" areas is necessary. The difficulty remains in finding the people who are trained and able to carry out this task.

Much of the excellent work done in the past has been undertaken by researchers at the Cawthron and Oceanographic Institutes, MAF and health authorities, but they are not able to adequately monitor the whole of New Zealand's coastal waters.

Specialist courses are needed to train most workers in identification and culture techniques, and more jobs are needed in the Crown Research Institutes, regional councils and universities for those trained in these exacting disciplines.

There remain a great many questions to be answered and, given the increasing number of algal blooms in New Zealand waters, these questions need answering quickly.

Dr Vivienne Cassie Cooper is a Research Associate in marine and freshwater microalgae with Landcare Research.