The enigma of the blue algae

EUROPA : Research Information Centre

2006-05-22   Source: RTD Info

Formerly known as ‘blue algae’ and widely dispersed in freshwater – both drinking water and bathing water – cyanobacteria are unicellular organisms that secrete dangerous toxins. Researchers on the European project PEPCY (PEPtides in CYanobacteria) are trying to gain a better understanding of the harmful effects of these surprising creatures by concentrating on the peptides they synthesize. Their results are bringing about progress in a field where many questions still remain unanswered.

It is difficult to like cyanobacteria when you come across the repulsive greenish moss that their proliferation generates. Yet the truth is we have much to thank these primitive creatures for. These very ancient life forms – they first appeared over 3 billion years ago – played a major part in making the Earth's atmosphere breathable by releasing into it vast quantities of oxygen. It was, in fact, cyanobacteria that ‘invented' photosynthesis, well before algae.

None of this alters the fact that today there is growing evidence that these cyanobacteria pose a very real, and not very well understood, health risk. It is now accepted that microcystins in particular – which are among the best known of these ‘blue algae', to give them their earlier and more attractive name – are able to secrete very active toxins that affect the nervous system and the liver in particular. A number of laboratory and field tests also show that cyanobacteria are responsible for the poisoning of all kinds of vertebrae, from dogs to fish and birds. Finally, it is known that these creatures produce a great diversity of peptides (small proteins) whose function and properties remain essentially unknown.

Focusing on peptides

"It is this situation that led to the setting up of the PEPCY   European research project," explains Ingrid Chorus of the German Federal Environment Agency (UBA), the project coordinator. “The toxins produced by cyanobacteria are among the five toxin varieties – along with arsenic, fluorine, lead and certain pesticides – found most commonly in European waters and in concentrations that pose real problems to human health.”

As is the case with many organisms generally described as ‘primitive', such as bacteria and mushrooms, cyanobacteria are in fact highly complex chemical factories. The PEPCY researchers chose to focus their attention on the peptides these cyanobacteria synthesize. This is because they are present in large numbers and also include most known toxins. “We were expecting to find a great diversity,” continues Chorus, “but we were nevertheless impressed. We found there were several hundred and no doubt a thousand different molecules. An almost monospecific proliferation of Planktothrix agardii, for example, was shown to contain over 255 distinct substances, principally peptides. All these compounds consist of eight or nine families and we tried to study in detail at least one member of each family.”

The toxicity stakes

The results of the study were as significant as they were surprising. These unknown peptides seem to have a very low impact! They are nothing like as toxic as microcystins or nodularins, the two most notoriously toxic families. However, there is reason to reserve final judgement. The PEPCY researchers did not carry out any experiments on animals, such as rats or mice. “Tests of that type raise ethical issues and over the past decade we have tried to avoid them whenever possible. We prefer to use human cell lines, or even tissue, invertebrates or the eggs of fish or amphibians to measure toxicity. This means we cannot be 100% certain that these peptides are harmless as it is possible that the cell lines used were not the right ones. It is conceivable that microcystins, which are known to be toxic for the liver, could penetrate to a cell interior by transport mechanisms designed for other compounds. A kind of molecular mistaken identity if you like. But when applied in a laboratory situation to a muscle cell, for example, such a toxin will not reveal its real effect.” 

In addition to this result, the PERCY researchers also developed many molecular tools with which to identify, in a short space of time, the genetic types of cyanobacteria found in a given area of water. We can therefore hope to begin to understand what elements of the environment – temperature, nutrients, oxygenation, for example – favour a particular category of blue algae, and, thus, the production of a particular peptide. Behind the process there lies a fundamental mystery: what can be the purpose of the hundreds of protein molecules the cyanobacteria so diligently secrete? It is probably not to poison vertebrates, as that would bring no apparent benefit.

“Perhaps these peptides serve to send messages within the colony, or to communicate with other organisms that live in a symbiotic relationship with the cyanobacteria,” postulates Chorus. “But these hypotheses have not yet been tested. I regret that we do not know what function these molecules serve as that would certainly help us to understand when they are produced – and when they are absent.”    

Navigating legislative waters

The ‘public health' strand of the project is not closed. Although the peptides studied proved to have little effect, this is not the case for the organisms as a whole. “A study of crude extract of cyanobacteria, as opposed to peptides alone, shows interference with the development of the eggs of amphibians and fish as well as other toxic effects that remain unexplained. So were we right to concentrate on the peptides?” wonders the researcher. “After all, these cells also secrete other molecules (alkaloids, polysaccharides, etc.). No doubt we will need further projects to explore all that.” 

It is probable that, despite PEPCY's generally reassuring results, we have not heard the last of cyanobacteria – especially as they could well be mentioned in the European Drinking Water Directive that is due for revision. They are already mentioned in the Bathing Water Directive. 

To make the job easier for the European legislator, the PEPCY team compiled a compendium of national legislation on the cyanobacteria problem that is now available on the internet. They have also issued recommendations stressing the need to carry out a case-by-case risk analysis. This is because the level of risk posed by using a particular water resource – drinking water or an area of water used for fishing, bathing, windsurfing, water skiing, etc. – is different in each case. The conclusion is that there remain many mysteries to solve before we can set our minds at rest on the subject of these strange unicellular organisms that seem determined to make life difficult for researchers.

Organisms boosted by human activity? 
Why are cyanobacteria so omnipresent, at least in freshwater? The answer lies in the well-known process known as eutrophication or the build-up of excessive quantities of nutrients in the river system. The root cause of this is human activity. Agriculture is one source, due to the fertilisers that are not absorbed by crops and end up entering freshwater resources. Another source is domestic waste, especially the presence of phosphorous.  

These feed the cyanobacteria – as indeed they feed all plant plankton – and the result can sometimes be very spectacular, with rapid proliferations, known as blooms. The effect increases dramatically at this point, with the water taking on a strange colour, usually various shades of green but, in the case of some species, a distinct red.

“Matters are made worse by the growing number of dams,” explains Chorus. “Cyanobacteria do not develop in fast flowing water and the risk increases as the current slows.” Over recent decades, a growing number of devices have been constructed that, combined, have a slowing effect on the flow of rivers – from the barriers erected by farmers to hold back the water in streams to the large areas of water for leisure activities developed on the outskirts of towns. Another fear is that global warming will further boost the proliferation of cyanobacteria, although there is, as yet, no recorded evidence in the field to confirm this. 

Original URL : http://ec.europa.eu/research/rtdinfo/49/article_4095_en.html