page 28 Lab Times 5-2012 Journal Club

Modern lichenology in Helsinki, Finland

In cyanolichens, fungi and cyanobacteria live happily together – with either of the partners providing their share for a successful partnership. Ulla Kaasalainen and Jouko Rikkinen discovered that parts of the bacteria’s contribution are of a toxic nature, hepatotoxins called microcystins.

Toxic RelationshipsToxic Relationships

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Lichens are rather unusual creatures. They are a combination of two very different organisms living together

in great intimacy – it’s like an alliance be-tween the two species. Most of the lichen component consists of fungal filaments but it’s the algal cells that make the symbiotic partnership complete. Those algae are usu-ally green algae but sometimes cyanobacte-ria can take their place, thus attributing a special name to the lichens – cyanolichens.

The most common cyanobacteria in li-chen association belong to the genus Nos-toc. They are not only able to photosynthe-sise for sugars but can also fix atmospheric nitrogen and convert it into ammonia, ni-trites and nitrates, which their fungal part-ners can readily absorb as their nutrition.

Conquering the worldSince lichens have no roots and do not

need a continuous supply of water, they can easily settle down in places lacking soil, like naked rock or artificial structures, such as walls and monuments. This characteris-tic has also enabled them to conquer the most extreme environments that are, nor-mally, inhospitable for most plants. Geo-graphically, lichens are widely distributed over the world. Do they really have any spe-cific requirement for their growth? The answer is no! Apparently, they grow equally well in such varible en-vironments, such as hot Africa or cold Scandinavia, which makes them more or less omnipresent!

This omnipresence made it easy for Jouko Rikkinen, Ulla Kaasalain-en from the University of Helsinki and their co-workers to find and col-lect about 803 specimens from differ-ent parts of the world, ranging from the coast of Hawaii to Japan’s shores. They collected the most specimens in their home country, Finland, followed by the Taita hills in Kenya. Although

the quantity and diversity of lichens varied from location to location, all of the lichens surprisingly contained toxins. Surprising because, if you think about symbiosis, only good things come to mind. However, there are some scientists who attribute the term ‘symbiosis’ to any kind of biological inter-action, which could either be mutualism, commensalism or parasitism.

In the lichens Rikkinen and Kaasalainen studied, the toxins are most likely harmless to the partners but harmful to, for example, animals with an appetite for fungal-bacteri-al delights. By the way, specimens with the highest percentage of toxins in their thalli were recorded for Scotland, Norway and the Oregon province in the United States of America.

A common productWhat is now the chemical nature of

those toxins? They are non-ribosomal pep-tides called microcystins, produced by the cyanobacterial partner and known to be toxic to both plants and animals. Their tox-ic effect unfolds after; in, for example, hu-mans, they are ingested and transported to the liver, where they are stored – but they can also remain in the blood stream to un-leash more evil on other tissues of the body.

On a molecular biological level, micro-cystins are potent protein phosphatase in-hibitors, which can inhibit both protein phosphatases 1 and 2A by covalently bind-ing to them. Protein phosphatases are in-volved in dephosphorylation processes. The binding of microcystins to protein phos-phatases renders the enzymes inactive and thus interferes with cellular control pro-cesses by excess phosphorylation. In the liver, this inhibition leads to the disruption of the cytoskeleton in hepatocytes. Subse-quent hepatocyte necrosis causes pooling of blood and acute cases of poisoning, in-trahepatic haemorrhagic and hypovolemic shock that can lead to death.

Toxically equippedUntil now, it was thought that only li-

chens associated with aquatic blooms pro-duce the toxic inhibitors but Kaasalainen and co. showed that also “terrestrial lichens from all over the world commonly produce microcystins”. By analysing their widely travelled subjects of study, they detected “either the biosynthetic genes for making microcystins or the toxin itself in 12% of all analysed lichen specimens”.

In a previous study from 2009 (Mol Plant Microbe Interact, 22 (6): 695-702),

Kaasalainen and Rikkinen dem-onstrated that the symbiotic Nostoc strain of the terrestri-al cyanolichen named Peltigera leucophlebia has all the genetic requirements – the microcystin synthetase gene mcyE – to pro-duce those microcystins and it indeed does so. “It was the first direct evidence of in situ produc-tion of microcystins in lichens or plant-cyanobacterial symbios-es and it could explain why cy-anolichens and symbiotic bryo-phytes are not among the pre-ferred food sources of most ani-Non-toxic working relationship: Ulla Kaasalainen & Jouko Rikkinen

Lab Times5-2012 page 29Journal Club

mal grazers”, concluded Kaasalainen et al. back then. A toxin for self-defence?

Lichen collectorsHowever, they were not sure whether

this toxin production was a frequent phe-nomenon in terrestrial cyanobacteria or whether they had just encountered an ex-ceptional case. Thus, the Finnish lichenol-ogists carried out a further study, analys-ing hundreds of cyanobacterial species for their ability to produce microcystins and nodularins. Nodularin is also a cyclic pep-tide, which has caused animal poisoning around the world.

In this present study (PNAS, 109 (15):5886-91), Kaasalainen, Rikkinen and their colleagues collected hundreds of lichen thalli, rep-resenting about 23 different cyanolichen genera from dif-ferent geographical locations. Using standard techniques like LC-MS and phylogenetic analysis, the team could clear-ly demonstrate and confirm that yes, cyanobacterial sym-bionts of terrestrial lichens can indeed produce microcystins.

The study was performed in two differ-ent labs, in the Department of Biosciences and the Department of Food and Environ-mental Sciences, both, however, located at the University of Helsinki in close proximi-ty. For Ulla Kaasalainen, who did the lion’s share of the work, all this to-ing and fro-ing wasn’t very easy. “I had to run here and

there to do the experiments but I’m glad that, at least, there was no problem with respect to funding”, she says.

A delicacy with downsidesEven though lichenology seems to be

rather a niche topic, learning more about the symbiotic partnership between fungi and algae or cyanobacteria could be a huge advantage to mankind. It has been known that lichens are a good source of carbohy-drate and protein (not only for an-imals) and for exactly that

reason, they are a delicacy in several cul-tures. There are, however, a couple of dis-advantages when using lichens as food – their polysaccharides are generally indi-gestible to humans and, apart from micro-cystin, they also contain further, second-

ary metabolites, like vulpinic acid – courte-sy of the fungal partner. Those metabolites may not be as toxic as microcystins but they aren’t exactly safe either. The good thing is that poisonous lichens are easy to spot. Most of them are yellow in colour.

Additionally, some lichen species were used as medicines during ancient and me-dieval times. Although regulated scientific studies are missing, certain metabolites are believed to act against gram-positive bac-teria. Lichen parts are also used in some ointments for topical application or in folk medicine as herbal tea infusions. Further-

more, their brilliant colours are exploited in extraction of dyes such as orchil and crottal.

Attractive appearanceUlla Kaasalainen per-

formed her studies as a part of her doctoral work, so the nat-ural thought is what will hap-pen to this interesting study, if she graduates and moves to another lab. When asked, Ulla smiles and says, “I like this re-

search. Maybe I’ll continue to work in this field, in my country or anoth-er.” But Ulla can also see herself doing work related to lichens and microcystins, which may not be directly connected to her pre-sent research interests.

But one thing is for sure, one look at these colourful lichens and anyone will fall in love with them; their vibrant colours at-tract and call you to work with them!

Vasudharani Devanathan

Just four of the 803 specimens the two lichenologists studied.