Toxic Cyanobacteria

Here’s an interesting highlight from the Cyanobacterial Workshop. I mentioned yesterday that most of us cyanobacterial researchers were working toward understanding these organisms to engineer them to do useful biochemistry for us and to learn more about the fundamental biochemistry of photosynthetic eukaryotes. One talk, however, was concerned more with eradicating cyanobacteria from the environment. Why, you ask? Because the biochemical prowess of cyanobacterial strains can also make them quite dangerous to other species. I’ve posted before on the contribution of cyanobacteria to hypoxic dead zones, where their overgrowth and ultimate collapse creates an out of control spiral towards lack of oxygen in an aquatic ecosystem. They can also just create an absolute mess, check out this link*.

Today, I’m talking about something even more sinister and intentional on behalf of some species of cyanobacteria. Some species are capable of producing toxins that can harm and kill other aquatic animals and humans. They come in many different flavors- neurotoxins, hepatotoxins, and cytotoxins. So, when warm weather and nutrient loading of waterways create favorable conditions for overgrowth of cyanobacteria, the ‘blooms’ of these organisms can be quite dangerous. They are generically called ‘harmful algal blooms’ (HABs)**. Some of these blooms are also known colloquially as ‘red tides,’ which some of my coastal readers may recognize. They are also the reason behind the ‘R’ rule*** for consuming oysters (and now broadly applied to other shellfish too).

The bottom line is that overgrowth of some cyanobacterial strains is dangerous, and it would be nice to have a way of specifically controlling their proliferation in our freshwater ecosystems. As it turns out, cyanobacterial researchers have noticed a weak point specific to cyanobacterial physiology. Because of the photosynthetic light problem, cyanobacteria have systems in place to deal with excess light energy before it creates irreversible oxidative damage to the cells. Plants and other eukaryotes must deal with this problem as well, but they have an added layer of protection that makes them more resistant to treatment with hydrogen peroxide. So, the related result is that cyanobacteria are more sensitive to hydrogen peroxide treatment than everything else in the freshwater aquatic ecosystem- more so than algae, plants, zooplankton and macrofauna (fish). This gave some researchers in the Netherlands an idea…

Could you treat a freshwater ecosystem with just enough hydrogen peroxide to stave off an HAB without harming everything else?

If it could work, this would represent a relatively cheap and low environmental risk prevention strategy for HABs. Hydrogen peroxide is considerably cheaper than treatment with other available cyanobacterial algicides. Algicides are also problematic, in that, while they kill cyanobacteria, the cyanobacteria tend to release all of their toxins upon treatment. Since some of these toxins can be quite stable and take some time to degrade naturally, water treated by this method is still not safe on a reasonable timescale. Hydrogen peroxide decays to water and oxygen by biological and chemical redox reactions within hours to a few days. So it does not persist in the environment for an appreciable amount of time and decays into biologically neutral byproducts.

So, researcher Hans Matthijs performed a few small scale experiments in the lab to optimize the conditions for hydrogen peroxide treatment to kill toxic cyanobacterial species. He was able to kill cyanobacteria without a concomitant release of cyanotoxins. These were increased to small enclosures at a local lake as a proof of concept on a smaller scale. These experiments demonstrated that the hydrogen peroxide treatment scheme worked on an isolated ecosystem- killing the cyanobacteria without major effects on the other organisms in the water. One treatment was enough to keep the cyanobacteria from recovering for seven weeks. These were the results reported in a 2012 article in Water Research.

At the meeting Matthijs reported a real test case for Lake Koetshuis which suffers from infestation with the toxic cyanobacterium Planktothrix agardhii. These organisms had reached such high levels that the lake had to be routinely closed for recreational purposes due to the hazardous conditions they caused. At this point, there was significant inconvenience to the human population in the area, because they would like to use the lake for water skiing and those businesses were losing money. There was not much to lose at this point. The scientists consulted with engineers that deigned a system for delivering and mixing the amount of hydrogen peroxide necessary to effectively but selectively kill the Planktothrix. It was basically a boat with an elaborate piping system and pumps. He reported a successful treatment of the lake, which was monitored over the course of the summer. The key was treating at just the right time before the cyanobacteria grew out of control. They were able to reduce the amount of toxic cyanobacteria and its toxin without any deleterious effects on other aquatic life in the lake. No closures were required that summer and everyone was able to water ski as much as they liked.

This is definitely a promising report, but there are still some hurdles to putting this into general practice. These include, knowing the effectiveness of hydrogen peroxide on the particular nuisance cyanobacteria for a given freshwater system and logistically delivering the necessary amount of hydrogen peroxide on the appropriate scale. Also, it is unknown how long term the cyanobacteria may adapt to these treatments such that they are no longer effective. However, as of now, it seems like the best option.


*OK, well technically, that one is algae (photosynthetic eukaryotic organism) and not cyanobacteria, but it is still pretty impressive. I just had to share it.

** You don’t have to tell me about the misnomer; I wrote the resources page on non-plant photosynthetic organisms. Cyanobacteria can be the real culprits, but it’s less of a mouthful to say harmful algal bloom than harmful cyanobacterial bloom. Actually, both of those are kind of a mouthful, so let’s just keep it  HABs.

*** I had to include another Louisiana reference today, although I’m sure many readers from coastal areas will recognize the ‘R rule.’  For those of you still wondering: Thou shalt only eat oysters in months containing an R. So abstain from eating oysters May through August when conditions are most favorable for red tides, which will generate toxins to contaminate your raw seafood. I don’t think this has anything to do with another rule strictly adhered to by Southerners concerning wearing white after Labor Day. These things just are not done.



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