Haloarchaea and Haloviruses| MIKE DYALL-SMITH'S RESEARCH
Cyanobacterial bloom in Lake Albert, Wagga, 2012-2013Mike Dyall-Smith
Lake Albert in Wagga is an artificial freshwater lake that is used for recreation purposes. It is around 125 ha, has an average depth of 3.6 m, and is situated within the suburb of the same name, Lake Albert. The walking/cycling path around the edge of the lake is well used, and the water is used for swimming, fishing, water-skiing and sailing. Over the summer of 2012-2013 there was a bloom of cyanobacteria, forming a green surface layer, and warnings were placed in the newspapers to avoid the water untll the bloom disappeared. Such blooms are most likely to occur in warmer months and when nutrient levels increase (particularly phosphate). Nutrient sources are not hard to find, as around the lake is a well-tended park with picnic areas, a large golf-course and a restaurant, and immediately beyond the park are suburban houses. There are also numerous water birds. After heavy rains, the runoff from the surrounding land would all drain into the lake.
While walking around the lake in mid-January of this year, I noticed the bloom had concentrated at the draining end of the lake (northern end). It formed an eye-catching bright green surface layer (picture right >). This floating layer was also being blown up against the retaining weir by the prevailing winds. Closer inspection showed that the layer was not a homogeneous green, but particulate, with green flocs (or 'clots') of varying size (picture below). I took a sample and examined it by bright field microscopy.
As shown in the pictures below, the flocs are composed of almost pure cultures of one particular type of cell, each being about 4-5 µm diameter (there is10 micron scale bar at the bottom left in the x100 picture).
The featuresof these cells, as seen in the micrographs below, are typical of a particular species of unicellular photosynthetic Gram-negative bacterium, called Microcystis aeruginosa. This is not an alga but a true bacterium (Bacteria >Cyanobacteria > Chroococcales > M. aeruginosa). The local council also reports that this is the major cyanobacterium present in the lake at this time, and stated the health risk as 'amber level (alert mode)' on samples taken on 14 Jan, 2013. But it is not a problem that is restricted to Wagga, or to Australia; Microcystis aeruginosa is "one of the most common bloom-forming cyanobacteria in freshwater ecosystems" around the world (Straub et al. 2011).
In the left-most picture shown below (low magnification, x10 obj. lens) you see the cells are held together in clumps (flocs), but you cannot see the gel-like substance that is holding them together. It is a clear polymer that the cells secrete, composed mainly of galacturonic acid, rhamnose and xylose (Nakagawa et al. 1987). One reason for the cells to live in this jelly is to avoid being eaten by grazing protists, crustaceans and other hungry eukaryotes. Predators like to eat individual cells, and it is much harder work trying to dig into the gel and suck out Microcystis. The transparecy of the gel also allows sunlight to penetrate the flocs so the cells can use it for photosynthesis. If you look closely at individual cells (high power and oil immersion pictures, below), you can see bright/clear areas within the cells (collections of gas vesicles) as welll as coloured areas (green/red; photosynthetic membranes). The gas vesicles are protein covered bubbles that are used to alter cell buoyancy, enabling cells to float to the surface of the water, where the sunlight is strongest, or to sink down from very strong light (with its damaging UV component).
One of the amazing attributes of this species is that by altering its buoyancy, it can change its vertical position in the water column over the course of the day, so called 'vertical migration'. The flocs rise to the surface in the morning (as seen in the 10 am picture above right), and then sink slowly during the day. The photosynthetic pigments can then harvest light energy, and use this to fix CO2. Note the green and red colours seen inside individual cells (best seen in the middle and right pictures, below). These are pigments of different colours that are related to photosynthesis: chlorophyll-a (green), carotenoid (orange-red), phycocyanin (light blue), and allophycocyanin (red).
The trouble with this organism is that it synthesises intracellular toxins which can kill animals, including humans. The toxins are called microcystins, and there are numerous types. The most dangerous is microcystin-LR, a cyclic (non-ribosomal) peptide that is synthesised not by the translation of mRNA, but by specialised enzymes that couple amino acids together in sequential order. Because this avoids ribosomes and tRNAs, it means that atypical residues can be joined together (e.g. D- rather than L-amino acids). Microcystins have a common structure: cyclo(-d-Ala1-l-X2-d-isoMeAsp3-l-Z4-Adda5-d-isoGlu6-Mdha7), where the superscripts refer to the conventional numbering of the residues. The structure of microcystin LR is shown below as a rotating molecule (pdb structure 1LCM).
After ingestion of contaminated water, the toxin can enter the bloodstream and reach the liver, where it causes apoptosis (death) of liver cells. Specifically, it binds to and inhibits type 1 and type 2A protein phosphatases, perturbs the cell cytoskeleton, and also induces ceramide generation (Li et al., 2012). These effects lead to damage and death of liver cells. At the animal level, the toxin is implicated in intrahepatic bleeding, liver necrosis and liver cancer, and has been most commonly known to kill livestock, but there are documented cases of significant numbers of human deaths (Carmichael et al., 2001).
Of course the organism has not evolved to kill livestock and humans, so what is the natural purpose of these toxins? Most likely, they are to prevent the cells being eaten by protozoa (e.g. amoeba) and other bacteria-feeders ('grazers'), such as Daphnia, present in the water. The toxins, in combination with the thick capsular polysaccharide, are protective mechanisms to enable them to avoid predation, and so to flourish (Trabeau et al., 2004). Since the toxins are directed at animals, it makes sense that they are also toxic for larger animals, like us.
While these cyanobacteria are ubiquitous, and a world-wide and signficant problem, there remain many unknowns. It is difficult to predict when such blooms will occur, and it is very much a problem trying to get rid of them when they do. The toxins can last for weeks after the bloom has visibly disappeared. What one would like to do is try and prevent them happening in the first place, such as by limiting the inflow of P and N, but no satisfactory control method is yet available for 'natural lakes' once a bloom occurs. It is obviously not possible to simply add chemicals to kill or suppress these microorganisms, as would be done at a public swimming pool or for drinking water. Possibly a biological control agent might be developed (e.g. a natural virus or a microcystin-resistant strain of Daphnia), which would specifically target this species, and so not cause problems elsewhere in the ecosystem. Currently, the local health authorities monitor lakes and rivers for toxic blooms, and when present, they put up signs to warn the public, and wait until the bloom disappears. The exact effects of the toxins on animals (including humans) are still not fully understood, and there are many chemical variants known, with new ones are being discovered every year.
An important take-home message is not to drink water that is green, or has recently been green, or has signs regarding blue-green algal blooms. The chlorophyll is fine, but the microcystins can be lethal.
Mike D-S, Jan. 19, 2013.