Scientists have found out the causes of water blooming in transparent lakes
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- Scientists have found out the causes of water blooming in transparent lakes
Cyanobacterium Gloeotrichia echinulata is capable of forming noticeable clusters even in clear lakes, which is why its appearance causes concern among vacationers and water quality specialists. However, the analysis of six genomes from US reservoirs did not reveal the genes necessary for the production of well-known classical cyanotoxins, including microcystin. At the same time, researchers have discovered the genes of a substance capable of spoiling the taste and smell of water. About what exactly scientists have found out and why the results do not allow us to consider any blooming lake safe, see the "Izvestia" article.
Why did Gloeotrichia raise concerns
Cyanobacteria are microorganisms capable of photosynthesis. Their mass reproduction in reservoirs is commonly referred to as water blooming. Previously, cyanobacteria were often called blue-green algae, although from the point of view of biology they are bacteria.
Gloeotrichia echinulata is especially noticeable in clear lakes. Its clusters look like greenish or white rounded formations that can gather in dense masses. For a body of water that looks outwardly clean, such a picture looks alarming. In the northeastern United States, this cyanobacterium is found, among other things, in lakes used for recreation, and the period of its greatest abundance may coincide with the bathing season.
An additional cause for concern was the results of previous work suggesting that Gloeotrichia could produce microcystin. It is one of the most well-known cyanotoxins that affect the liver. The authors of the new study decided to check whether the studied populations have the genetic apparatus necessary for its synthesis.
How was the ability to produce toxins tested
Researchers from the Bigelow Ocean Sciences Laboratory and the University of Oregon studied samples of Gloeotrichia echinulata from lakes in the western and northeastern United States. In total, they collected six genomes: four complete, one almost complete, and one rough one. The material came from reservoirs in California, Oregon, New Hampshire and Maine.
The genome allows you to check whether the body has a set of genes for the production of a certain substance. This is different from measuring the concentration of a substance in lake water: DNA analysis shows the possibility of its synthesis, but by itself does not establish the composition of the water on a particular day.
The work began with a sample from Lake Winnipesaukee in New Hampshire. Robin Slate, one of the lead authors of the study, did not find microcystin synthesis genes in it. He then joined forces with Oregon State University researcher Theo Dreher, who studied cyanobacteria on the west coast of the United States. To obtain detailed data on the northeastern samples, the team used Oxford Nanopore DNA sequencing technology.
The comparison showed that all six genomes studied lacked genes for the synthesis of classical cyanotoxins, including microcystin. At the same time, genomes from remote regions turned out to be very similar: their average identity at the genome-wide level was 98-99%. The authors consider them as a close group within the same species.
"It may be producing unexplored toxins or substances that we don't understand yet. But for the toxins known to science, which pose the most serious acute danger to human health, we have not seen obvious ways to synthesize them," explained Slate.
Why Cyanobacteria can still affect water quality
The absence of studied toxin synthesis genes does not mean that the mass reproduction of Gloeotrichia takes place without a trace for the reservoir. In all six genomes, scientists have found genes related to the production of geosmin— a substance that gives water its characteristic earthy smell and taste.
The genetic result indicates the ability of cyanobacteria to produce geosmin, but does not show exactly how much of this substance turned out to be in each of the lakes studied. Nevertheless, its possible presence is important for organizations involved in the preparation of drinking water: the smell and taste may require additional treatment.
Scientists have also found features in the genomes that help explain the life of Gloeotrichia in clear lakes with few nutrients. According to their data, cyanobacteria are able to actively grow on the surface of bottom sediments, and then move into the water column.
The identified genes are associated, in particular, with the capture of light at depth, movement along the surface of sediment and the formation of gas bubbles that help to stay in the water. This is a possible explanation for how the species persists in the lake and then becomes visible near the surface.
Can flowering be considered safe
The study concerns six studied genomes of one species, not all cyanobacteria in lakes. The authors admit that there are as yet unexplored populations of Gloeotrichia capable of producing toxins. There is also the possibility of the formation of substances, the toxicity of which has not yet been studied.
There is another reason not to evaluate the safety of a reservoir by one species: other cyanobacteria can multiply along with Gloeotrichia, including those capable of producing dangerous compounds. Therefore, the results of the genetic analysis do not cancel out the observation of flowering and checking the water in lakes used for bathing or water supply.
The work helps to more accurately identify the source of a possible threat. In the studied American populations of Gloeotrichia echinulata, there were no genetic signs of the production of well-known classical cyanotoxins, but a potential cause of the unpleasant smell of water was discovered. To understand the risk in a particular lake, it is necessary to take into account the entire composition of the bloom and the monitoring results of the reservoir itself.
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