Featured Image Caption: Cyanobacteria come in all shapes and sizes. They can be filamentous, like the species depicted here. They have specialized cells to store nutrients, and in some species, to fix nitrogen. Photo credit: Katie Gregor
Lin, W., Zhang, Z., Chen, Y., Zhang, Q., Ke, M., Lu, T., & Qian, H. (2023). The mechanism of different cyanobacterial responses to glyphosate. Journal of Environmental Sciences, 125, 258-265. https://doi.org/10.1016/j.jes.2021.11.039
Let’s face the facts: algae have a bad reputation, in part due to their ability to form ‘toxic blooms’ in ponds and lakes. However, mixed in with the algae is another group, known as microalgae. These microorganisms, which also contribute to toxic blooms, are amazingly complex and deserve some appreciation. The term ‘microalgae’ encompasses cyanobacteria, which are bacteria that can photosynthesize; they share a close evolutionary history with land plants. Some cyanobacteria can even fix nitrogen, an ability very few organisms possess. Nitrogen fixation is an impressive feat for these microscopic bacteria; the process includes capturing gaseous nitrogen from the air and converting it into a usable form for growth. Many land plants rely on this usable nitrogen that is produced by nitrogen-fixing bacteria. Cyanobacteria are usually found in aquatic environments, which are unfortunately often polluted from human activity. Bodies of water such as ponds and lakes can receive runoff from nearby areas, thereby collecting an unappealing combination of fertilizer, pesticides, and antibiotics. How do cyanobacteria survive in these harsh conditions? Lin et al. set out to study one angle of this question.
The scientists first studied how six different species of cyanobacteria grew in the presence of glyphosate, a chemical found in herbicides such as Roundup®. Glyphosate is a popular component for an herbicide, as it targets an enzyme that mammals do not possess. While this chemical is therefore relatively harmless to mammals, it is destructive to aquatic life since it rapidly degrades into toxic components. The researchers wanted to specifically target cyanobacteria’s reaction to glyphosate contamination, especially given their similarity to land plants. To do this, they set up small, individual containers that each held a single cyanobacterial species. Glyphosate treatments were then applied; this involved the addition of 5 respective glyphosate concentrations, ranging from 10 micrograms per liter (abbreviated µg/L) to 5,000 µg/L. This was repeated for six distinct cyanobacteria species. A control treatment container for each cyanobacterium was also established without the addition of glyphosate. The scientists tracked cyanobacterial growth rate for all concentrations of tested glyphosate for over two weeks. Secondly, an additional “natural water” experiment was performed. Water from a nearby lake was collected, and the native bacterial community, both photosynthetic and non-photosynthetic bacteria, was tracked after the addition of 1,000 µg/L glyphosate. Water quality, total phosphorus and nitrate, and photosynthetic output were also measured.
To Grow or Not to Grow?
Cyanobacteria, like all living things, require nutrients to grow. Such nutrients include phosphorous and nitrogen. When those are not readily available, cyanobacteria will either adapt or die. Interestingly, unlike plants, glyphosate can be metabolized by cyanobacteria; the chemical compound is broken down and formed into amino acids. Alternatively, cyanobacteria can simply use glyphosate as a phosphorous source to continue to grow. Cyanobacteria that possess capabilities to utilize glyphosate as a growth source could potentially reduce the toxicity of pollutants, thereby contributing to the overall health of an aquatic ecosystem. The trade-off though is a faster growth rate and more robust growth of various cyanobacteria species, including harmful ones.
Cyanobacteria can not only survive in the presence of glyphosate, but sometimes, they grow even better in this environment than in control conditions! When the six species were grown separately in media supplemented with glyphosate, only one of the six tested species seemed to suffer in the presence of glyphosate. This cyanobacterial species was the one most closely related to land plants. The remaining five species exhibited stimulated growth. Of special interest in this study was the high growth rate of Microcystis aeruginosa in the presence of glyphosate. This cyanobacterium is a particularly problematic microbe as it is one of the species responsible for the harmful effects of cyanobacteria blooms. Microcystis species produce several toxins, including microcystin, that can wreak havoc in humans and animals alike.
Glyphosate’s Effect on Aquatic Bacterial Communities
Pond water is a delicate ecosystem, held together by many microscopic organisms. Among these are bacteria that do not perform photosynthesis. In their second study, Lin et al also investigated the effect of glyphosate on these other bacterial populations in an experiment using lake water. He found that when exposed to glyphosate, cyanobacteria had a much higher relative abundance (7.74%) than in control conditions (2.35%). In the control environment, on Day 7, other non-photosynthetic bacteria, such as those belonging to Proteobacteria and Bacteroidetes, ranked highest. These bacteria were still present in the glyphosate treatments, but at a much lower percentage. This difference in the abundance of bacteria can have far-reaching consequences for the ecosystem. With enough time, cyanobacteria can continue to outcompete the other prokaryotes, leading to water quality and health hazards.
Figure caption: Cyanobacteria can be grown for testing in bioreactors. This is similar Lin et al’s setup; here we have a pure culture in specialized media. A bubbler is added for carbon dioxide input; this is required for photosynthesis. Photo credit: Maria Marlin
Business as Normal
Recall that cyanobacteria are capable of photosynthesis, meaning they have special pigments to carry out this process. Chlorophyll a is an example of such a pigment, and measuring the quantity of this pigment is oftentimes used to gauge photosynthetic capacity in ecosystems. In the lake water experiment, the addition of 1000 µg/L glyphosate did not affect the chlorophyll a content in the water. One can conclude that even in the presence of this toxic compound, the cyanobacteria’s ability to photosynthesize was not impaired. Likewise, when the amount of nitrate in the water was measured, no significant difference was found between the glyphosate treatment and control conditions (plain lake water without glyphosate treatment). Cyanobacterial uptake of nitrogen continued unimpeded as well. Phosphorus levels in the treated water were elevated compared to the control. This indicates that the cyanobacteria were instead using glyphosate as a phosphorus source, leaving excess phosphorus in solution.
Cyanobacteria: Friend or Foe?
The cyanobacteria that produce toxins are well studied, but their diversity extends far beyond that small group of species. Cyanobacteria may have unknown roles in an aquatic ecosystem. Contaminants from humans can pose a threat to vertebrates, invertebrates, and the microscopic world alike, exacerbating threats to these groups that are already present in the environment. Some cyanobacteria can participate in glyphosate uptake, thereby reducing the hazards to other aquatic life. Unfortunately, though, we cannot control which cyanobacteria succeed and which do not. Especially disastrous is the increase in growth of toxic species, in part, due to excess nutrients in run-off. Each cyanobacterial species fulfills a unique role in its environment; striving to control run-off will reduce disruptions to the balance of aquatic ecosystems.