An idyllic lake turns threatening when heavy rainfall causes a sewage treatment plant to overflow. Within 24 hours, once-benign microbes turn into virulent pathogens, breeding incessantly and attacking the embryos of lake fish. As much as that may sound like the synopsis of a movie on Mystery Science Theater, this is an impact on lake ecosystems that is actually occurring.
It is widely accepted that nutrient enrichment and pollution in streams and lakes cause oxygen depletion, called eutrophication. For example, sewage effluents can cause bacterial growth in nearby streams, increasing the number of oxygen-gulping organisms in the water and forcing fish (and other organisms) to compete for dissolved oxygen. The lack of oxygen then weakens and increases these organisms’ susceptibility to disease.
However, this is just one effect pollution can have on these ecosystems, say Claus Wedekind and colleagues from Switzerland in a study published in the May issue of Ecology. Specifically, the scientists discovered that, in addition to depleted dissolved oxygen, elevated bacterial levels can have a direct impact on the egg development of the whitefish Coregonus suitieri, a keystone salmonid in some lake ecosystems. As the authors put it:
The primary causes of embryo mortality both in the wild and in hatcheries are often unclear, but microbial pathogens appear to play a major role. Accordingly, whitefish embryos show adaptations such as precocious hatching to avoid infection and early immunological defense…
The ecologists caught whitefish from Lake Lucerne in Switzerland during spawning season and took several water samples from the lake as well. They used the fish’s gametes for in vitro fertilization, incubated more than 1000 randomly-selected, fertilized eggs and sterilized the water taken from the lake. They then dissolved a meat extract and peptone, a nutrient medium used to grow bacteria, in the sterilized water and inserted the eggs.
Then, in some of the wells containing eggs, the scientists administered antimicrobial agents and compared the results to the untreated water wells. The researchers measured dissolved water levels, bacterial growth, egg infection rates, embryo mortality and hatching rates. Initially, Wedekind and colleagues found that hatching rates were higher in the antimicrobial wells; however, they could not determine whether that was a result of increased oxygen in the water (since a reduced number of microbes would take up less dissolved oxygen) or relief from the direct impact of the microbes.
To determine the likely cause of egg survival, the researchers focused on the first 24 hours after administering the antimicrobial compounds. They discovered that oxygen concentrations did not significantly decline in wells without antimicrobial compounds within the first 24 hours. In other words, while the eggs were hatching in the antimicrobial wells, the infested eggs were not, and this was not due to a lack of dissolved oxygen in the water. The likely source then? Opportunistic pathogens, say Wedekin and co-authors:
Microbes that are typically benign but have the potential to become virulent to immunologically compromised hosts are called ‘opportunistic pathogens.’ Thermal or oxygen stress, high host density or mechanical injuries are among the possible stressors that affect host susceptibility to such infectious diseases…The typical pattern is that chronic or acute stress increases host susceptibility, especially so in the case of ‘opportunistic’ microbial pathogens.
An infestation of microbes depletes the dissolved oxygen, but not enough to outright kill the eggs. However, the slight change in the microhabitat could be enough to turn opportunistic pathogens into virulent ones, attacking the embryos before the effects of oxygen depletion even take hold. Says Wedekin:
Microhabitat changes may frequently affect important characteristics of symbiotic microbial communities… Resource enrichment is likely to change not only growth patterns but also the social environment of microbes…Our results demonstrate that changes in the ecological environment of microbes, and possibly linked changes in their social environment, can be sufficient to turn benign microbe communities into virulent ones.
Wedekind, C., Gessner, M., Vazquez, F., Maerki, M., & Steiner, D. (2010). Elevated resource availability sufficient to turn opportunistic into virulent fish pathogens Ecology, 91 (5), 1251-1256 DOI: 10.1890/09-1067.1