Abstract
Competition between microbes is extremely common, with many investing in mechanisms to harm other strains and species. Yet positive interactions between species have also been documented. What makes species help or harm each other is currently unclear. Here, we studied the interactions between 4 bacterial species capable of degrading metal working fluids (MWF), an industrial coolant and lubricant, which contains growth substrates as well as toxic biocides. We were surprised to find only positive or neutral interactions between the 4 species. Using mathematical modeling and further experiments, we show that positive interactions in this community were likely due to the toxicity of MWF, whereby each species' detoxification benefited the others by facilitating their survival, such that they could grow and degrade MWF better when together. The addition of nutrients, the reduction of toxicity, or the addition of more species instead resulted in competitive behavior. Our work provides support to the stress gradient hypothesis by showing how harsh, toxic environments can strongly favor facilitation between microbial species and mask underlying competitive interactions.
Highlights
Competition between microbes is extremely common, with many investing in mechanisms to harm other strains and species
Here we used a synthetic community composed of 4 bacterial species that has been applied to the bioremediation of highly alkaline and polluting liquids used in the manufacturing industry, called metal working fluids (MWF) [24,25,26]
Positive interactions were most common in the toxic fluid, and, if we made survival easier, for example by adding nutrients, bacteria began to compete
Summary
Competition between microbes is extremely common, with many investing in mechanisms to harm other strains and species. Facilitation [11] is more prevalent, since it encompasses cooperation as well as commensalism, where one species accidentally benefits from another, for example by cross-feeding off its waste products [12,13,14,15] It appears, that microbial life is mostly competitive: Microbes have evolved a great number of ways to harm other strains and species, which gives them a competitive advantage for available resources, be they nutrients, oxygen, or space [16]. By quantifying MWF degradation efficiency and mapping it to species composition and their interactions, this model system can help answer another key question in microbial ecology: How do interspecies interactions affect ecosystem functioning?
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