Global change goes beyond temperature as synthetic contaminants increasingly permeate our world, yet ecology and evolution research tends to focus on changing climate alone. An alarming amount of plastic enters the environment as trash, where it fragments into smaller and smaller particles. These particles persist, and have under-explored effects on organisms and their interactions, such as plants and their associated microbes. Some of my recent work seeks to understand effects of plastic.
Tire tread wear is a common microplastic, accumulates near roads, and leaches contaminants toxic to some animals into water. Dosing duckweed with his leachate increased growth but caused normally beneficial interactions with the microbiome to become costly, and induced plant-microbe fitness conflicts. Of course, the effects of tire wear particle leachate also depended on other manipulated aspects of global change: temperature and carbon dioxide. Check it out at Environmental Research, or the associated preprint to learn more.
Relatedly, work lead by Leah Chibwe identified that leachate from tire wear particles is more toxic to fathead minnows when particles leach for longer, or when fish are simultaneously exposed to the particles themselves. While leachate from tire wear particles is a highly complex mixture with many unknown compounds, the paper newly out at Environmental Toxicology and Chemistry, also identifies known chemical components of the leachate that are more correlated with toxic effects.
In work led by recent Master’s graduate Yawen Guo, we explored how background pollution with nanoscale plastic fragments may alter the effectiveness of hydrogen peroxide as a control measure for harmful algal blooms. Nanoplastics interfered with some of the toxic effects of hydrogen peroxide, but also had a toxic effect on algae themselves. We therefore expect that hydrogen peroxide will remain an effective control measure, though, as with duckweed, ecological context (light, temperature) matters. Out recently at ES & T Water.
One key insight is that counter-intuitiively, conflict can increase mutualism. When hosts and microbes have fitness peaks at different trait values, this evolutionary conflict can actually cause hosts to depend MORE on microbes for optimal trait expression, thereby enhancing the degree of mutualism we would obsesrve.
Another key insight is that the evolution of traits in multiple genomes may favor local adaptation. If hosts are paired with microbes with which they did not share an evolutionary history, this would likely lead to trait mis-expression and appear as a signal of increased fitness with local microbes.
Work led by Rebecca T Batstone shows that microbes evolve to benefit local hosts. One neat aspect of our work is that benefits occurred because loci increasing microbe fitness on local hosts also increased the fitness of local hosts — e.g. microbes were neither “altruistic” nor “cheaters.” Indeed alleles underlying mutual benefits more commonly contributed to evolved microbial differences for host and microbe growth than expected by chance.
A favorite thesis chapter is online at Evolution! Microbes may alter plant adaptation to climate: root microbes alter teosinte's expression & genetic covariance of traits adaptively diverged across environmentshttps://t.co/8CDIGdBCpT w/@jrossibarra@systrauss & Ruairidh Sawers
Teosinte phenotypes that differ from low elevation, warm sites (later flowering, larger root mass) to high elevation, cold sites (early flowering, small roots) are shifted by root microbes. More interesting yet, microbes shift the genetic variation and covariation between traits, potentially altering plant responses to selection pressures, e.g. climate change.
Hubbard et al 2019 find that root microbes drive a substantial amount of plant defense in Boechera stricta, and that plant and root-microbe pathways to defense are different. I opine here on what separate pathways mean for evolution of plant defense, and what alternate knock-on effects of microbe- and plant- driven defense on insects could mean for tri-trophic interactions.