PS 47-214 - Initial steps in testing how soil selenium influences bacterial tolerance

Abstract: The Stress Gradient Hypothesis (SGH) states that interspecific interactions should shift from competitive to facilitative interactions as environmental stress increases. Selenium (Se) is a naturally occurring metalloid that acts as an environmental pollutant and as an essential micronutrient. Due to its metallic properties, Se acts as an oxidative stress and can be toxic to plants and animals. Given this, Se in soil systems acts as an abiotic stress that influences species composition and species interactions. Within soils, bacteria and fungi have evolved to tolerate Se via detoxification and Se processing for respiration and studies show that metal detoxification traits like this can provide community benefits for (i.e. facilitate) non-tolerant species. Thus, this is an opportunity to test whether the SGH applies to microbial systems. First, however, we need to determine thresholds for Se toxicity in microbes. Here we examine the Se toxicity thresholds for bacteria from soils with high and low Se across three geological formations that give rise to different soil types. We collected soil from 18 paired soil sites of naturally high and low Se concentrations across 3 geological formations within the Rocky Mountains and determined soil bulk density, pH, and nitrogen content. We then extracted 10 bacterial isolates from each site and tested their growth rates when exposed to Se at 0 to 500 ppm Se (100 ppm increments) to determine their Se tolerance thresholds and understand if soil pH, soil bulk density, nitrogen, geological formation, and original soil Se (high or low) influence these thresholds. Our data suggests that there is variation in Se tolerance thresholds both within and between sites. This variation may influence a tolerant isolate’s capacity to provide facilitative benefits to a non-tolerant neighbor.