Abstract
Bacterial communities exhibit various classes of interspecies interactions, ranging from synergistic to competitive. As these interaction classes play a crucial role in determining characteristics of bacterial communities, including species composition and community stability, understanding the mechanisms that shape them is important. Whereas several studies have suggested that synergistic interactions are rare, a study focused on single-carbon-source environments reported them to be relatively common. This discrepancy highlights the potential role of carbon source diversity in shaping interaction classes, although the quantitative relationship remains unclear. To elucidate this relationship, we examined 896 interspecies interactions amongst 28 synthetic bacterial pairs, isolated from various environments, under 32 conditions with varying levels of carbon source diversity. As a result, we frequently observed synergistic interactions in single-carbon-source environments, with the interactions shifting to competitive as the carbon source diversity increased. Further analyses suggested that this shift was driven by processes occurring in environments with an increased diversity of carbon sources, such as resource competition. Our findings provide new insights into how environmental factors, particularly carbon source diversity, shape interspecies interactions in bacterial communities.