Environment-mediated interactions cause an externalized and collective memory in microbes

Microbes typically live in complex communities, interacting with numerous other microbial species. These interactions determine who can persist in a community and how the overall community forms and functions. Microbes often exert interactions by chemically changing the environment, like taking up nutrients or producing toxins. These environmental changes can persist over time. We demonstrate here that such lasting environmental changes can induce a “memory effect,” where current growth conditions influence interaction outcomes in the future. This memory is only stored in the environment and not inside bacterial cells. Only the collective effort of many bacteria can build up this memory, making it an emergent property of bacterial populations. This externalized and collective memory can also impact the assembly of more complex communities and lead to different final compositions depending on the system's past. Overall, we show that to understand interaction outcomes fully, we not only have to consider the interacting species and abiotic conditions but also the system's history