Environment-mediated interactions cause an externalized and collective memory in bacteria.

Bacteria usually live in complex communities interacting with many other microbial species. These interactions determine who can persist in a community and how the overall community forms and functions. Bacteria often exert interactions by chemically changing the environment, like taking up nutrients or producing toxins. These environmental changes can persist over time. We show here that such lasting environmental changes can cause a "memory effect" where current growth conditions alter 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 have to consider not only the interacting species and abiotic conditions but also the system's history.