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