The impact of tropodithietic acid on microbial physiology under varying culture complexities.

Understanding marine bacterial physiology under environmentally relevant conditions requires the study of biotic interactions across systems of varying complexities. Here, we examine how the capability of Phaeobacter inhibens bacteria to produce tropodithietic acid (TDA), a secondary metabolite, influences microbial physiology and interactions. Our systematic approach, which includes progressing from bacterial monocultures to co-cultures and tri-cultures involving algal hosts, allows us to evaluate the impact of the tdaB gene and the TDA metabolite on microbial interactions. Our findings show that deleting the tdaB gene resulted in no detectable TDA production and affected bacteria-bacteria interactions in co-culture but not in tri-cultures with the algal host. Additionally, our data reveal that algal death was delayed in cultures containing P. inhibens ΔtdaB mutants compared to those with wild-type bacteria, although no TDA was detected in these tri-cultures. The findings of our study highlight the importance of microbial complexity in the study of bacterial physiology and point to the understudied role of TDA in microbial interactions.IMPORTANCELaboratory model systems enable controlled studies of marine microbial processes; however, the microbial complexity of the culture can influence the outcome. In this study, we employ a systematic approach to assess the impact of the bacterial ability to produce the antibiotic TDA in laboratory cultures with varying microbial complexities (from bacterial monocultures to bacterial co-cultures and algal-bacterial tri-cultures). Our findings demonstrate altered effects of the tdaB gene deletion with increasing microbial complexity, showing distinct impacts on microbial fitness. Since antibiotics like TDA mediate microbial interactions, it is important to examine them within ecologically relevant model systems that reflect inter- and intra-trophic interactions, including bacteria-bacteria and algae-bacteria relationships. Overall, our study highlights the importance of accounting for culture complexity when designing laboratory experiments to investigate microbial interactions and the compounds that mediate them.