Heme auxotrophy in abundant aquatic microbial lineages

Abstract

Significance Heme is essential for respiration. As a cofactor of cytochromes, heme functions as a main electron carrier in all respiratory electron transport chains. Therefore, it is natural to expect all respiring and free-living microorganisms to make heme. Against this expectation, here we show that the acI lineage, one of the most abundant bacterial groups in freshwater environments, is unable to biosynthesize heme and requires exogenous heme. Furthermore, we provide genomic evidence for putative heme auxotrophy among many not-yet-cultured aquatic microbial groups. Heme should be the focus of future research on the metabolic dependency among microorganisms and the role of exchangeable metabolites in structuring diverse ecosystems, and would also be a media component that must be considered when cultivating novel microbes.