Metabolic Responses of a Phototrophic Co-Culture Enriched from a Freshwater Sediment on Changing Substrate Availability and its Relevance for Biogeochemical Iron Cycling

Abstract

Metabolic flexibility controls microbial survival and shapes ecological structures. Although the utilization of a variety of substrates has been studied for individual metabolically versatile microorganisms, only little is known about the response of microbial communities toward substrate fluctuation. Here, we exposed a phototrophic co-culture (Rhodopseudomonas sp. and Chlorobium sp.) that was isolated from a littoral freshwater sediment to a variety of substrates (acetate, Fe(II), or H2) and followed the abundance of individual microbial partners based on their characteristic light absorption pattern. Chlorobium sp. dominated the culture when Fe(II) was supplied, whereas Rhodopseudomonas sp. decreased below detection. Rhodopseudomonas sp. dominated when the co-culture was transferred onto acetate (Chlorobium sp. below detection). When grown on H2, the dominance pattern depended on pre-culture conditions. Pre-cultures on Fe(II) or acetate resulted in dominance of Chlorobium sp. or Rhodopseudomonas sp., respectively. Although the relative abundance of the two partner strains strictly relies on substrate availability, we were incapable of isolating Chlorobium sp. from the co-culture. Our study shows how substrate fluctuations shape microbial distribution and points toward potential community interactions that enable microbial survival.