Purple phototrophic bacteria release crotonate as a metabolic overflow pathway

Purple phototrophic bacteria (PPB) exhibit diverse metabolic strategies to cope with excess carbon and metabolic constraints. Here, we report a previously unrecognized overflow mechanism in PPB: the transient extracellular release of crotonate as a metabolic escape valve. In batch photoheterotrophic cultures of enriched PPB, crotonate accumulated under carbon-excess conditions when conventional metabolic sinks were constrained. Crotonate excretion coincided with depletion of polyhydroxybutyrate and H₂ production and was reversible upon addition of an alternative electron acceptor (DMSO), indicating a regulated overflow rather than irreversible fermentation. Metaproteomic analysis showed that dominant Rhodopseudomonas species redirect acetyl-CoA metabolism toward crotonyl-CoA formation when glyoxylate shunt activity is suppressed. A CoA-transferase was identified as a candidate enzyme enabling conversion of crotonyl-CoA to free crotonate for excretion. These findings reveal crotonate excretion as a naturally emerging overflow phenotype in wild-type PPB under defined metabolic constraints.