Figure 3. Examples of cross-feeding in biofilms.
Four distinct cases of metabolite cross-feeding are depicted as biofilms that each contain two metabolic subpopulations (beige and green). Metabolic pathways for each subpopulation are indicated by selected intermediates (arrows may represent multiple pathway steps). All biofilms shown are growing under an oxic atmosphere and form an O2 gradient such that cells are O2-limited at the biofilm base. In the biofilms shown in (A), cells at the base convert glucose into an intermediate that is released. The intermediate is taken up by cells in the oxic zone and provides a source of carbon for the tricarboxylic acid (TCA) cycle. In the biofilms shown in (B), cells are releasing compounds that can be used as electron acceptors. Left: In our group’s model of P. aeruginosa biofilms growing under an oxic atmosphere, on medium with nitrate (NO 3-), the first step of denitrification is carried out in the oxic zone, and subsequent steps are carried out in the O2-limited zone. Right: Some P. aeruginosa phenazines are specifically produced in the oxic zone of the biofilm, then oxidized by O2 (blue arrow). Phenazines are reduced by cells at the biofilm base and can serve to balance the intracellular redox state.