Phototrophic communities of autotrophic microalgae and heterotrophic bacteria perform complex tasks of nutrient acquisition and tackling environmental stress but remain underexplored as a basis for the bioremediation of emerging pollutants. In industrial monoculture designs, poor iron uptake by microalgae limits their productivity and biotechnological efficacy. Iron supplementation is expensive and ineffective because iron remains insoluble in an aqueous medium and is biologically unavailable. However, microalgae develop complex interkingdom associations with siderophore-producing bacteria that help solubilize iron and increase its bioavailability. Using dye degradation as a model, we combined environmental isolations and synthetic ecology as a workflow to design a simplified microbial community based on iron and carbon exchange. We established a mutualism between the previously non-associated alga Chlorella sorokiniana and siderophore-producing bacterium Ralstonia pickettii . Siderophore-mediated increase in iron bioavailability alleviated Fe stress for algae and increased the reductive iron uptake mechanism and bioremediation potential. In exchange, C. sorokiniana produced galactose, glucose, and mannose as major extracellular monosaccharides, supporting bacterial growth. We propose that extracellular iron reduction by ferrireductase is crucial for azoreductase-mediated dye degradation in microalgae. These results demonstrate that iron bioavailability, often overlooked in cultivation, governs microalgal growth, enzymatic processes, and bioremediation potential. Our results suggest that phototrophic communities with an active association for iron and carbon exchange have the potential to overcome challenges associated with micronutrient availability, while scaling up bioremediation designs.

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Mutualism between microalgae and bacteria is ubiquitous, but remains underexplored as a basis for biodegradation of anthropogenic pollutants. In industrial systems, poor iron uptake by microalgae limits growth, bioprocessing efficacy, and bioremediation potential. Iron supplementation is costly and ineffective because iron remains insoluble in aqueous medium and biologically unavailable. In aquatic environments, microalgae develop an association with bacteria that solubilize iron by production of siderophore, which increases the bioavailability of iron as a public good. Algae, in exchange, provides dissolved organic matter to bacteria to sustain such interkingdom associations. Therefore, using a case study of azo dye degradation, we combine environmental isolations and synthetic ecology as a workflow, establishing a microbial community to degrade industrially relevant Acid Black 1 dye. We create a mutualism between previously non-associated chlorophyte alga Chlorella sorokiniana and siderophore-producing bacterium Ralstonia pickettii, based on the eco-evolutionary principle of exchange of iron and carbon. This siderophore-mediated increased iron bioavailability increases reductive iron uptake, growth rate, and azoreductase-mediated dye degradation of microalga. In exchange, C. sorokiniana produces galactose, glucose, and mannose as major extracellular monosaccharides, supporting bacterial growth. We propose a mechanism whereby extracellular ferrireductase is crucial for azoreductase-mediated dye degradation in microalgae. Our work demonstrates that bioavailability of iron, which is often overlooked in industrial bio-designs, governs microalgal growth and enzymatic processes. Our results suggest that algal-bacterial consortia based on the active association are a self-sustainable mechanism to overcome existing challenges of micronutrient availability in bioremediation systems and their industrial translation.

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