<p>A pair of new strains was obtained from a natural environment and identified as the filamentous non-heterocystous cyanobacterium SZ2 and the rhizobia <i>Ensifer</i>/<i>Sinorhizobium</i> sp. ST1. This pair was of particular interest because it could potentially perform both photosynthesis and nitrogen fixation. Interactions and material production were investigated in a new co-culture system using the "natural pair" of these strains. Cocultivation was favorable under both mixotrophic (coMC) and autotrophic conditions (coAC) when material production was assessed using heptadecane (C₁₇H₃₆) from the SZ2 strains. Under coAC, where nitrogen is depleted, some soluble factor(s) produced by the co-culture—possibly including ammonium ions—appear to function as biomass-increasing factors (BIFs), contributing to the enhanced accumulation of chlorophyll <i>a</i> in SZ2, along with extracellular polysaccharides, lipids, carbohydrates, and proteins in the total biomass. A notable feature of this "aquatic" coAC system is that it overcomes biomass reduction under nitrogen-deficient conditions, which is difficult in monoculture systems and can contribute to economic material production for biorefineries.</p>