The cyanobacterium Acaryochloris marina is the only known phototroph harboring chlorophyll (Chl) d. It is easy to cultivate it in a planktonic growth mode, and A. marina cultures have been subject to detailed biochemical and biophysical characterization. In natural situations, A. marina is mainly found associated with surfaces, but this growth mode has not been studied yet. Here, we show that the A. marina type strain MBIC11017 inoculated into alginate beads forms dense biofilm-like cell clusters, as in natural A. marina biofilms, characterized by strong O(2) concentration gradients that change with irradiance. Biofilm growth under both visible radiation (VIS, 400 to 700 nm) and near-infrared radiation (NIR, ∼700 to 730 nm) yielded maximal cell-specific growth rates of 0.38 per day and 0.64 per day, respectively. The population doubling times were 1.09 and 1.82 days for NIR and visible light, respectively. The photosynthesis versus irradiance curves showed saturation at a photon irradiance of E(k) (saturating irradiance) >250 μmol photons m(-2) s(-1) for blue light but no clear saturation at 365 μmol photons m(-2) s(-1) for NIR. The maximal gross photosynthesis rates in the aggregates were ∼1,272 μmol O(2) mg Chl d(-1) h(-1) (NIR) and ∼1,128 μmol O(2) mg Chl d(-1) h(-1) (VIS). The photosynthetic efficiency (α) values were higher in NIR-irradiated cells [(268 ± 0.29) × 10(-6) m(2) mg Chl d(-1) (mean ± standard deviation)] than under blue light [(231 ± 0.22) × 10(-6) m(2) mg Chl d(-1)]. A. marina is well adapted to a biofilm growth mode under both visible and NIR irradiance and under O(2) conditions ranging from anoxia to hyperoxia, explaining its presence in natural niches with similar environmental conditions.