Contrasting irradiation of senescent cells of the diatom Thalassiosira sp. in association with the bacterium Pseudomonas stutzeri showed the effect of intensity of irradiance on the transfer of singlet oxygen ( 1 O 2 ) to bacteria attached to phytoplanktonic cells. Under low irradiances, 1 O 2 is produced slowly, favors the oxidation of algal unsaturated lipids (photodynamic effect), and limits 1 O 2 transfer to attached bacteria. However, high irradiances induce a rapid and intense production of 1 O 2 , which diffuses out of the chloroplasts and easily reaches the attached bacteria, where it efficiently oxidizes their unsaturated membrane components. Analysis of numerous sinking particle samples collected in different regions of the Canadian Arctic showed that the photooxidation state of attached bacteria increased from ice‐covered areas to open water, in agreement with in vitro results. Photooxidation of bacteria appeared to be particularly intense in sea ice, where the sympagic algae–bacteria association is maintained at relatively high irradiances for long periods of time.