Patricia Bonin

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.

Several studies set out to explain the presence of high proportions of photooxidation products of cis-vaccenic acid (generally considered to be of bacterial origin) in marine environments. These studies show that these oxidation products result from the transfer of singlet oxygen from senescent phytoplankton cells to the bacteria attached to them in response to irradiation by sunlight. This paper summarizes and reviews the key findings of these studies, i.e., the demonstration of the process at work and the effect of different parameters (intensity of solar irradiance, presence of bacterial carotenoids, and presence of polar matrices such as silica, carbonate, and exopolymeric substances around phytoplankton cells) on this transfer. A large part of this review looks at how this type of alteration of bacteria can affect the preservation of algal material in the marine environment, especially in polar regions where conditions drive increased transfer of singlet oxygen from sympagic algae to bacteria.