The origin of 11-hydroxyoctadec-trans-12-enoic and 12-hydroxyoctadec-trans-10-enoic acids (photodegradation products of cis-vaccenic acid) in the marine environment was investigated. cis-Vaccenic acid is commonly used as a bacterial biomarker; however, in heterotrophic bacteria the observed rates of cis-vaccenic acid photodegradation are negligible. Here, two hypotheses explaining the source of the photoproducts were tested. According to the first hypothesis, the photoproducts originate from aerobic anoxygenic bacteria, i.e., photoheterotrophic organisms using bacteriochlorophyll-containing reaction centers. Alternatively, the photoproducts come from a heterotrophic bacterial community closely associated with senescent phytoplanktonic cells. cis-Vaccenic acid photodegradation was detected in both experimental setups. However, a detailed comparison of the cis-vaccenic acid photodegradation patterns with those observed in particulate matter samples of the DYFAMED station (Mediterranean Sea) suggests that photodegradation of heterotrophic bacteria attached to senescent phytoplanktonic cells constitutes the more likely source of cis-vaccenic acid oxidation products detected in situ.