Haynesina germanica is a coastal benthic foraminifer known to sequester chloroplasts from benthic pennate diatoms. This study investigates its cellular organization, as well as the oxygen consumption and production rates under dark and light conditions. The implications of the sequestered chloroplasts are subsequently discussed at an individual and ecological level. Living specimens collected in Arcachon Basin showed well-preserved chloroplasts in the cytoplasm in observations using transmission electron microscopy. Microelectrodes were used to measure oxygen consumption and production rates in incubated specimens in the laboratory. In the dark, oxygen consumption rates ranged from 662 +/- 12 to 1082 +/- 59 pmol O-2 ind(-1) d(-1). Under illuminated conditions (i.e., about 300 mu mol photon(-1) m(-2) s(-1)), we observed and quantified substantial photosynthetic activity, up to 199 pmol O-2 ind(-1) d(-1). It appears that H. germanica combines autotrophic and heterotrophic nutritional modes, thereby reflecting a mixotrophic strategy. In Arcachon Basin, H. germanica potentially uses chloroplasts acquired from microphytobenthos to carry out photosynthesis and produce oxygen and organic matter. Stored chloroplasts could constitute an additional food source during impoverished periods, providing substantial competitive advantage to this species. This putative behavior could also affect the biogeochemistry of the sediment at a microscale by creating oxygenated microniches around individual foraminifera.