Contributions of different microbial groups to absorption, backscattering, and marine reflectance ((a(λ), bb(λ) and R(λ) respectively) were quantified during an El Niño-Southern Oscillation cold phase in the equatorial Pacific during the Etude du Broutage en Zone Equatoriale cruise on board the R/V L'Atalante. In situ data were collected at every degree of latitude from 8°S to 8°N, 180°(26 October to 13 November 1996), and satellite reflectances were available from POLDER-ADEOS for the 1-10 November 1996 decade. Bulk absorption and backscattering coefficients were estimated at 440 nm for the major microbial groups enumerated in the upper surface layer (heterotrophic bacteria, Prochlorococcus, Synechococcus, and <20-mm eukaryotic algae). Total absorption and backscattering coefficients were retrieved from space by a new inverse method. The observed ecosystem was typical of a well-developed equatorial upwelling, with maximal values of 0.4 mg m^-3 for Tchl a_, 0.026 m^-1 for a_p (440), 0.023 m^-1 for a_phy(440), and a low in situ a_det (<14% of a_p). Prochlorococcus and nanoeukaryotic algae (3.4-mm mean diameter) were the dominant absorbers (97%), contributing about equally to a_phy. The retrieved total absorption coefficient, a_sat (440), from POLDER (maximum of 0.03 m^-1) was higher than a p (440), as it included absorption by CDOM (estimated to be 15% of a p (440) + a_w (440), where a_w = absorption by pure water). Heterotrophic bacteria were the dominant contributors (73%) to total simulated microbial backscattering, b_bmic (maximum = 3.7 x 10^-4 m^-1), but b_bmic was negligible compared to the inverted total backscattering by particles, b_bp (2.7 x 10^-3 m^-1), indicating that unidentified small nonliving particles contributed most to the satellite signal.