Prochlorococcus is an important component of phototrophic biomass in oligotrophic areas. In Such systems, diel variations in optical properties have been reported. In order to better understand these natural variations, the optical (absorption, attenuation, and scattering) and biochemical (carbon and pigment) properties of an axenic clone of Prochlorococcus, PCC 9511, grown in a turbidostat set under light conditions that simulated those found near the ocean surface, were monitored every 2 h for several consecutive days. All optical parameters showed pronounced diel patterns except the divinyl-chlorophyll a (Dv-Chl a) specific absorption coefficient at 676 nm, a*(676), which remained stable (0.019 m(2) mg Dv-Chl a(-1)). The diel oscillations of the Dv-Chl a specific absorption coefficient at 440 nm, a*(440) (43% increase between sunrise and sunset), were essentially governed by variations in the zeaxanthin to Dv-Chl a ratio (52% increase of this ratio between sunrise and sunset). The scattering cross section at 555 nm, sigma(b)(555), showed oscillations with the largest amplitude (182% increase between sunrise and sunset). Finally, the carbon-specific attenuation coefficient at 660 nm, c(c)*(660) (1.04 m(2) gC(-1)) is less than half that of the other algal groups. This estimation, together with the diel fluctuations in c(c)*(660) highlighted in the present study, is discussed in the context of using in situ measurements of optical properties to infer biogeochemical stocks (vegetal or detrital biomass) or processes (primary production).