The silicic acid (DSi) is a dissolved nutrient used by diverse marine organisms to build their skeletons of biogenic silica (BSi). This consumption, mostly due to diatoms, largely determines the availability of DSi in the photic ocean. Yet growing evidence suggests that Si consumers traditionally disregarded, such as the siliceous sponges, may also play a role. This study investigated the kinetics of DSi utilization by two demosponges as a function of both DSi availability and duration of the incubation period (24 h vs. 48 h). Consumption increased with increasing DSi availability following a saturable Michaelis–Menten kinetics. Haliclona simulans saturated at about 70 μM (Km = 45.9) and Suberites ficus around 130 μM (Km = 108.2). Forty-eight hour incubations yielded more conservative consumption rates than 24 h incubations, particularly when DSi availability was far below saturation. DSi concentrations in the sponge natural habitats (0.2–15 μM) were consistently much lower than required for efficient elaboration of the BSi skeleton, suggesting a chronic DSi limitation. The DSi consumption kinetics was combined with quantifications of sponge biomass in the Bay of Brest (France), which was used as case study. In this system, sponges consume daily 0.10 0.19 mmol Si m−2 and about 6.4 × 106 mol Si yearly. This activity represents 7.6% of the net annual BSi production in the Bay, a figure overlooked in previous nutrient balances based only on diatoms. Since the world marine Si cycle does not yet incorporate the contribution of sponges, its global BSi production budget may also be underestimated.