LAUSR

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 and about 6.4 × 10 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. Silicic acid (DSi), the only biologically assimilable dissolved form of silicon (Si), is a pivotal nutrient to ocean primary productivity. Its availability facilitates the growth of diatoms, which polycondensate DSi to elaborate their skeletons of biogenic silica (BSi). Diatoms are fundamental primary producers in the ocean food web, also the main DSi consumers and BSi producers in the photic ocean, largely determining the interplay between particulate (i.e., BSi) and dissolved (i.e., DSi) forms of Si in the marine biogeochemical cycle of this element (DeMaster 1981; Nelson et al. 1995; Tréguer et al. 1995). Over the last decades, the concern is rising that at least another group of Si-using organisms, namely marine siliceous sponges, may also play a relevant global role regarding the conversion of DSi into BSi in the ocean (Reincke and Barthel 1997; Maldonado et al. 2005, 2010, 2011, 2012b; Tréguer and De La Rocha 2013; López-Acosta et al. 2016). Sponges are abundant and even dominant organisms in many marine benthic communities, both in shallow-water and deep-sea habitat (e.g., Maldonado et al. 2017). Approximately, about 80% of the 8900 known sponge species have silica skeletons, produced from the silicic acid dissolved in the seawater. Unlike in the case of diatoms, the sponge DSi consumption consistently deals with the DSi pool in demersal water masses rather than that in the open water column. Despite the potential of sponges as DSi users, quantitative approaches to their functional role are scarce. The scarcity of this basic information is, in turn, preventing the understanding of their function within the global marine Si cycle. To date, the kinetics of DSi consumption has been evaluated only for six sponge species: Halichondria panicea (Reincke and Barthel 1997), Axinella damicornis, A. polypoides, A. verrucosa (Maldonado et al. 2011), and Hymeniacidon perlevis and Tethya citrina (López-Acosta et al. 2016). All species show consumption kinetics that fit a saturable Michaelis–Menten model. Yet large between-species variability has been noticed in the value of the parameters that govern that Michaelis– Menten kinetics, that is, maximum DSi transport velocity (Vmax) and half-saturation constant (Km), defined as the DSi *Correspondence: [email protected] Additional Supporting Information may be found in the online version of this article.