AQUATIC MICROBIAL ECOLOGY Vol. 32: 299–306, 2003 Published July 14 Aquat Microb Ecol Marine diatom growth on different forms of particulate silica: evidence of cell/particle interaction Antonella Penna1,*, Mauro Magnani1, Ivana Fenoglio2, Bice Fubini2, Carlo Cerrano3, Marco Giovine4, Giorgio Bavestrello5 1Centro Biologia Ambientale, Università di Urbino, Viale Trieste 296, 61100 Pesaro, Italy 2Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Università di Torino, Via P. Giuria 9, 10125 Torino, Italy 3Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26, 16132 Genova, Italy 4Istituto Policattedra Chimica Biologica,Università di Genova, Viale Benedetto XV, 16132 Genova, Italy 5Istituto di Scienze del Mare, Università di Ancona, Via Brecce Bianche, 60131 Ancona, Italy ABSTRACT: The influence of particulate silica sources on growth and silicon uptake of 3 diatom spe- cies: Cylindrotheca fusiformis, Navicula sp., and Skeletonema costatum, was investigated. Each diatom strain was incubated in controlled conditions, with mineral (quartz sand and 2 pure quartz dusts with variable degree of hydrophilicity/hydrophobicity) and biogenic (diatomaceous earth and sponge spicules) silica substrates. Mineral sources were all crystalline, while the biogenic substrates were mostly amorphous. Each diatom species showed a different growth pattern with the various particulate silica substrates, none of which related to the rate of silicon dissolution in the growth medium: S. costatum grew better in presence of the quartz sand; while Navicula sp. and C. fusiformis showed higher growth values with the hydrophobic quartz particles. In contrast, low levels of growth of the 3 diatoms were found in the presence of the biogenic amorphous silica substrates. The high values of the silicon uptake of all diatom species in the presence of the crystalline substrates in cul- ture conditions seemed to confirm the preferred exploitation of the dissolved silicon from crystalline sources with respect to the amorphous mineral substrates. Thus, it could be hypothesized that the dis- solved silicon uptake by marine diatoms was not only mediated by solubilized oligomeric silica, but a direct interaction between diatom cell and particulate mineral substrates. Such chemical/physiologi- cal interaction can be highly specific either for the different diatom species or silica particles. A chem- ical model of a possible role of some organic compounds involved in the uptake of dissolved silicon from particulate silica sources by marine diatoms was suggested, and the potential ecological signif- icance of these findings is discussed. KEY WORDS: Biomineralization · Diatoms · Mineral substrates · Silicon uptake · Crystalline amorphous silica · Biogenic amorphous silica · Hydrophilicity/Hydrophobicity Resale or republication not permitted without written consent of the publisher INTRODUCTION regulation of diatom growth, as it constitutes the diatom cell wall in its polymerized form, opaline silica (Shipe & Diatoms are the predominant siliceous organisms in Brzezinski 1999). Silicon is supplied to the ocean both the marine environment and the major component of dissolved as monomeric or oligomeric form or as solid phytoplankton. They account for 40% of total primary particulates in variable size and crystallinity. Dissolved production in the sea, and tend to deplete the limiting silicon can be loaded by river runoff (Millero & Sohn inorganic nutrient supply in the photic zone (Barnes & 1997) as well as by the dissolution of particulate silicon Mann 1991). Silicon availability is a key factor in the present in the lithogenic and biogenic systems. In the *Email: [email protected] © Inter-Research 2003 · www.int-res.com 300 Aquat Microb Ecol 32: 299–306, 2003 sea the biogeochemical dissolution of this mineral is face interaction, or whether the response is due to the controlled by temperature, zooplankton grazing, di- variations in solubility of the different silica forms. It atom sinking (Nelson et al. 1996) and bacterial activity can be suggested that organic compounds, such as (Bidle & Azam 1999). In turn, the requirement of silicon ascorbate and/or catechol, are excreted by the cell and by diatom growth negatively affects the silicon balance partially mediate the solubility of the silica mineral in the marine ecosystem (Jézéquel et al. 2000). substrates in the medium. Studies on the uptake of dissolved silicon by diatom cells (Paasche 1973) have demonstrated that active silicon transport and the specific Si-dependent cell- MATERIALS AND METHODS division rate follow Michaelis-Menten kinetics (Ta- guchi et al. 1987, Gensemer et al. 1993). Silicon trans- Diatom cultures. The diatom species Skeletonema port inside the cell seems to be carrier-mediated, and costatum, Strain CBA1 (Centro Biologia Ambientale), in marine diatoms the ion-carrier can be Na+ depen- Cylindrotheca fusiformis, Strain CBA2 and Navicula dent (Bhattacharya & Volcani 1980) with metabolic sp., Strain CBA4, were maintained in f/2 enriched sea- energy supply (Lewin 1955, Azam et al. 1974). The car- water medium with added solution of sodium silicate rier function of silicon transport inside the cell can be (Guillard 1975) at 17 ± 1°C under a photon flux of mediated by the action of a protein: the silicon trans- 100 µE m–2 s–1 in a 14:10 h light:dark cycle. porter (SIT), present in marine and freshwater diatoms Si-free media. Artificial seawater, as ASPM (Artifi- (Hildebrand et al. 1998). Silica deposition and morpho- cial Seawater Provasoli-McLachlan) base (Guillard genesis of the cell wall seems to occur in a specialized 1975), was prepared to produce Si-free seawater. The compartment: the Silica Deposition Vesicle, known as above seawater was enriched as f/2, without added SDV (Vrieling et al. 1999). In the SDV, the acidic envi- dissolved silicon (dSi). The pH of the final medium was ronment contributes to the silicic acid polymerization 8.0. Bacteria-free medium was obtained by filtering into amorphous hydrated silica onto an organic tem- 100 ml aliquots of the medium through 0.2 µm cellu- plate (proteins, polysaccharides, lipids), that drives the lose acetate filters (Nalgene) into 550 ml sterile poly- frustule silicification and morphogenesis (Kröger et al. styrene flasks. 2000). Silica mineral substrates. Diatoms were grown in the Generally it is assumed that diatoms take up silicon presence of silicon supply from different silica mineral as undissociated silicic acid, Si(OH)4, at pH 8.0 from substrates. The silica minerals used have either a min- the marine environment (Lewin 1962, Del Amo & eral crystalline (natural quartz sand or hydrophilic and Brzezinski 1999). This is the dominant form, constitut- hydrophobic quartz dusts) or biogenic amorphous ori- ing 97% of total dissolved Si (dSi); the remainder is gin (diatomaceous earth and sponge spicules). Natural – essentially SiO(OH)3 . Despite numerous studies quartz sand (BDH) is comprised of spherical grains, 125 focused on characterizing the chemistry of silicon to 250 µm in diameter, with an internal texture com- uptake and deposition (Volcani 1981, Riedel & Nelson prised of aggregates of polygonal granuloblastic 1985, Brzezinski et al. 1990), the influence on diatom quartz. Hydrophilic quartz (Min-U-Sil 5, Berkeley growth of different particulate silica forms as a silicon Springs Plant, ITC) is a highly pure commercial dust source has not been studied so far. This issue is of high (SiO2, 98.7%) obtained by crushing quartz crystals: the interest in marine coastal water systems, where dis- grinding process causes the exposure of fresh surfaces solved silicon can be depleted during diatom blooms, with a high reactivity due to the presence of surface although both siliceous lithogenic material and bio- radicals and trace metal impurities. Such quartz parti- genic silica from settled diatoms are abundant. cles (mean diameter 1.7 µm) are characterized by a The aim of this work is to test the ability of 3 common high hydrophilicity due to the abundance of silanols at benthic and planktonic diatom species, Navicula sp. the surface, generated by the contact of water vapour and Cylindrotheca fusiformis and Skeletonema costa- with the newly formed surfaces (Fubini et al. 1999). Hy- tum, respectively, to take up silicon from various litho- drophobic quartz dust is obtained from the above hy- genic and biogenic amorphous sources, such as crys- drophilic quartz by heating it to 800 ± 1°C in a quartz talline quartz (natural quartz sand, hydrophilic and container connected to a vacuum line. This treatment hydrophobic quartz dusts), diatomaceous earth and causes the partial condensation at the surface of the hy- sponge spicules by measuring the diatom biomass drophilic silanols (Si-OH) into hydrophobic siloxanes increase and silicon uptake in culture conditions. Fur- (Si-O-Si). The degree of hydrophilicity of the 2 dusts thermore, a hypothetical chemical model was investi- have been evaluated by measuring the heat of water gated to explain whether the response of marine- vapour adsorption using the microcalorimetric tech- diatom biomass-increases toward the different nique (Bolis et al. 1992). Diatomaceous earth (Sigma- particulate silica substrates can be mediated by a sur- Aldrich, D-5384) was purified by acid washing and then Penna et al.: Diatom growth and silicon sources 301 calcined (SiO2, 97.5%), according to the manufacturer’s Navicula sp. and Skeletonema costatum were grown in instructions.