Algal growth response to particle-bound orthophosphate and zinc

Dissolved nutrient concentrations in natural waters may at times be controlled by interactions between particulate and solution phases. Effects of Zn [O-l PM total Zn(II)] and orthophosphate (8-12 PM total P) additions on growth indices for the chlorophyte Selenastrum capricornutum Printz were examined in a synthetic growth medium containing 50 mg liter-l colloidal titania. Over the Zn(H) concentration range used, detrimental growth and yield effects were observed. Addition of P to a synthetic growth medium (S-3) increased stationary phase cell density, but had minimal effect on growth rate and duration of lag phase. Presence of TiO, particles in culture media significantly reduced Zn and P dissolved fractions. Although adsorbed Zn and P were less available to Selenastrum, desorption of both solutes increased their availability. Rapid desorption of Zn(I1) from TiO, particles served in effect to buffer Zn2+ free ion concentration, until Zn became partitioned primarily with the algal fraction as cell concentration approached stationary phase density. Although phosphate desorption from TiO, in nonbiological systems was negligible, Sefenastrum was able to scavenge some P initially adsorbed onto TiO,. Accurate primary productivity predictions in nature may therefore require an understanding of equilibrium and reaction rates involved in the partitioning of nutrients and toxic substances between dissolved and particulate phases. The ability of environmental models to effectively describe and predict natural events depends on accurate identification and quantification of the myriad processes that control these events. For example, it is apparent from previous work (Bencala 1983; Kuwabara et al. 1984) that physical transport processes (e.g. dispersion and diffusion) may interact with chemical processes (e.g. complexation, adsorption, and precipitation) as well as biological processes (e.g. nutrient uptake, exudate production, and seston transport) to regulate nutrient transport in aquatic bodies. The degree of importance of these interdependent processes may vary greatly with time and location (Bencala et al. 1983), so that understanding them is important in determining their environmental effects. Interactive effects of the essential elements zinc and phosphorus on algal growth have been observed and quantified in laboratory studies (Rana and Kumar 1974; Kuwabara 1985), including the interference of P metabolism by elevated Zn activity (Bates et al. 1982). It has been suggested that zinc ion activities in natural waters may control algal growth (Anderson et al. 1978; Kuwabara 198 1). Although dissolved Zn and P concentrations may be regulated by many chemical processes, it is clear from work in marine environments that many nutrient concentrations may be controlled by interactions with particles (Turekian 1977). In most aquatic toxicological studies in chemically defined media, however, synthetic chelators have been used to buffer micronutrient ion activities and avoid precipitation of metal oxides (Morel et al. 1979; Kuwabara 19 8 2). Because chemical speciation directly affects nutrient or toxicant bioavailability (Sunda and Guillard 1976; Anderson et al. 1978; Luoma 1983; Leland and Kuwabara 1984), the following environmentally significant questions arise: Does the presence of particles affect biological response? To what extent can suspended particulate matter buffer nutrient concentrations and do suspended particles behave like the synthetic chelators used in other algal growth studies? Given growth models describing biological response to nutrient concentrations in media without particles, and given chemical models describing sorption of nutrients on particles, can one then directly predict biological response to nutrient concentrations in media containing particles that affect nutrient bioavailability? In the present study we sought to answer these questions. Selenastrum capricornuturn Printz growth was examined in chemically defined media where dissolved Zn and