The rising concern about the potential toxicity of synthetic gold nanoparticles (AuNPs) in aquatic environments requires a rigorous estimation of physico-chemical parameters of reactions between AuNPs and major freshwater microorganisms. This study addresses the interaction of 10-nm size, positively charged AuNPs with periphytic freshwater diatoms (Eolimna minima). The adsorption experiments on viable cells were performed in 10 mM NaCl and 5 mM NaCl + 5 mM NaHCO₃ solution at a variable pH (3–10), at an AuNPs concentration from 1 µg/L to 10,000 µg/L, and an exposure time from a few minutes to 55 days. Three types of experiments, adsorption as a function of time (kinetics), pH-dependent adsorption edge, and constant-pH “Langmuirian” type isotherms, were conducted. In addition, long-term interactions (days to weeks) of live diatoms (under light and in the darkness) were performed. The adsorption was maximal at a pH from 3 to 6 and sizably decreased at a pH of 6 to 10. Results of adsorption experiments were modeled using a second order kinetic model, a Linear Programming Model, Freundlich isotherm, and a ligand binding equation for one site competition. The adsorption of AuNPs(+) most likely occurred on negatively-charged surface sites of diatom cell walls such as carboxylates or phosphorylates, similar to previously studied metal cations. Under light exposure, the AuNPs were stabilized in aqueous solution in the presence of live cells, probably due to the production of exometabolites by diatoms. The adsorbed amount of AuNPs decreased after several days of reaction, suggesting some AuNPs desorption. In the darkness, the adsorption and assimilation were stronger than under light. Overall, the behavior of positively charged AuNPs at the diatom–aqueous solution interface is similar to that of metal cations, but the affinity of aqueous AuNPs to cell exometabolites is higher, which leads to the stabilization of nanoparticles in solution in the presence of diatoms and their exudates. During photosynthetic activity and the pH rising above 9 in the vicinity of diatom cells, the adsorption of AuNPs strongly decreases, which indicates a decreasing potential toxicity of AuNPs for photosynthesizing cells. The present study demonstrates the efficiency of a thermodynamic and kinetic approach for understanding gold nanoparticles interaction with aquatic freshwater peryphytic microorganisms.

Riverine particulates dominate the transport of vital nutrients such as Si, Fe or P to the ocean margins, where they may increase primary production by acting as slow-release fertilizer. Furthermore, the supply of particulate surface area to the ocean is considered to be a major control of organic carbon burial. Taken together, these observations suggest a close link between the supply of riverine particulate material and the organic carbon cycle. To explore this link, we conducted microcosm experiments to measure the growth of the marine diatom Thalassiosira weissflogii in the presence and absence of different types and concentrations of riverine particulate material. Results demonstrate a strong positive effect of riverine particulate material on diatom growth with increased total diatom concentrations and slowed post-exponential death rates with increasing particulate concentration. Moreover, SEM and optical microscope investigations confirm that riverine particulates facilitate organic carbon burial through their role in the aggregation and sedimentation of phytoplankton. The supply of riverine particulate material has been shown to be markedly climate sensitive with their fluxes increasing dramatically with increasing global temperature and runoff. This pronounced climate sensitivity implies that riverine particulates contribute substantially in regulating atmospheric CO₂ concentrations through their role in the organic carbon cycle.

Elasticity is a trait of biofilm physiognomy which relates to cell clustering and can be measured by means of an electrochemical assay based on rotating disc electrode (RDE). This study aimed at testing the hypothesis according to which exposure of phototrophic biofilm to toxicant could reduce its elasticity. We compared biofilms developed for 21 days, in four sets of 6 replicated experimental units, in absence and presence of isoproturon at two concentrations of the inoculating suspension of biofilm, 103 and 104 diatom cell mL-1. Biofilm thickness and elasticity were measured based on RDE assay, bacterial and diatom density were measured by microscope-based numerations.Very thin biofilms (< 10 µm) were obtained as compared with a previous study. This might be linked with the way we selected the initial biofilm providing the suspension and the way we developed its growth. The biofilm elasticity mean values in the presence of isoproturon was quasi twice lower (60 ± 10 and 60 ± 41 µm rpm0.5) than the treatment without isoproturon (138 ± 93 and 115 ± 104 µm rpm0.5), for initial biofilm concentration of 103 and 104 respectively, but there was no significant difference between the mean values of each treatment. Nevertheless, the present preliminary study demonstrated the feasibility of an experiment dedicated to assessing biofilm elasticity changes as a response to toxicant exposure.

Riverine particulates dominate the transport of vital nutrients like Si, Fe or P to the ocean mar-gins, where they may increase primary production by acting as slow release fertilizer. Further-more, the supply of particulate surface area to the ocean is considered to be a major control or-ganic carbon burial. Taken together, these observations suggest a close link between the supply of riverine particulate material and the organic carbon cycle. To explore this link, we conducted microcosm experiments to measure the growth of the marine diatom Thalassiosira weissflogii in the presence and absence of different types and concentrations of riverine particulate material. Results demonstrate a strong positive effect of riverine particulate material on diatom growth with increased total diatom concentrations and slowed post-exponential death rates with in-creasing particulate concentration. Moreover, SEM and optical microscope investigations con-firm that riverine particulates facilitates organic carbon burial through their role in the aggrega-tion and sedimentation of phytoplankton. The supply of riverine particulate material has been shown to be markedly climate sensitive with their fluxes increasing dramatically with increas-ing global temperature and runoff. This pronounced climate sensitivity implies that riverine particulates contribute substantially in regulating atmospheric CO2 concentrations through their role in the organic carbon cycle.