The influence of a number of N-containing macromolecules on the polycondensation of silicic acid to form amorphous silica is studied by the combined use of 29Si NMR spectroscopy and the silicomolybdate test. Polymeric additives include poly(allylamine hydrochloride) (PAH), the poly(aminoamide) dendrimer of generation 1 (PAMAM-1), poly(ethyleneimine) (PEI), and poly(vinylpyrrolidone) (PVP). These studies were performed under biologically relevant conditions (pH 5.4 and 7.0) using aqueous solutions of isotope-labeled sodium [29Si]metasilicate as the precursor compound. It was found at pH 5.4 that all additives accelerate silicic acid polycondensation, except for PVP, which exerts a minor silicic acid stabilizing effect. At pH 7.0, polycondensation is much faster in the presence of PAMAM-1, PEI, and PAH. However, PVP significantly stabilizes mono- and disilicic acid. Silica precipitates were also studied by 29Si NMR spectroscopy. The effect observed for PVP is striking and indicates that the silicic acid polycondensation is slowed, although the oligomers are immobilized by the PVP polymer. In contrast, the charged PAH attracts the oligomeric species and enhances the silicic acid polycondensation.

Numerous publications report the existence of intracellular “Si” storage pools in diatoms representing intracellular concentrations of ca. 19–340 mM depending on the species. “Si” storage pools in diatom cells, if present, are supposed to accumulate “Si” for the production of new valves. The accumulated “Si” is then transported into the silicon deposition vesicle (SDV) where the new cell wall is synthesized. Interestingly, the reported concentrations of intracellular “Si” within the storage pool sometimes strongly exceed the solubility of monosilicic acid (ca. 2 mM pH <9). Various types of “Si” storage pools are discussed in the literature. It is usually assumed that “Si” species are stabilized by the association with some kind of organic material such as special proteins, thus forming a soluble silicic acid pools inside the cells. In an effort to mimic the above phenomenon, we have used a variety of neutral or cationic polymers that stabilize two soluble forms of “Si,” silicic and disilicic acids. These polymers include amine-terminated dendrimers, amine-containing linear polymers (with primary, secondary or tertiary amines), organic ammonium polymers, polyethylene glycol (PEG) neutral polymers, co-polymers (containing neutral and cationic parts) and phosphonium end-grafted PEG polymers. All the aforementioned polymeric entities affect the rate of silicic acid polycondensation and also the silica particle growth. Synergistic combinations of cationic and anionic polymers create in situ supramolecular assemblies that can also affect the condensation of silicic acid. Possible mechanisms for their effect on the condensation reaction are presented, with an eye towards their relevance to the “Si pools,” from a bioinspired/biomimetic point of view.

A methacrylate-based polyampholyte homopolymer was synthesized starting\ud from N-methacryloyl-L-histidine (MHist). This paper reports the inhibitory effects of poly-Nmethacryloyl-L-histidine\ud (poly-MHist) on the in vitro silicic acid condensation. In particular,\ud the ability of poly-MHist to retard silicic acid condensation in aqueous supersaturated\ud solutions at three pH values, 5.5, 7.0, and 8.5, is studied. The direct role of the imidazole ring\ud was confirmed by substantial changes in silicic acid stabilization efficiency based on the\ud following observations: (a) the protonation degree of the imidazole ring affects stabilization. At\ud a relatively low pH of 5.5, the imidazole is protonated and the entire polymer acquires a\ud zwitterionic character (−COO− is also present). Inhibitory activity increases considerably. In\ud contrast, at a high pH of 8.5, the imidazole ring is neutral and the polymer backbone is anionic\ud due to the presence of −COO− moieties. This results in total inactivity with respect to silicic\ud acid stabilization. (b) The use of a similar, but pH-insensitive derivative poly(N-acryloyl-Lphenylalanine)\ud (poly-PHE), which contains a phenylalanine instead of histidine, results in total\ud loss of silicic acid stabilization activity. Finally, poly-MHist also shows effects on colloidal silica\ud particle morphology. Increasing the poly-MHist concentration results in a reduced size of silica particles

Fish kills are events of massive and usually sudden mortality of fish populations in a localized area and are almost always causes of concern to human communities associated with the affected areas.Fish kills have been recorded worldwide in many different types of water bodies, including sea water, rivers, ponds and lakes (La & Cooke, 2011). There are many causes of fish kills with oxygen depletion and harmful algal blooms being the most commonly recorded.Causes related to anthropogenic activities are also often associated with these events and pollution and toxicity by hazardous wastes are usually suspected and investigated especially when these events occur in water bodies which are used by humans. Such an event was recorded in August 2019 at the artificial lake of the Aposelemis Dam which is the biggest dam in the island of Crete, Greece. The dam was constructed to provide drinking water to the urban areas of Norther Crete and particularly Heraklion city and Agios Nikolaos which are also major touristic centres.The fish kill was recorded on the 20th of August 2019, when hundreds of goldfish, Carassius auratus, were found dead in a pond of a side stream which provides water to the dam (File S1). The mortality was reported to the Institute of Marine Biology, Biotechnology and Aquaculture (IMBBC) of the Hellenic Centre for Marine Research by the Organization for the Development of Crete (OAK S.A.) responsible for the management of Aposelemis Projects (dam, reservoir, pipelines network and water treatment plant). IMBBC is in close vicinity to the affected area and researchers of Institute visited the site to take samples and investigate the causes of the incidence with the cooperation of the OAK S.A. scientific staff.In this paper, we present our findings regarding the causes of this fish kill and discuss its impacts on the ecosystem and on the local community.