Bacterially mediated removal of phosphorus and cycling of nitrate and sulfate in the waste stream of a "zero-discharge" recirculating mariculture system.

Simultaneous removal of nitrogen and phosphorus by microbial biofilters has been used in a variety of water treatment systems including treatment systems in aquaculture. In this study, phosphorus, nitrate and sulfate cycling in the anaerobic loop of a zero-discharge, recirculating mariculture system was investigated using detailed geochemical measurements in the sludge layer of the digestion basin. High concentrations of nitrate and sulfate, circulating in the overlying water (∼15 mM), were removed by microbial respiration in the sludge resulting in a sulfide accumulation of up to 3 mM. Modelling of the observed S and O isotopic ratios in the surface sludge suggested that, with time, major respiration processes shifted from heterotrophic nitrate and sulfate reduction to autotrophic nitrate reduction. The much higher inorganic P content of the sludge relative to the fish feces is attributed to conversion of organic P to authigenic apatite. This conclusion is supported by: (a) X-ray diffraction analyses, which pointed to an accumulation of a calcium phosphate mineral phase that was different from P phases found in the feces, (b) the calculation that the pore waters of the sludge were highly oversaturated with respect to hydroxyapatite (saturation index = 4.87) and (c) there was a decrease in phosphate (and in the Ca/Na molar ratio) in the pore waters simultaneous with an increase in ammonia showing there had to be an additional P removal process at the same time as the heterotrophic breakdown of organic matter.

Nature of phosphorus limitation in the ultraoligotrophic eastern Mediterranean.

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.

Detection and removal of dissolved hydrogen sulphide in flow-through systems via the sulphidation of hydrous iron (III) oxides.

A novel automated warning and removal system for hydrogen sulphide in aqueous flow-through systems has been developed based on the sulphidation of ferrihydrite sorbed to zeolite substrate. The system consists of a small flow-through reaction cartridge with photo-sensors positioned at the base. During the reaction, sulphide is initially oxidised to elemental sulphur by the ferrihydrite, and Fe2+ is subsequently released to solution. This Fe2+ then reacts with additional dissolved sulphide to form solid phase iron monosulphide. The colour change from orange ferrihydrite to black iron monosulphide is continuously monitored by the photo-sensors, which provide a rapid and reproducible response (via a voltage change) to pulses of sulphidic water. The response of the photo-sensors is linear with respect to inflowing sulphide concentration, while the most rapid response to dissolved sulphide occurs at a flow rate of approximately 200 ml min(-1) (equivalent to a hydraulic loading rate of 21 cm min(-1). The presence of phosphate in solution substantially decreases reaction rates due to adsorption to reactive surface sites. However, the response time of the photo-sensors remains sufficient to provide a rapid indication of sulphidic conditions even in systems with high concentrations of dissolved phosphate. The cartridge has the advantage of partially or completely removing sulphide (depending on flow rate and substrate mass) from an initial pulse of water. At the optimal flow rate for the successful use of the cartridge as a sulphide warning system (200 ml min(-1)), required substrate masses for the complete removal of dissolved sulphide (over the experimental range of 0-1000 microM) are relatively small (0.5-2 kg).

Dissolution of pyroxenes and amphiboles during weathering.

Augite, hypersthene, diopside, and hornblende all undergo dissolution during weathering by means of the formation, growth, and coalescence of distinctive, parallel, lens-shaped etch pits. Similar etch features can be produced if these minerals are treated in the laboratory with concentrated hydrofluoric acid plus hydrochloric acid. These pits most likely form at dislocation outcrops, and their shape and orientation are controlled primarily by the crystallography of the underlying mineral. The results are similar to those found for soil feldspars and suggest that silicate weathering, in general, takes place by selective etching and not by general attack of the surface with consequent rounding as necessiated by bulk diffusion-type weathering theories.