A fresh look at the evolution and diversification of photochemical reaction centers

In this review, I reexamine the origin and diversification of photochemical reaction centers based on the known phylogenetic relations of the core subunits, and with the aid of sequence and structural alignments. I show, for example, that the protein folds at the C-terminus of the D1 and D2 subunits of Photosystem II, which are essential for the coordination of the water-oxidizing complex, were already in place in the most ancestral Type II reaction center subunit. I then evaluate the evolution of reaction centers in the context of the rise and expansion of the different groups of bacteria based on recent large-scale phylogenetic analyses. I find that the Heliobacteriaceae family of Firmicutes appears to be the earliest branching of the known groups of phototrophic bacteria; however, the origin of photochemical reaction centers and chlorophyll synthesis cannot be placed in this group. Moreover, it becomes evident that the Acidobacteria and the Proteobacteria shared a more recent common phototrophic ancestor, and this is also likely for the Chloroflexi and the Cyanobacteria. Finally, I argue that the discrepancies among the phylogenies of the reaction center proteins, chlorophyll synthesis enzymes, and the species tree of bacteria are best explained if both types of photochemical reaction centers evolved before the diversification of the known phyla of phototrophic bacteria. The primordial phototrophic ancestor must have had both Type I and Type II reaction centers

Early detection of preferential channeling in reverse electrodialysis

Membrane applications often experience fouling, which prevent uniform flow distribution through the feed water compartments, i.e. preferential channeling may occur. This research shows the effect of preferential channeling on energy generation from mixing salt water and fresh water using reverse electrodialysis (RED). The experimentally obtained power density, electrical resistance and pressure drop are evaluated for artificially controlled preferential channeling. The obtained power density decreases significantly when part of the feed water compartment is inaccessible for flow; a blockage of only 10% of the feed water compartments decreases the net power density by approximately 20%. When 80% of the feed water channels is inaccessible, the net power densities are only marginally positive. This decrease in power density is due to an increase in non-ohmic resistance, which is related to the concentration changes in the feed water compartments when ions are transported from the seawater to the river water side. Chronopotentiometric measurements show that the typical response time to establish a non-ohmic overpotential is an even more sensitive and easily scalable parameter to detect preferential channeling (and the presence of possible fouling) in an early stage. In practical applications, this response time can thus be used as an indicator for preferential channeling and serve to decide on and selectively apply cleaning in RED and other applications

Fouling in reverse electrodialysis under natural conditions

Renewable energy can be generated from mixing salt water and fresh water in reverse electrodialysis. The potential for energy generation from mixing seawater and river water is enormous. To investigate the effect of fouling when such natural feed waters are used, the performance of three different setups for reverse electrodialysis was evaluated for 25 days using seawater and river water as feed water, with no other (pre-)treatment than a 20 μm filter. Due to the absence of other anti-fouling treatments, a mixture of fouling is observed on the membranes, composed of remnants of diatoms, clay minerals, organic fouling and scaling. The fouling type was dependent on the different membrane types. The anion exchange membranes attract mainly diatoms and clay minerals, whereas scaling was only found on the cation exchange membranes. As a reference, plastic sheets without charge were used, which results in significant cleaner surfaces. Additionally, the setups without spacers in between the membranes (i.e. profiled membranes) appear significant less sensitive to fouling. This was quantified by the pressure drop over the feed waters and the power density obtained from the membrane piles. The pressure drop increases four times slower and the power density remains higher when profiled membranes are use instead of flat membranes with spacers. Although the obtained power density reduced with approximately 40% in the first day under these conditions, caused by organic fouling, several strategies are available to maintain a high power output using reverse electrodialysi

Periodic feedwater reversal and air sparging as antifouling strategies in reverse electrodialysis

\u3cp\u3eRenewable energy can be generated using natural streams of seawater and river water in reverse electrodialysis (RED). The potential for electricity production of this technology is huge, but fouling of the membranes and the membrane stack reduces the potential for large scale applications. This research shows that, without any specific antifouling strategies, the power density decreases in the first 4 h of operation to 40% of the originally obtained power density. It slowly decreases further in the remaining 67 days of operation. Using antifouling strategies, a significantly higher power density can be maintained. Periodically switching the feedwaters (i.e., changing seawater for river water and vice versa) generates the highest power density in the first hours of operation, probably due to a removal of multivalent ions and organic foulants from the membrane when the electrical current reverses. In the long term, colloidal fouling is observed in the stack without treatment and the stack with periodic feedwater switching, and preferential channeling is observed in the latter. This decreases the power density further. This decrease in power density is partly reversible. Only a stack with periodic air sparging has a minimum of colloidal fouling, resulting in a higher power density in the long term. A combination of the discussed antifouling strategies, together with the use of monovalent selective membranes, is recommended to maintain a high power density in RED in short-term and long-term operations.\u3c/p\u3

Membrane resistance:The effect of salinity gradients over a cation exchange membrane

\u3cp\u3eIon exchange membranes (IEMs) are used for selective transport of ions between two solutions. These solutions are often different in concentration or composition. The membrane resistance (R\u3csub\u3eM\u3c/sub\u3e) is an important parameter affecting power consumption or power production in electrodialytic processes. In contrast to real applications, often R\u3csub\u3eM\u3c/sub\u3e is determined while using a standard 0.5M NaCl external solution. It is known that R\u3csub\u3eM\u3c/sub\u3e increases with decreasing concentration. However, the detailed effect of a salinity gradient present over an IEM on R\u3csub\u3eM\u3c/sub\u3e was not known, and is studied here using alternating and direct current. NaCl solution concentrations varied from 0.01 to 1.1M. The results show that R\u3csub\u3eM\u3c/sub\u3e is mainly determined by the lowest external concentration. R\u3csub\u3eM\u3c/sub\u3e can be considered as two resistors in series i.e. a gel phase (concentration independent) and an ionic solution phase (concentration dependent). The membrane conductivity is limited by the conductivity of the ionic solution when the external concentration, c\u3csub\u3eext\u3c/sub\u3eext≥0.3M, then differences of R\u3csub\u3eM\u3c/sub\u3e are small. A good approximation of experimentally determined R\u3csub\u3eM\u3c/sub\u3e can be obtained. The internal ion concentration profile is a key factor in modeling R\u3csub\u3eM\u3c/sub\u3e.\u3c/p\u3