Robert W. Lovitt

Placing microalgae on the biofuels priority list: a review of the technological challenges

Microalgae provide various potential advantages for biofuel production when compared with ‘traditional’ crops. Specifically, large-scale microalgal culture need not compete for arable land, while in theory their productivity is greater. In consequence, there has been resurgence in interest and a proliferation of algae fuel projects. However, while on a theoretical basis, microalgae may produce between 10- and 100-fold more oil per acre, such capacities have not been validated on a commercial scale. We critically review current designs of algal culture facilities, including photobioreactors and open ponds, with regards to photosynthetic productivity and associated biomass and oil production and include an analysis of alternative approaches using models, balancing space needs, productivity and biomass concentrations, together with nutrient requirements. In the light of the current interest in synthetic genomics and genetic modifications, we also evaluate the options for potential metabolic engineering of the lipid biosynthesis pathways of microalgae. We conclude that although significant literature exists on microalgal growth and biochemistry, significantly more work needs to be undertaken to understand and potentially manipulate algal lipid metabolism. Furthermore, with regards to chemical upgrading of algal lipids and biomass, we describe alternative fuel synthesis routes, and discuss and evaluate the application of catalysts traditionally used for plant oils. Simulations that incorporate financial elements, along with fluid dynamics and algae growth models, are likely to be increasingly useful for predicting reactor design efficiency and life cycle analysis to determine the viability of the various options for large-scale culture. The greatest potential for cost reduction and increased yields most probably lies within closed or hybrid closed–open production systems.

Dielectric permittivity of microbial suspensions at radio frequencies: a novel method for the real-time estimation of microbial biomass

The radiofrequency dielectric properties of microbial suspensions are a direct and monotonic function of the radius and volume fraction of the particles constituting the suspended phase. Measurement of these properties therefore permits the direct estimation of microbial biomass during fermentations, in situ and in real time. The present approach to biomass estimation does not suffer significant interference from non-cellular particulate matter and retains its linearity at volume fractions two orders of magnitude greater than those at which the Beer-Lambert law fails.

Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes

Non-contact atomic force microscopy has been used to investigate the surface pore structure of Cyclopore and Anopore microfiltration membranes in air. Three Cyclopore membranes and three Anopore membranes of different pore sizes were studied. Excellent high resolution images were obtained. Analysis of the images gave quantitative information on the surface pore structure, in particular the pore size distribution. Non-contact AFM is an excellent means of obtaining such information for microfiltration membranes.

A new technique for membrane characterisation: direct measurement of the force of adhesion of a single particle using an atomic force microscope

Abstract An Atomic Force Microscope (AFM) has been used to quantify directly the adhesive force between a colloid probe and two polymeric ultrafiltration membranes of similar MWCO (4000 Da) but different materials (ES 404 and XP 117, PCI Membrane Systems (UK)). The colloid probe was made from a polystyrene sphere (diameter 11 μm) glued to a V shaped AFM cantilever. Measurements were made in 10 −2 M NaCl solution at pH 8. It was found that the adhesive force at the ES 404 membrane was more than five times greater than that at the XP 117 membrane. As it allows direct quantification of particle/membrane interactions, this technique should be invaluable in the development of new membrane materials and in the elucidation of process behaviour.

Application of atomic force microscopy to the study of micromechanical properties of biological materials

Atomic force microscopy (AFM) has been used to study the micromechanical properties of biological systems. Its unique ability to function both as an imaging device and force sensor with nanometer resolution in both gaseous and liquid environments has meant that AFM has provided unique insights into the mechanical behaviour of tissues, cells and single molecules. As a surface scanning device, AFM can map properties such as adhesion and the Youngs modulus of surfaces. As a force sensor and nanoindentor AFM can directly measure properties such as the Youngs modulus of surfaces or the binding forces of cells. As a stress-strain gauge AFM can study the stretching of single molecules or fibres and as a nanomanipulator it can dissect biological particles such as viruses or DNA strands. The present paper reviews key research that has demonstrated the versatility of AFM and how it can be exploited to study the micromechanical behaviour of biological materials.

Visualisation of an ultrafiltration membrane by non-contact atomic force microscopy at single pore resolution

Abstract Non-contact atomic force microscopy (AFM) has been used to investigate the furface pore structure of a polyethersulfone ultrafitration membrane of specified molecular weight cut off (MWCO) 25 000 (ES625, PCI Membrane Systems). Excellent images at up to single pore resolution were obtained. This is the first time that AFM images of a membrane at such high resolution have been presented. Analysis of the images gave a mean pore size of 5.1 nm with a standard deviation of 1.1 nm. The results have been compared to previously published studies of membranes of comparable MWCO using contact AFM and electron microscopy. Non-contact AFM is a powerful means of studying the surface pore characteristics of ultrafiltration membranes.

An atomic force microscopy study of the adhesion of a silica sphere to a silica surface—effects of surface cleaning

Abstract An atomic force microscope (AFM) has been used to quantify directly the adhesive interactions between a silica sphere and a planar silica surface. Electrostatic double-layer interactions have also been quantified through analysis of approach curves. The surfaces of the sphere and planar surface were treated prior to measurements either by ethanol washing or by plasma treatment. Adhesion forces were then measured in 0.01 M NaCl solutions at pH 3 and 8. The adhesion force did not vary greatly with pH for a given cleaning procedure. However, the magnitudes of the adhesion forces were substantially less for the plasma treated surfaces. The adhesion forces did not vary systematically with the loading force. Agreement of the adhesion measurements with theory (DLVO, using a non-retarded Hamaker constant based on the latest interpretation of spectroscopic data for water) was good for the ethanol treated surface at pH 3 — conditions where double layer interactions are negligible. However, the plasma treated surface at pH 3 showed adhesion an order of magnitude lower than calculated. In contrast, adhesion at pH 8 was in both cases greater than theoretical expectations, though the lower adhesion for the plasma treated surface was in quantitative agreement with the increased electrostatic double–layer interactions induced by plasma treatment. The results show that the adhesion of such surfaces is a complex phenomenon and that non-DLVO interactions probably play a substantial overall role.

Effect of CO partial pressure on cell-recycled continuous CO fermentation by Eubacterium limosum KIST612

Eubacterium limosum KIST612 was cultivated on carbon monoxide using a cell-recycled continuous fermentation system to produce organic acids. A bubble column reactor system (kLa=72 h−1) was used with a membrane to allow on-line products removal with cell recycle. When cell concentration reached 5.25 g l−1 in the reactor, the cell concentration did not increase at carbon monoxide (CO) partial pressures lower than 74 kPa though CO was consumed with the acidic products formation. At this stage, the overall CO mass transfer rate (kLa[C*−CL]) was lower than required for the maximum cell growth (qCOmaxX), but higher than that to meet the maintenance requirement (msX). When the CO mass transfer rate was maintained higher than maintenance requirement by increasing CO partial pressure, the cell concentration increased to 9.5 g l−1.

The gradostat: A bidirectional compound chemostat and its application in microbiological research

A multistage continuous culture system is described in which solutes are transferred between vessels in opposite directions simultaneously. The system, called a gradostat, produces opposing solute gradients and is a good laboratory model of many natural microbial ecosystems in which solute gradients are important. Theoretical predictions concerning solute transfer were confirmed under steady-state and non steady-state conditions, using a coloured dye. Paracoccus denitrificans grew anaerobically in the gradostat at the intersection between opposing gradients of succinate and nitrate. Opposing gradients of glucose and oxygen separated the growth of a Bacillus sp. (a facultative anaerobe) and Clostridium butyricum (an obligate anaerobe). Viable counts for both species fell exponentially away from their growth positions at the ends of the gradostat. The potential value of the gradostat and possible alternative conformations are discussed.

Atomic force microscope studies of membranes: force measurement and imaging in electrolyte solutions

Abstract An atomic force microscope has been used to study the electrical double layer interactions between a silicon tip (with an oxidised surface) and two polymeric membranes, one microfiltration (nominally 0.1 μm) and the other ultrfiltration (25 000 MWCO), in aqueous NaCl solutions. Force-distance curves were measured for the two membranes at four ionic strengths. The membranes were also imaged under the same conditions using electrical double layer repulsive forces of differing magnitudes —; “electrical double layer mode” imaging. Image analysis was used to determine surface pore size distributions. The force-distance curves, together with numerically calculated potential profiles at the entrance to a charged pore, allow an explanation and identification of the optimum imaging conditions. The best images were obtained at high ionic strength with the tip close to the membrane surface.