The effect of mechanical agitation on the microalgae Phaeodactylum tricornutum and Porphyridium cruentum was investigated in aerated continuous cultures with and without the added shear protectant Pluronic F68. Damage to cells was quantified through a decrease in the steady state concentration of the biomass in the photobioreactor. For a given aeration rate, the steady state biomass concentration rose with increasing rate of mechanical agitation until an upper limit on agitation speed was reached. This maximum tolerable agitation speed depended on the microalgal species. Further increase in agitation speed caused a decline in the steady state concentration of the biomass. An impeller tip speed of >1.56 m s(-1) damaged P. tricornutum in aerated culture. In contrast, the damage threshold tip speed for P. cruentum was between 2.45 and 2.89 m s(-1). Mechanical agitation was not the direct cause of cell damage. Damage occurred because of the rupture of small gas bubbles at the surface of the culture, but mechanical agitation was instrumental in generating the bubbles that ultimately damaged the cells. Pluronic F68 protected the cells against damage and increased the steady state concentration of the biomass relative to operation without the additive. The protective effect of Pluronic was concentration-dependent over the concentration range of 0.01-0.10% w/v.

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The influence of solar irradiance and carbon dioxide molar fraction of injected CO2–air mixtures on the behavior of outdoor continuous cultures of the microalga Phaeodactylum tricornutum in tubular airlift photobioreactors was analyzed. Instantaneous solar irradiance, pH, dissolved oxygen, temperature, biomass concentration, and the mass flow rates of both the inlet and outlet oxygen and carbon with both the liquid and gas phases were measured. In addition, elemental analysis of the biomass and the cell‐free culture medium was performed. The oxygen production rate and carbon dioxide consumption rate increased hyperbolically with the incident solar irradiance on the reactor surface. Carbon losses showed a negative correlation with the daily variation of the carbon dioxide consumption rate. The maximum CO2 uptake efficiency was 63% of the CO2 supplied when the CO2 concentration in the gas supplied was 60% v/v. Carbon losses were >100% during the night, due to CO2 production by respiration, and hyperbolically decreased to values of 10% to 20% in the midday hours. An increase in the carbon fixed in the biomass with the solar cycle was observed. A slight daily decrease of carbon content of the cell‐free culture medium indicated the existence of carbon accumulation in the culture. A decrease in CO2 molar fraction in the injected gas had a double benefit: first, the biomass productivity of the system was enhanced from 2.05 to 2.47 g L−1 day−1 by reduction of CO2 inhibition and/or pH gradients; and second, the carbon losses during the daylight period were reduced by 60%. The fluid dynamics in the reactor also influenced the carbon losses: the higher the liquid flow rate the higher the carbon losses. By using a previous mass transfer model the experimental results were simulated and the usefulness of this method in the evaluation and scale‐up of tubular photobioreactors was established. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 67: 465–475, 2000.

B-phycoerythrin from the red alga Porphyridium cruentum was crystallized using the technique of capillary counter-diffusion. Crystals belonging to the space group R3 with almost identical unit cell constants and diffracting to 1.85 and 1.70 Å were obtained at pH values of 5 and 8, respectively. The most important difference between structures is the presence of the residue His88a in two different conformations at pH 8. This residue is placed next to the chromophore phycoerythrobilin PEB82a and the new conformation results in the relocation of the hydrogen-bond network and hydration around PEB82a, which probably contributes to the observed pH dependence of the optical spectrum associated with this chromophore.Comparison with the structures of B-phycoerythrin from other red algae shows differences in the conformation of the A-ring of the chromophore PEB139a. This conformational difference in B-phycoerythrin from P. cruentum enables the formation of several hydrogen bonds that connect PEB139a with the chromophore PEB158b at the (ab) 3 hexamer association interface. The possible influence of these structural differences on the optical spectrum and the ability of the protein to perform energy transfer are discussed, with the two pH-dependent conformations of His88a and PEB82a being proposed as representing critical structural features that are correlated with the pH dependence of the optical spectrum and transient optical states during energy transfer. DatabaseStructural data have been deposited in the Protein Data Bank under accession numbers 3V58 and 3V57. Structured digital abstract• B-phycoerythrin beta and B-phycoerythrin alpha bind by x-ray crystallography (View interaction)Abbreviations PDB, Protein Data Bank; PEB, phycoerythrobilin; PUB, phycourobilin.

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