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Engineering analyses combined with experimental observations in horizontal tubular photobioreactors and vertical bubble columns are used to demonstrate the potential of pneumatically mixed vertical devices for large-scale outdoor culture of photosynthetic microorganisms. Whereas the horizontal tubular systems have been extensively investigated, their scalability is limited. Horizontal tubular photobioreactors and vertical bubble column type units differ substantially in many ways, particularly with respect to the surface–to–volume ratio, the amount of gas in dispersion, the gas–liquid mass transfer characteristics, the nature of the fluid movement and the internal irradiance levels. As illustrated for eicosapentaenoic acid production from the microalga Phaeodactylum tricornutum, a realistic commercial process cannot rely on horizontal tubular photobioreactor technology. In bubble columns, presence of gas bubbles generally enhances internal irradiance when the Sun is low on the horizon. Near solar noon, the bubbles diminish the internal column irradiance relative to the ungassed state. The optimal dimensions of vertical column photobioreactors are about 0.2 m diameter and 4 m column height. Parallel east–west oriented rows of such columns located at 36.8°N latitude need an optimal inter-row spacing of about 3.5 m. In vertical columns the biomass productivity varies substantially during the year: the peak productivity during summer may be several times greater than in the winter. This seasonal variation occurs also in horizontal tubular units, but is much less pronounced. Under identical conditions, the volumetric biomass productivity in a bubble column is ∼60% of that in a 0.06 m diameter horizontal tubular loop, but there is substantial scope for raising this value.

A well established analysis of energy dissipation in the riser, downcomer and the bottom sections of split-cylinder airlift bioreactors (aspect ratio=7.6 and 14.5; equal riser-to-downcomer cross-sectional area ratios of 1.0) was used to characterize shear rates in those systems for application to animal cell culture. Shear rates were evaluated for suspensions of typical microcarriers (loading =0–30 kg m −3 ; particle diameter=(150–300)×10 −6 m; density 1030–1050 kg m −3 ) encountered in anchorage-dependent cell culture and for microcarrier-free liquids. For the reactors tested, the highest shear rates were encountered in the bottom zone; the riser had lower shear rate values, while the downcomer was the most quiescent. The shear rates in various zones ranged over 0–12 000 s −1 for a riser superficial gas velocity range of 0–6.7×10 −3 m s −1 which is typical for cell culture. In all zones, the shear rates increased with increasing aeration rate. Shear rates declined with increasing loading of microcarriers, but were not substantially affected by the carrier diameter or density. Relative to the microcarrier free system, even small amounts of carriers (6 kg m −3 ) lowered the maximum prevailing shear rate to about 4000 s −1 . The shear rates were extremely sensitive to the length scale of the fluid eddies when the eddy length-to-carrier diameter ratio was less than or equal to unity. The results showed quantitatively how the shear rate in various zones of airlift reactors may be manipulated by modifications to operational and geometric parameters. The methodology presented allowed for characterization of shear rates in the bulk flow, unlike existing studies that provide information only on wall shear rates which are not particularly relevant to shear sensitive bioprocesses.

Growth and fatty acid composition of Isochrysis galbana ALII-4 in batch and semicontinuous outdoor culture in a closed tubular photobioreactor are reported. Specific growth rate at the exponential phase was 0.38 d −1 , higher than that obtained in previous indoor experiments. At biomass concentration above 2 g l −1 net biomass productivity obtained in three successive semicontinuous cultures was 0.32 g l −1 d −1 . A daily cyclic variation in the fatty acid profile was observed. From morning to afternoon, 14:0, 16:0 and 16:1 n 7 content increased, although it decreased during darkness renewing the cycle again with daylight. On the other hand, no daily variation was observed in polyunsaturated fatty acid content. The EPA generation rate during linear growth, with an average content of 2.56%, was 8.2 mg l −1 d −1 . The present study was conducted uninterruptedly in February and March, 1994, in Almeria, Spain (latitude 36° 50′ N, longitude 2° 27′ W).

Explants of the tropical sponge Pseudosuberites andrewsi were fed with the marine diatom Phaeodactylum tricornotum. The food was supplied either as intact algae or as a filtered crude extract. Growth (measured as an increase in underwater weight) was found in both experiments. The explants fed with intact algae increased to an average underwater weight of 255% of the initial weight in 45-60 days. The explants fed with crude extract increased to an average of 200% of the initial weight in 30 days. These results show that it is possible to grow a sponge using a single microorganism species as a food source. In addition, it was demonstrated that sponges are also capable of growing on non-particulate food. Therefore, this study is an important step forward towards the development of controlled, in vivo sponge cultures. (C) 2002 Elsevier Science B.V. All rights reserved.