To evaluate the effect of increased expression of genes involved in flower pigmentation, additional dihydroflavonol-4-reductase (DFR) or chalcone synthase (CHS) genes were transferred to petunia. In most transformants, the increased expression had no measurable effect on floral pigmentation. Surprisingly, however, in up to 25% of the transformants, a reduced floral pigmentation, accompanied by a dramatic reduction of DFR or CHS gene expression, respectively, was observed. This phenomenon was obtained with both chimeric gene constructs and intact CHS genomic clones. The reduction in gene expression was independent of the promoter driving transcription of the transgene and involved both the endogenous gene and the homologous transgene. The gene-specific collapse in expression was obtained even after introduction of only a single gene copy. The similarity between the sense transformants and regulatory CHS mutants suggests that this mechanism of gene silencing may operate in naturally occurring regulatory circuits.

The phytoplankton community of eutrophic shallow lakes is often dominated by filamentous cyanobacteria of the family Oscillatoriaceae. In this paper we follow two independent approaches to show that this situation is likely to be one of two alternative stable states of the algal community. First we analyze patterns of cyanobacterial dominance observed in the field, and show that these patterns imply that the algal community is a hysteretic system with two alternative equilibria. Then, we construct a simple competition model to show that hysteresis should in fact be expected from differences in physiology between cyanobacteria and algae. The basic mechanism is that cyanobacteria are the superior competitors under conditions of low light, but also promote such conditions, as they can cause a higher turbidity per unit of phosphorus than other algae. This mechanism of hysteresis offers an explanation for the resistance of cyanobacteria dominance in shallow lakes to restoration efforts by means of nutrient reduction.

Enclosed outdoor photobioreactors need to be developed and designed for large-scale production of phototrophic microorganisms. Both light regime and photosynthetic efficiency were analyzed in characteristic examples of state-of-the-art pilot-scale photobioreactors. In this study it is shown that productivity of photobioreactors is determined by the light regime inside the bioreactors. In addition to light regime, oxygen accumulation and shear stress limit productivity in certain designs. In short light-path systems, high efficiencies, 10% to 20% based on photosynthetic active radiation (PAR 400 to 700 nm), can be reached at high biomass concentrations (>5 kg [dry weight] m(-3)). It is demonstrated, however, that these and other photobioreactor designs are poorly scalable (maximal unit size 0.1 to 10 m(3)), and/or not applicable for cultivation of monocultures. This is why a new photobioreactor design is proposed in which light capture is physically separated from photoautotrophic cultivation. This system can possibly be scaled to larger unit sizes, 10 to >100 m(3), and the reactor liquid as a whole is mixed and aerated. It is deduced that high photosynthetic efficiencies, 15% on a PAR-basis, can be achieved. Future designs from optical engineers should be used to collect, concentrate, and transport sunlight, followed by redistribution in a large-scale photobioreactor.

(16). Populations of these species are known to include both microcystin-producing and -nonproducing strains (25,26,33).

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m2 was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake. 2. The phytoplankton shifted from a cyanobacteria‐dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non‐buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy. 3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m–2) increased in the artificially mixed lake. 4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification. 5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.