Efficiency of light utilization of Chlamydomonas reinhardtii under medium-duration light/dark cycles.

The light regime inside a photobioreactor is characterized by a light gradient with full (sun)light at the light-exposed surface and darkness in the interior of the bioreactor. Consequently, depending on the mixing characteristics, algae will be exposed to certain light/dark cycles. In this study the green alga Chlamydomonas reinhardtii was cultivated under five different light regimes: (1) continuous illumination; (2) a square-wave light/dark cycle with a light fraction (epsilon) of 0.5 and a duration (t(c)) of 6.1 s; (3) epsilon=0.5, t(c)=14.5 s; (4) epsilon=0.5, t(c)=24.3 s and (5) epsilon=0.8, t(c)=15.2 s. The biomass yield on light energy, protein per photons, decreased under light/dark cycles (epsilon=0. 5) in comparison to continuous light (CL), from 0.207 (CL) to 0.117-0.153 g mol(-1) (epsilon=0.5). Concomitantly, the maximal specific photosynthetic activity, oxygen production per protein, decreased from 0.94 (CL) to 0.64-0.66 g g(-1) h(-1) (epsilon=0.5). Also the quantum yield of photochemistry, yield of the conversion of light energy into chemical energy, decreased from 0.47 (CL) to 0. 23 (epsilon=0.5, t(c)=24.3 s). Apparently, C. reinhardtii is not able to maintain a high photosynthetic capacity under medium-duration light/dark cycles and since specific light absorption did not change, light utilization efficiency decreased in comparison to continuous illumination.

Application of light-emitting diodes in bioreactors: flashing light effects and energy economy in algal culture (Chlorella pyrenoidosa).

Light-emitting diodes (LEDs) were used as the sole light source in continuous culture of the green alga Chlorella pyrenoidosa. The LEDs applied show a peak emission at 659 nm with a half-power bandwidth of 30 nm. Selection of this wavelength range, which is optimal for excitation of chlorophylls a and b in their "red" absorption bands makes all photons emitted potentially suitable for photosynthesis. No need for additional supply of blue light was found. A standardized panel with 2 LEDs cm(-2) fully covered one side of the culture vessel. At standard voltage in continuous operation the light output of the diode panel appeared more than sufficient to reach maximal growth. Flash operation (5-mus pulse duration) enables potential use of higher operating voltages which may render up to three times more light output. Flat airlift fermentor-type continuous culture devices were used to estimate steady state growth rates of Chlorella pyrenoidosa as a function of the light flux (micromol photons x m(-2) x s(-1)) and the flashing frequency of the light-emitting diodes (which determines the duration of the dark "off" time between the 5-micros "on" pulses). At the fixed voltage and turbidostat setting applied a 20-kHz frequency, which equals dark periods of 45 mus, still permitted the maximum growth rate to become nearly reached. Lower frequencies fell short of sustaining the maximal growth rate. However, the light flux decrease resulting from lowering of the flash frequency appeared to reduce the observed growth rates less than in the case of a similar flux decrease with light originating from LEDs in continuous operation. Flash application also showed reduction of the quantum requirement for oxygen evolution at defined frequencies. The frequency domain of interest was between 2 and 14 kHz. LEDs may open interesting new perspectives for studies on optimization of mixing in mass algal culture via the possibility of separation of interests in the role of modulation on light energy conversion and saturation of nutrient supply. Use of flashing LEDs in indoor algal culture yielded a major gain in energy economy in comparison to luminescent light sources. (c) 1996 John Wiley & Sons, Inc.

Photobiology of bacteria.

The field of photobiology is concerned with the interactions between light and living matter. For Bacteria this interaction serves three recognisable physiological functions: provision of energy, protection against excess radiation and signalling (for motility and gene expression). The chemical structure of the primary light-absorbing components in biology (the chromophores of photoactive proteins) is surprisingly simple: tetrapyrroles, polyenes and derivatised aromats are the most abundant ones. The same is true for the photochemistry that is catalysed by these chromophores: this is limited to light-induced exciton- or electron-transfer and photoisomerization. The apoproteins surrounding the chromophores provide them with the required specificity to function in various aspects of photosynthesis, photorepair, photoprotection and photosignalling. Particularly in photosynthesis several of these processes have been resolved in great detail, for others at best only a physiological description can be given. In this contribution we discuss selected examples from various parts of the field of photobiology of Bacteria. Most examples have been taken from the purple bacteria and the cyanobacteria, with special emphasis on recently characterised signalling photoreceptors in Ectothiorhodospira halophila and in Fremyella diplosiphon.

The relationship of a prochlorophyte Prochlorothrix hollandica to green chloroplasts.

It is generally accepted that chloroplasts arose from one or more endosymbiotic events between an ancestral cyanobacterium and a eukaryote. Such an origin fits well in the case of the chloroplasts of rhodophytes that, like cyanobacteria, contain chlorophyll a and phycobilin pigments. The green chloroplasts from higher plants, green algae, and euglenoids however, contain chlorophyll b as well as chlorophyll a, and lack phycobilins. Consequently, it has been suggested that they arose independently of the rhodophyte chloroplasts, from an ancestral prokaryote containing that complement of pigments. The 'prochlorophytes' Prochloron didemni (an exosymbiont on didemnid ascidians) and Prochlorothrix hollandica (a recently discovered, free-living, filamentous form) have been suggested to be modern counterparts of the ancestor of the green chloroplasts because they are prokaryotes that also contain both chlorophylls a and b, and lack phycobilins. We report here a 16S rRNA-based phylogenetic analysis of P. hollandica. The organism is found to fall within the cyanobacterial line of descent, as do the green chloroplasts, but it is not a specific relative of green chloroplasts. Thus, similar pigment compositions do not necessarily reflect close evolutionary relationships.

Chlorophyll-protein composition of the thylakoid membrane from Prochlorothrix hollandica, a prokaryote containing chlorophyll b.

The chlorophyll-protein complexes of the thylakoid membrane from Prochlorothrix hollandica were identified following electrophoresis under nondenaturing conditions. Five complexes, CP1-CP5, were resolved and these green bands were analyzed by spectroscopic and immunological methods. CP1 contains the photosystem I (PSI) reaction center, as this complex quenched fluorescence at room temperature, and had a 77 K fluorescence emission peak at 717 nm. CP4 contains the major chlorophyll-a-binding proteins of the photosystem II (PSII) core, because this complex contained polypeptides which cross-reacted to antibodies raised against Chlamydomonas PSII proteins 5 and 6. Furthermore, fluorescence excitation studies at 77 K indicated that only a Chl a is bound to CP4. Complexes CP2, CP3 and CP5 contained functionally bound Chl a and b as judged by absorption spectroscopy at 20 degrees C and fluorescence excitation spectra at 77 K. CP2, CP3 and CP5 all contain polypeptides of 30-33 kDa which are immunologically distinct from the LHC-II complex of higher plant thylakoids.

Some aspects of the ecophysiology of cyanobacteria.

Fresh waters rich in nutrients often show mass development of cyanobacteria. The kind of cyanobacteria to be found depends on the properties of the lake. In non-stratified shallow lakes, the most common species Oscillatoria agardhii. In stratified lakes, cyanobacteria can be found in restricted zones of the deeper part of the lakes, and always possess cells with very active gas vacuoles. The most common species are Microcystis aeruginosa, Oscillatoria agardhii var isothrix, Oscillatoria "var red" and different Anabaena species. In lakes with prolonged nitrogen limitation, genera such as Anabaena and Aphanizomenon are common. Two important factors determine the distribution of these organisms: the light climate and the availability of nutrients. We have limited our discussion in this paper to the influence of the light climate. This influence of the light climate on growth can be determined in different ways. The influence of light intensity on the growth rate of Oscillatoria agardhii can be described using a Monod-like relation with light inhibition at light intensities above 40 W m-2. Studies with other species of cyanobacteria like Microcystis aeruginosa and Aphanizomenon flos-aquae support the assumption that most cyanobacteria living in fresh water are extremely sensitive to high light intensities. The energy balance of phototrophic growth can be described by the equation mu = qE . c - mue where mu is the specific growth rate, mue the specific maintenance energy rate and qE the specific light energy uptake rate; c represents the growth efficiency factor with which light energy is converted into cell material. In this equation, there are two factors which determine the growth of the organisms: ue and c. It was found that the value of mue of Oscillatoria was extremely low (mue approximately 0.001 h-1) compared with a eukaryotic organism (Scenedesmus: mue 0.008 h-1). The value of c was found to be dependent on the growth rate, but did not greatly differ from values found in eukaryotic algal cells. Data obtained from cultures of Oscillatoria grown under light-dark cycles, so that total energy input over 24 h was growth-limiting, showed good regulation of both the carbohydrate synthesis rate and the growth rate. In comparing the data of light-limited growth of cyanobacteria, the conclusion can be drawn that these organisms are favoured at low light intensities by their low specific maintenance energy rate and their pigment composition. A number of competition experiments support the conclusion that the light climate is the most important steering factor for the distribution of cyanobacteria in fresh waters.