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.

In vivo manipulation of the xanthophyll cycle and the role of zeaxanthin in the protection against photodamage in the green alga Chlorella pyrenoidosa.

Chlorella pyrenoidosa was grown in steady-state continuous cultures in either high or low light. Samples of these cultures were incubated in darkness (violaxanthin state) or in saturating light (zeaxanthin state). These samples were kept in the respective preadapted states throughout the entire photodamage treatment. Photodamage involved exposure to single-turnover flashes fired at a low (non-actinic) frequency. The damage caused by the light stress thus applied was monitored by changes in photosynthetic properties and pigment composition. Cells preadapted in the light resisted photodamage better than those kept in darkness. The low light grown cells were more vulnerable to photodamage than the high light grown cells. Our experimental approach permitted the equilibria between the components that participate in the xanthophyll cycle to be set without addition of inhibitors. Regardless of the total amount of violaxanthin being present, its conversion to anthera- and zeaxanthin is a prerequisite for protection. The protection is most effective for photosystem II. It appeared that antheraxanthin accumulates as a result of photodamaging flashes provided that these are fired in the presence of background light, i.e. with zeaxanthin present. From this, it is newly derived that the xanthophyll cycle operates in full in the light, including epoxidation of zeaxanthin. The latter conversion was also demonstrated in vitro, via nonenzymatic oxygen-dependent turnover of zeaxanthin into violaxanthin.

Relationship between Methanogenic Cofactor Content and Maximum Specific Methanogenic Activity of Anaerobic Granular Sludges.

In this study we investigated whether a relationship exists between the methanogenic activity and the content of specific methanogenic cofactors of granular sludges cultured on different combinations of volatile fatty acids in upflow anaerobic sludge blanket or fluidized-bed reactors. Significant correlations were measured in both cases between the contents of coenzyme F(420)-2 or methanopterin and the maximum specific methanogenic activities on propionate, butyrate, and hydrogen, but not acetate. For both sludges the content of sarcinapterin appeared to be correlated with methanogenic activities on propionate, butyrate, and acetate, but not hydrogen. Similar correlations were measured with regard to the total content of coenzyme F(420)-4 and F(420)-5 in sludges from fluidized-bed reactors. The results indicate that the contents of specific methanogenic cofactors measured in anaerobic granular sludges can be used to estimate the hydrogenotrophic or acetotrophic methanogenic potential of these sludges.