Influences of external factors on the energy conversion and productivity of Scenedesmus Sp. in mass culture

Experiments about the influence of external factors on the energy conversion in mass cultures of Scenedesmus are described in this thesis. Several types of culture vessels were used in the laboratory as well as in the open. Demonstration models of Miele washing machines with a volume of 50 l were used for experiments in the open. In the laboratory the algae were cultivated in flasks on the rocking table, in washing machines under mercury light, in 'continuous culture' tubes and in small culture tubes with a volume of 100 ml in a thermostated bath. A mixture of 5 % CO 2 and air was continuously bubbled through the suspension, and the temperature was kept constant in all culture methods.

The growth rate of Scenedesmus was independent of the concentrations of SO42-and H 2 PO4-above a certain minimum. This was not the case with KNO 3 (Cf. Chapter 3). An optimum for growth was found in culture media containing KNO 3 at 5 mM in flasks on the rocking table. The average light intensity received by an individual cell was low in this case. The inhibition of growth by KNO 3 -concentrations higher than 5 mM could only partially be explained osmotically. No optimal KNO 3 -concentration was found in other types of culture vessels with a comparatively high average light intensity for an individual cell. It was concluded that tolerance for KNO 3 depended on the energy supply for the individual cell. Therefore, the incident radiation, the density of the algal suspension and shape and dimensions of the culture vessel are of importance.

An increase of the energy conversion of 30% was found when the rate of photosynthesis was light limited and when ammonium salts in well buffered media or urea was used instead of KNO 3 . It was concluded, that the yield at low light intensities can be improved with about 30 % by bypassing nitrate reduction.

The growth rate was slightly enhanced by the addition of yeast extract, whereas the growth rate was not influenced by soil extract in our experiments.

Section 4.2 describes the light distribution over the surfaces of the washing machines. The theoretical models for calculation of the total production per unit of surface, which are described in literature suppose that light only enters via the upper surface. The presence of horizontal and vertical surfaces in washing machines necessitated to adapt such models to the situation existing in these machines. The total global radiation measured in the field only was valid for a horizontal plane. To estimate light distribution for any complex surface is theoretically possible when the ratio between direct and diffuse light is known for every moment of the day. Calculation of the distribution of the incident radiation was simplified in our case in that the energy supply over the vertical surface was expressed as if it was an enlargment of the horizontal surface. This surface was called 'apparent horizontal surface'.

Growth rates depend on the incident energy under light-limited conditions. Algal production in washing machines with incident radiation via the horizontal surface only, was compared with that in washing machines with incident radiation over the total surface. When light limitation exists, the ratio between these productions measured over a period of several days enabled to estimate the 'apparent horizontal surface' for several parts of the growing season. Since the contribution of the vertical surface depended on the height of the sun, it is feasible that a minimum 'apparent horizontal surface' was obtained around the longest day of the year. Decrease of light energy in the vessels was calculated with LAMBERT- BEER'S law. The extinction coefficient k was determined experimentally. The stirring was insufficient to expect an appreciable increase in energy conversion. An increase in algal density at the start of the experiments decreased the energy conversion, which was atributed to respiratory losses (cf. Ch. 4.3).

A comparison between three series of experiments showed that respiration varied significantly under the different experimental conditions. Daily respiratory losses were determined by means Of WARBURG measurements and dry matter determinations. These losses ranged from 8.7 % to 18.6 % of the total amount of dry matter present.

The energy conversion corrected for shading by the buildings, reflection and absorption by the culture vessels was 6.0 % on the average for the period from March to October in the years 1962-1965. Energy conversion was 10.2% on the basis of incident light in July and August.

The efficiency of stirring was computed on the basis of the stirring velocity. A relative utilisation of 20 % of the maximum efficiency of the flash, i.e. 5 % on the basis of photosynthetically active radiation, was expected.

The energy conversion could be enchanced in washing machines under mercury light by decreasing the volumes of the suspension in the vessels (cf. section 4.3. 1). A more effective stirring was obtained in using smaller depths of layer at the same rate of stirring.

Influence of daylength on the energy conversion in mass cultures were examined in the four combinations: thick/thin layers; high/low cell densities (cf. Ch. 5). The energy conversion on the basis of incident light decreased with shorter daylengths under light saturated conditions in experiments lasting 24 hours, i.e. in thin/thick layers with low cell densities.

The culture as a whole was light limited in thick layers with high cell densities. A lower energy conversion as compared with continuous light was obtained only with a light/dark ratio of 8/16. This was atributed to respiratory losses.

A completely different situation occurred when cultures were exposed to a fixed light/dark treatment during several days. A strong discrepancy in light/ dark ratio between pretreatment and experiment caused a decrease in energy conversion in cultures in flasks on the rocking table. Optimum absolute growth rates and growth rates per hour light were found at light/dark ratios of 12/12 in 'continuous culture' tubes in which the algal suspension was kept at a constant optical density.

It was concluded from the experiments described in Chapter 5 that cells pretreated in continuous light exhibited a simple reaction pattern during the first 24 hours after the pretreatment. An increase in energy conversion on the basis of incident light was found when the cell concentration at the start of the experiment was increased, at relatively high average light intensities for the individual cell. A decrease in energy conversion can be expected in thick layers and high cell concentrations at very short daylengths (i.e. L/D = 8/16), which is caused by respiratory losses. Longer lasting pretreatments with intercalation of dark periods were most favourable when the daylength approximated a light/dark ratio which is necessary for synchronisation; i.e. L/D = 12/12, causing partial synchronisation. A temperature optimum of 35°C was found for photosynthesis, respiration and growth when cells were precultivated at 30°C (cf. Ch. 6). No shift in temperature optimum was obtained with shorter daylengths.

Higher energy conversion was found in cultures receiving variable night temperatures, when non-buffered culture solutions containing KNO 3 were utilised. Culture solutions containing NH 4 NO 3 did not show differences in energy conversion between the two temperature treatments, whereas a buffered culture solution containing KNO 3 showed highest efficiencies in the constant temperature series. The increase in energy conversion at variable night temperatures in non- buffered culture solutions was attributed to a higher amount of total available CO 2 , or soluble phosphate, as compared with cultures kept at a constant temperature. When phosphate or CO 2 do not limit growth, as is the case in buffered culture solutions, a low night temperature probably inhibits cell division which reduces the efficiency.

The results of the experiments described so far are compared with literature in Chapter 7. Considering the energy conversion values obtained in our experiments and the efficiencies measured in agricultural crops with a closed crop surface, it is concluded that algae in dense packing contain a comparable amount of chlorophyll per unit of surface. The energy conversion is about the same in algal cultures and in crops with a closed canopy. An increase in efficiency would theoretically be possible in higher plants if the CO 2 concentration would have been increased.

At last a possible application of algae as food plants or as purifiers of waste waters is discussed.