Growth of microalgae adapted to high-light temperature conditions in different types of wastewater.

To obtain microalgae strains adapted to wastewater in the Mediterranean region, microalgae present in the Nile River were cultivated at both high-light temperature (HLT) and low-light temperature (LLT) conditions. It was found that the species which became dominant under HLT was chlorophyta of the genus Scenedesmus. In contrast, under LLT, bacillariophyta became dominant. The microalgae strain (Scenedesmus arcuatus) was subsequently isolated and cultivated in different types of primary treated wastewater under HLT and LLT conditions. The different types of primary treated wastewater were black water (BW), grey water (GW), and sewage water (SW). Growth rates reached during the exponential phase at HLT using GW, BW, and SW were between 0.38 and 0.4 (day-1). At LLT, 1.5-2.7 folds of lower growth rates were determined due to limitation by CO2. Removal of COD and inorganic N and P from sewage wastewater reached up to 88, 96, and 100%, respectively. Results thus indicate that microalgae adapted to the climate conditions can be efficiently used for COD removal and nutrient recovery from wastewater in the Mediterranean.

Responses of photosystems I and II of Acutodesmus obliquus to chemical stress caused by the use of recycled nutrients.

Nutrients derived from hydrothermal gasification of Acutodesmus obliquus were tested on its biological compatibility to support growth of the same microalgae. Photosynthetic parameters of photosystems I and II (PS I and PS II) were investigated to study physiological effects on the microalgal cell. The nutrients were collected as liquid residues. Dilutions of 1:500 showed no effect on both photosystems. Lower dilutions affected PS II initially and later also PS I. Cyclic electron flow around PS I compensated for loss of electrons due to partially inhibited PS II. The highest tested concentration of liquid residue erased any photosynthetic activity of PS II after 28 min and onwards. In contrast, PS I remained active. The results suggest that PS I is less susceptible than PS II and that the mixture of chemicals in the liquid residue did not directly affect PS I but PS II. The toxicants in the residues seemed to interfere with linear electron flow of PS II even though light-driven formation of radicals and subsequent damage to one of the photosystems can be excluded as demonstrated in darkness. Lowered photosynthetic activity of PS I during actinic irradiation was caused due to lack of supply of electrons from PS II. The cyclic electron flow might play a key role in delivering the energy needed to restore PS II activity and to biodegrade the toxicants when linear electron flow failed. These negative effects of liquid residue towards microalgal cells require a remediation step for direct application of the liquid residue to substitute commercial fertilizers in microalgal mass cultures.

The effect of discontinuous airlift mixing in outdoor flat panel photobioreactors on growth of Scenedesmus obliquus.

Discontinuous airlift mixing was realized by injecting pressured air at time intervals with a frequency between 0.033 and 0.25 Hz (at 80 kPa; i.e., every 4-30 s; valve opening time 800 ms) into outdoor flat panel photobioreactors ([Formula: see text]). This caused a flow velocity between 2 and 20 cm s(-1) of the culture medium within the photobioreactor and the mixing time was between 38 and 103.5 s, requiring 0.175-1.340 L(gas volume) L(photobioreactor volume)(-1) min(-1) pressured air. In order to detect the effect on growth of Scenedesmus obliquus during outdoor experiments and to be able to compare obtained results, a batch run with an airlift frequency of 0.25 Hz was simultaneously used as control. Growth at different airlift frequencies was measured by the increase of cell dry weight (CDW) during 3-5 days and biomass yield on light energy was calculated. With increasing airlift frequencies, growth increased from 52 to 91 % compared to the control. When CDW was at around 1.0-1.5 g L(-1), airlift frequency had no effect on growth, indicating that mass transfer gradients of nutrients and gas were not the limiting factors of growth. Above 1.5 g CDW L(-1), growth increased with increasing airlift frequency and light limitation for a single cell occurred. This effect was observed during low and high irradiance and it is concluded that a higher mean flow causes a better light distribution, resulting in an enhanced growth. Biomass productivity and demand of pressured air are correlated logarithmically, which enables to save mixing energy during cultivation.

Irradiance optimization of outdoor microalgal cultures using solar tracked photobioreactors.

Photosynthetic activity and temperature regulation of microalgal cultures (Chlorella vulgaris and Scenedesmus obliquus) under different irradiances controlled by a solar tracker and different cell densities were studied in outdoor flat panel photobioreactors. An automated process control unit regulated light and temperature as well as pH value and nutrient concentration in the culture medium. CO2 was supplied using flue gas from an attached combined block heat and power station. Photosynthetic activity was determined by pulse amplitude modulation fluorometry. Compared to the horizontal irradiance of 55 mol photons m(-2) d(-1) on a clear day, the solar tracked photobioreactors enabled a decrease and increase in the overall light absorption from 19 mol photons m(-2) d(-1) (by rotation out of direct irradiance) to 79 mol photons m(-2) d(-1) (following the position of the sun). At biomass concentrations below 1.1 g cell dry weight (CDW) L(-1), photoinhibition of about 35 % occurred at irradiances of ≥1,000 µmol photons m(-2) s(-1) photosynthetic active radiation (PAR). Using solar tracked photobioreactors, photoinhibition can be reduced and at optimum biomass concentration (≥2.3 g CDW L(-1)), the culture was irradiated up to 2,000 µmol photons m(-2) s(-1) to overcome light limitation with biomass yields of 0.7 g CDW mol photons(-1) and high photosynthetic activities indicated by an effective quantum yield of 0.68 and a maximum quantum yield of 0.80 (F v/F m). Overheating due to high irradiance was avoided by turning the PBR out of the sun or using a cooling system, which maintained the temperature close to the species-specific temperature optima.

Impact of green and blue-green light on the growth, pigment concentration, and fatty acid unsaturation in the microalga Monoraphidium braunii.

The spectral composition of light is an important factor for the metabolism of photosynthetic organisms. Several blue light-regulated metabolic processes have already been identified in the industrially relevant microalga Monoraphidium braunii. However, little is known about the spectral impact on this species' growth, fatty acid (FA), and pigment composition. In this study, M. braunii was cultivated under different light spectra (white light: 400-700 nm, blue light: 400-550 nm, green light: 450-600 nm, and red light: 580-700 nm) at 25°C for 96 h. The growth was monitored daily. Additionally, the FA composition, and pigment concentration was analyzed after 96 h. The highest biomass production was observed upon white light and red light irradiation. However, green light also led to comparably high biomass production, fueling the scientific debate about the contribution of weakly absorbed light wavelengths to microalgal biomass production. All light spectra (white, blue, and green) that comprised blue-green light (450-550 nm) led to a higher degree of FA unsaturation and a greater concentration of all identified pigments than red light. These results further contribute to the growing understanding that blue-green light is an essential trigger for maximized pigment concentration and FA unsaturation in green microalgae.

Blue-green light is required for a maximized fatty acid unsaturation and pigment concentration in the microalga Acutodesmus obliquus.

Blue-green light is known to maximize the degree of fatty acid (FA) unsaturation in microalgae. However, knowledge on the particular waveband responsible for this stimulation of FA desaturation and its impact on the pigment composition in microalgae remains limited. In this study, Acutodesmus obliquus was cultivated for 96 h at 15°C with different light spectra (380-700 nm, 470-700 nm, 520-700 nm, 600-700 nm, and dark controls). Growth was monitored daily, and qualitative characterization of the microalgal FA composition was achieved via gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI/MS). Additionally, a quantitative analysis of microalgal pigments was performed using high-performance liquid chromatography with diode array detection (HPLC-DAD). Spectra that included wavelengths between 470 and 520 nm led to a significantly higher percentage of the polyunsaturated fatty acids (PUFA) 18:3 and 16:4, compared to all other light conditions. However, no significant differences between the red light cultivations and the heterotrophic dark controls were observed for the FA 18:3 and 16:4. These results indicate, that exclusively the blue-green light waveband between 470 and 520 nm is responsible for a maximized FA unsaturation in A. obliquus. Furthermore, the growth and production of pigments were impaired if blue-green light (380-520 nm) was absent in the light spectrum. This knowledge can contribute to achieving a suitable microalgal pigment and FA composition for industrial purposes and must be considered in spectrally selective microalgae cultivation systems.

Differences in adaptation to light and temperature extremes of Chlorella sorokiniana strains isolated from a wastewater lagoon.

Presently, two Chlorella sorokiniana strains sampled during summer (CS-S) and winter (CS-W) from a maturation pond and isolated by dominance were studied on their behavior on temperature and light extremes in batch experiments. Although both strains showed no differences in their tolerance of temperatures up to 45 °C, the growth rates, pigment contents and fatty acid compositions in response to PAR at 700 and 1,500 µmol m-2sec-1 differed. CS-W was less affected by photoinhibition and maintained constantly high growth rates. High radiation resulted in both strains in an equivalent decrease of chlorophyll a and accessory pigments indicating that the latter did not function as a light filter. PUFAS (18:3 and 16:3) increased in CS-W at high radiation by > 60% and decreased in CS-S by 8 %. Results indicate that CS-W is highly favorable for mass cultivation particularly in outdoors, in which diurnal variations of solar radiation occur.

Growth and fatty acid composition of Acutodesmus obliquus under different light spectra and temperatures.

The combined impact of temperature and light spectra on the fatty acid (FA) composition in microalgae has been sparsely investigated. The aim of this study was to investigate the interactions of light and temperature on the FA composition in Acutodesmus obliquus. For this purpose, A. obliquus was cultivated with different temperatures (20, 30, and 35°C), as well as broad light spectra (blue, green, and red light). Growth and FA composition were monitored daily. Microalgal FA were extracted, and a qualitative characterization was done by gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI/MS). Compared to red light, green and blue light caused a higher percentage of the polyunsaturated fatty acids (PUFA) 16:4, 18:3, and 18:4, at all temperatures. The highest total percentage of these PUFA were observed at the lowest cultivation temperature and blue and green light. These data imply that a combination of lower temperatures and blue-green light (450-550 nm) positively influences the activity of specific FA-desaturases in A. obliquus. Additionally, a lower 16:1 trans/cis ratio was observed upon green and blue light treatment and lower cultivation temperatures. Remarkably, green light treatment resulted in a comparably high growth under all tested conditions. Therefore, a higher content of green light, compared to blue light might additionally lead to a higher biomass concentration. Microalgae cultivation with low temperatures and green light might therefore result in a suitable FA composition for the food industry and a comparably high biomass production.

Self-organization of dissolved organic matter to micelle-like microparticles in river water.

In aquatic systems, the concept of the 'microbial loop' is invoked to describe the conversion of dissolved organic matter to particulate organic matter by bacteria. This process mediates the transfer of energy and matter from dissolved organic matter to higher trophic levels, and therefore controls (together with primary production) the productivity of aquatic systems. Here we report experiments on laboratory incubations of sterile filtered river water in which we find that up to 25% of the dissolved organic carbon (DOC) aggregates abiotically to particles of diameter 0.4-0.8 micrometres, at rates similar to bacterial growth. Diffusion drives aggregation of low- to high-molecular-mass DOC and further to larger micelle-like microparticles. The chemical composition of these microparticles suggests their potential use as food by planktonic bacterivores. This pathway is apparent from differences in the stable carbon isotope compositions of picoplankton and the microparticles. A large fraction of dissolved organic matter might therefore be channelled through microparticles directly to higher trophic levels--bypassing the microbial loop--suggesting that current concepts of carbon conversion in aquatic systems require revision.