Photic Volume in Photobioreactors Supporting Ultrahigh Population Densities of the Photoautotroph Spirulina platensis

Potential application of a newly isolated microalga Desmodesmus sp. GXU-A4 for recycling Molasses vinasse

The development of cost-effective and energy-efficient technologies for the stabilization of organic wastewater by microalgae has been essential and sought after. In the current study, GXU-A4 was isolated from an aerobic tank treating molasses vinasse (MV) and identified as Desmodesmus sp. based on its morphology, rbcL, and ITS sequences. It exhibited good growth with a high lipid content and chemical oxygen demand (COD) when grown using MV and the anaerobic digestate of MV (ADMV) as the growth medium. Three distinct COD concentrations for wastewater were established. Accordingly, GXU-A4 removed more than 90% of the COD from molasses vinasse (MV1, MV2, and MV3) with initial COD concentrations of 1193 mgL^-1, 2100 mgL^-1, and 3180 mgL^-1, respectively. MV1 attained the highest COD and color removal rates of 92.48% and 64.63%, respectively, and accumulated 47.32% DW (dry weight) of lipids and 32.62% DW of carbohydrates, respectively. Moreover, GXU-A4 grew rapidly in anaerobic digestate of MV (ADMV1, ADMV2, and ADMV3) with initial COD concentrations of 1433 mgL^-1, 2567 mgL^-1, and 3293 mgL^-1, respectively. Under ADMV3 conditions, the highest biomass reached 13.81 g L^-1 and accumulated 27.43% DW of lipids and 38.70% DW of carbohydrates, respectively. Meanwhile, the removal rates of NH₄-N and chroma in ADMV3 reached 91.10% and 47.89%, respectively, significantly reducing the concentration of ammonia nitrogen and color in ADMV. Thus, the results demonstrate that GXU-A4 has a high fouling tolerance, a rapid growth rate in MV and ADMV, the ability to achieve biomass accumulation and nutrient removal from wastewater, and a high potential for MV recycling.

Microalgae cultivation in wastewater effluent from tilapia culture pond for enhanced bioethanol production

The large number of wastewaters are generated because of the various production processes. Vegetable and fish processing can be considered an important industry for wastewater generation. The essential method for completing this waste is to digest the organic matter using anaerobic digestion followed by aerobic wastewater treatment processes; however, wastewater from tilapia culture pond retains considerable quantities of inorganic substances, particularly nutrients like nitrogen and phosphorus. The optimal conditions for cultivating Chlorella vulgaris from wastewater treatment effluent from tilapia culture pond were investigated in this study. The appropriate conditions were found to be 10% initial stock suspension, 20 cm depth, and 12 days of culture conditions. C. vulgaris had an optical density of 0.649, a cell density of 17.68 × 10⁵ cells/mL, and biomass of 0.376 ± 94.21 mg/L after cultivation. Discharged wastewater from the fishpond was utilized for the improved growth of microalgae and obtained biomass was used for bioethanol production. This study verified that fishpond wastewater is the best source of nutrients for algal mass production and biofuel applications.

Model based optimization of high cell density cultivation of nitrogen-fixing cyanobacteria

In the present study, fed-batch cultivation of Cyanothece sp. ATCC 51142, a known hydrogen producer, was optimized for maximizing biomass production. Decline in growth of this organism in dense cultures was attributed to increased substrate consumption for maintenance and respiration, and photolimitation due to self shading. A model incorporating these aspects was developed, and by using control vector parameterization (CVP), substrate feeding recipe was optimized to achieve 12-fold higher biomass concentration. The optimization results were verified experimentally on shake flask and bioreactor. The latter resulted in greater exponential growth rate possibly by overcoming photolimitation by simulating flashing light effect. Such a strategy can be readily applied for mixotrophic cultivation of cyanobacterial cultures in the first stage followed by photoautotrophic growth at the production stage.

Stress-induced changes in optical properties, pigment and fatty acid content of Nannochloropsis sp.: implications for non-destructive assay of total fatty acids

In order to develop a practical approach for fast and non-destructive assay of total fatty acid (TFA) and pigments in the biomass of the marine microalga Nannochloropsis sp. changes in TFA, chlorophyll, and carotenoid contents were monitored in parallel with the cell suspension absorbance. The experiments were conducted with the cultures grown under normal (complete nutrient f/2 medium at 75 μmol PAR photons/(m(2) s)) or stressful (nitrogen-lacking media at 350 μmol PAR photons/(m(2) s)) conditions. The reliable measurement of the cell suspension absorbance using a spectrophotometer without integrating sphere was achieved by deposition of cells on glass-fiber filters in the chlorophyll content range of 3-13 mg/L. Under stressful conditions, a 30-50% decline in biomass and chlorophyll, retention of carotenoids and a build-up of TFA (15-45 % of dry weight) were recorded. Spectral regions sensitive to widely ranging changes in carotenoid-to-chlorophyll ratio and correlated changes of TFA content were revealed. Employing the tight inter-correlation of stress-induced changes in lipid metabolism and rearrangement of the pigment apparatus, the spectral indices were constructed for non-destructive assessment of carotenoid-to-chlorophyll ratio (range 0.3-0.6; root mean square error (RMSE) = 0.03; r (2) = 0.93) as well as TFA content of Nannochloropsis sp. biomass (range 5.0-45%; RMSE = 3.23 %; r (2) = 0.89) in the broad band 400-550 nm normalized to that in chlorophyll absorption band (centered at 678 nm). The findings are discussed in the context of real-time monitoring of the TFA accumulation by Nannochloropsis cultures under stressful conditions.

Sustainable, high-yielding outdoor mass cultures of Chaetoceros muelleri var. subsalsum and Isochrysis galbana in vertical plate reactors

Continuous cultures of Chaetoceros muelleri and Isochrysis galbana were grown outdoors in flat plate-glass reactors in which light-path length (LPL) varied from 5 to 30 cm. High daily productivity (13 to 16 g cell mass per square meter of irradiated reactor surface) for long periods of time was obtained in reactors in which the optical path as well as cell density were optimized. 'Twenty centimeters was the optimal LPL, yielding the highest areal productivity of cell mass (g m(-2)d(-1)), eicosapentaenoic acid, and docosahexaenoic acid, which was identical with that previously found for polysaccharide production of Porphyridium and not far from the optimal LPL affecting maximal productivity in Nannochloropsis species. Relating the energy impinging on a given reactor surface area to the appropriate number of cells showed that the most efficient light dose per cell, obtained with the 20-cm LPL reactor, was approximately 2.5 times lower than the light dose available per cell in the 5-cm LPL reactor, in which a significant decline in areal cell density accompanied the lowest areal output of cell mass. The most effective harvesting regimen was in the range of 10% to 15% of culture volume harvested daily and replaced with fresh growth medium, resulting in a sustainable culture density of 24 x 10(6) and 28 x 10(6) cells/ml of C. muelleri and I. galbana, respectively.

Enclosed outdoor photobioreactors: light regime, photosynthetic efficiency, scale-up, and future prospects

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.

Optimization of a flat plate glass reactor for mass production of Nannochloropsis sp. outdoors

The relationships between areal (g m(-2) per day) and volumetric (g l(-1) per day) productivity of Nannochloropsis sp. as affected by the light-path (ranging from 1.3 to 17.0 cm) of a vertical flat plate glass photobioreactor were elucidated. In general, the shorter the length of the light-path (LP), the smaller the areal volume and the higher the volumetric productivity. The areal productivity in relation to the light-path, in contrast, yielded an optimum curve, the highest areal productivity was obtained in a 10 cm LP reactor, which is regarded, therefore, optimal for mass production of Nannochloropsis. An attempt was made to identify criteria by which to assess the efficiency of a photobioreactor in utilizing strong incident energy. Two basic factors which relate to reactor efficiency and its cost-effectiveness have been defined as (a) the total illuminated surface required to produce a set quantity of product and (b) culture volume required to produce that quantity. As a general guide line, the lower these values are, the more efficient and cost-effective the reactor would be. An interesting feature of this analysis rests with the fact that an open raceways is as effective in productivity per illuminated area as a flat-plate reactor with an optimal light path, both cultivation systems requiring ca. 85 m(2) of illuminated surface to produce 1 kg dry cell mass of Nannochloropsis sp. per day. The difference in light utilization efficiency between the two very different production systems involves three aspects - first, the open raceway requires ca. 6 times greater volume than the 10 cm flat plate reactor to produce the same quantity of cell-mass. Second, the total ground area (i.e. including the ground area between reactors) for the vertical flat plate reactor is less than one half of that occupied by an open raceway, indicating the former is more efficient, photosynthetically, compared with the latter. Finally, the harvested cell density is close to one order of magnitude higher in the flat plate reactor, which carries economic significance. The advantage of vertical lamination of photoautotrophic cells provided by vertical plate reactors, is thereby clearly seen. The optimal population density (i.e. which results in the highest areal productivity) in the 10 cm plate reactor was obtained by a daily harvest of 10% of culture volume, yielding an annual average of ca. 12.1 g dry wt. m(-2) per day (on the basis of the overall illuminated reactor surfaces, i.e. front and back) or 240 mg l(-1) per day.

On recording the true absorption spectrum and the scattering spectrum of a turbid sample: application to cell suspensions of the cyanobacterium Anabaena variabilis

An integrating sphere is often used for recording the absorption spectrum of a turbid sample. If the sample is placed inside the sphere, scattering losses are eliminated, but the recorded spectrum suffers from other distortions. These distortions can be avoided by positioning the sample outside the sphere; but, since some of the scattered light escapes the detector, the recorded spectrum suffers from residual scattering losses. A method proposed by Latimer and Eubanks more than 30 years ago (Arch. Biochem. Biophys. 98 (1962) 274), is put to a quantitative examination, which has shown that one can obtain, by recording two spectra at different distances from the sphere, not only the true absorption spectrum but also the scattering spectra of the sample. Conditions for the validity of the basic assumption underlying the method are investigated by examining suspensions containing various concentrations of cells of the cyanobacterium Anabaena variabilis, and it is shown that the calculated absorbance is proportional to the number density of the cells. The application of the method for quantitative spectrophotometric analysis of pigments in cell suspensions is discussed.

Optical properties of Nannochloropsis sp and their application to remote estimation of cell mass

The absorption and scattering coefficients and reflectance spectra of ultra-high density Nannochloropsis occulata cultures were investigated in detail to identify the optical properties of the cultures and devise algorithms for remote estimation of dry cell mass in ultra-high cell density cultures. High-spectral resolution measurements of apparent absorption and attenuation as well as reflectance from 400 to 900 nm were carried out in relation to the dry weight, cell count, and pigment concentration in outdoor cultures. Indices calculated as (R(NIR) - R(red))/(R(NIR) + R(red)) and R(NIR)/R(red), in which R(NIR) is reflectance in the range from 750 to 800 nm and R(red) is reflectance in the range 670-680 nm, were used for remote assessment of dry cell mass. Remote estimation in the range 1 to 8 g/L was accomplished with an error of less than 0.66 g/L. A different index, i.e., (R(NIR) - R(red)) was employed for estimation of cell-chlorophyll concentration. This is the first report of in vivo specific absorption coefficient of chlorophyll-a and specific scattering coefficient per dry algal weight of Nannochloropsis sp., providing a basis for remote monitoring of dense phytoplankton masses.

Optical characteristics of the phototroph Thiocapsa roseopersicina and implications for real-time monitoring of the bacteriochlorophyll concentration

Optical characteristics of a Thiocapsa roseopersicina culture and environmental samples containing T. roseopersicina were investigated in the spectral range of 400 to 1,100 nm (absorption coefficient, diffuse attenuation coefficient, and reflectance). Specific absorption coefficients of T. roseopersicina at wavelengths of 480, 520, 550, 580, 805, 860, and 880 nm were determined. It is suggested that the optical properties of T. roseopersicina in the near-infrared range of 800 to 930 nm, confirmed in this study, may be used for development of remote sensing techniques for real-time monitoring of T. roseopersicina and other bacteriochlorophyll a-containing microbes.