R. Materassi

From open ponds to vertical alveolar panels: the Italian experience in the development of reactors for the mass cultivation of phototrophic microorganisms

The need to develop new concepts in reactor design and the growing interest inSpirulina prompted our group to abandon open ponds in the seventies and to focus interest mainly on closed systems. Two substantially different closed photobioreactors have been developed and are at present under investigation in our Research Centre: the tubular photobioreactor (made of rigid or collapsible tubes) and the recently devised vertical alveolar panel (VAP) made of 1.6-cm-thick Plexiglas alveolar sheets.The technical characteristics of the two systems are described and discussed in relation to the main factors which regulate the growth of oxygenic photosynthetic microorganisms in closed reactors.

Production of Spirulina biomass in closed photobioreactors

The results of a six year investigation on the outdoor mass culture of Spirulina platensis and S. maxima in closed tubular photobioreactors are reported. On average, under the climatic conditions of central Italy, the annual yield of biomass obtained from the closed culture units was equivalent to 33 t dry weight ha−1 year−1. In the same climatic conditions the yield of the same organisms grown in open ponds was about 18 t ha−1 year−1. This considerable difference is due primarily to better temperature conditions in the closed culture system. The main problems encountered relate to the control of temperature and oxygen concentration in the culture suspension. This will require an appropriate design and management of the photobioreactor as well as the selection of strains specifically adapted to grow at high temperature and high oxygen concentration.

Effect of temperature on yield and night biomass loss in Spirulina platensis grown outdoors in tubular photobioreactors

Outdoor experiments carried out in Florence, Italy (latitude 43.8° N, longitude 11.3° E), using tubular photobioreactors have shown that in summer the average net productivity of a Spirulina platensis culture grown at the optimal temperature of 35 °C was superior by 23% to that observed in a culture grown at 25 °C. The rates of night biomass loss were higher in the culture grown at 25 °C (average 7.6% of total dry weight) than in the one grown at 35 °C (average 5%). Night biomass loss depended on the temperature and light irradiance at which the cultures were grown, since these factors influenced the biomass composition. A net increase in carbohydrate synthesis occurred when the culture was grown at a low biomass concentration under high light irradiance or at the suboptimal temperature of 25 °C. Excess carbohydrate synthesized during the day was only partially utilized for night protein synthesis.

As integrated culture system for outdoor production of microalgae and cyanobacteria

Cultivation of microalgae in closed photobioreactors suffers from high temperatures, whereas in open raceway ponds the optimal temperature is seldom reached. With an integrated device coupling a raceway pond with a suitably sized and positioned closed system (alveolar panel), the heat accumulated in the panel was efficiently transferred to the pond and a near-optimum temperature regimen for Arthrospira platensis was maintained, with no need for any additional cooling device. The productivity obtained in the integrated system was higher than the sum of the productivities (g reactor−1 day−1) of the pond and panel systems operating separately.

Temperature as an important factor affecting productivity and night biomass loss in Spirulina platensis grown outdoors in tubular photobioreactors

Outdoor experiments using tubular photobioreactors have shown that in summer the average net productivity of a Spirulina platensis culture grown at the optimal temperature of 35°C was superior by 23% to that observed in another culture grown at 25°C. The rates of night biomass loss were higher in the culture grown at 25°C (average 7·6% of dry weight) than in the one grown at 35°C (average 5% of dry weight). We found that the night biomass loss was dependent on the temperature and light irradiance at which the cells were grown, since these factors influence the biomass composition. A net increase in carbohydrate synthesis was observed when the cells were grown under high light irradiance or at the suboptimal temperature of 25°C. The excess of carbohydrate synthesized during the day was only partially utilized for night protein synthesis.

DNA restriction fingerprint analysis of the soil bacterium Azospirillum

Total DNAs of 18 strains of Azospirillum from different sources and geographical areas were compared by restriction endonuclease pattern analysis. Fragments obtained with HindIII or BglII were separated by PAGE and stained with silver nitrate. Each strain possessed a unique and reproducible fingerprint with each enzyme, thereby facilitating strain recognition. UPGMA analysis recovered clusters of band patterns that were compared to the distribution of species within the genus Azospirillum.

Microbial biomass recovery using a synthetic cationic polymer

Abstract A synthetic cationic polymer (Praestol) for biomass recovery by flocculation of Tetraselmis suecica, Spirulina platensis and Rhodopseudomonas palustris was tested both in the laboratory and outdoors. The flocculating efficiency of Praestol increased with the dose used up to 1 mg litre−1 for all microorganisms tested. With this flocculant dose, the percentage of biomass separated was 86% for Rhodopseudomonas and 70% for Tetraselmis and Spirulina. The maximum flocculation efficiency was observed when the flocculant: biomass ratio was 1:1000 (by weight). Moreover, satisfactory flocculation efficiency was maintained when Praestol was used on cultures grown in sea- or brackish-water media. No inhibitory effect on the growth of the cultures was noted when the medium was recycled in the pond after flocculation.

STUDIES ON EXOPOLYSACCHARIDE RELEASE BY DIAZOTROPHIC BATCH CULTURES OF CYANOSPIRA CAPSULATA

SummaryDiazotrophic batch cultures of Cyanospira capsulata producing large amounts of a soluble exopolysaccharide (EPS) were studied over a period of about 30 days under continuous illumination. The thickness of the capsule surrounding the trichomes remained almost the same throughout the growth phases and the EPS was continuously released into the medium at a rate which was roughly constant throughout the culture period. A mean EPS productivity of about 6 g m−2 day−1 was attained. Purified EPS samples exhibited a saccharidic composition consisting of four neutral sugars (glucose, mannose, fucose and arabinose) and galacturonic acid in a molar ratio of 1:1:1:1:2, respectively. The EPS was also characterized by the presence of pyruvic residues and by a protein content of about 2%. O-Acetyl groups and sulphate residues were not detected. The massive release of this polysaccharidic material into the liquid medium made the cultures progressively more viscous.

Hydrogen production by immobilized cells—I. light dependent dissimilation of organic substances by Rhodopseudomonas palustris

Abstract Photoproduction of H2 from some organic acids by agar entrapped cells of Rhodopseudomonas palustris has been investigated. With an agar layer 3.5 mm thick, the rate of H2 evolution from dl -malate is limited by the diffusion of the substrate into the agar when the concentration of bacterial cells is as high as 1.7 mg cells dry weight cm−3 (ca. 0.59 mg cells d.w.cm−2) while no limitation occurs with 0.425 mg cells d.w.cm−3 (ca. 0.15 mg d.w.cm−2). At the latter concentration of bacterial cells in the agar layer, the rate of H2 evolution is light saturated at an energy flux of 2.7 × 103 erg cm−2 s−1. Immobilized cells retain their H2-producing activity much longer than the free cells kept in identical conditions. The conversion efficiency (H2-formed divided by the the H2-content of the substrate decomposed × 100) varied between 67% (succinate) and 40% (acetate).

Hydrogen production by immobilized cells—II. H2-photoevolution and waste-water treatment by agar-entrapped cells of Rhodopseudomonas palustris and Rhodospirillum molischianum

Abstract The photoproduction of molecular hydrogen by agar-entrapped cells of Rhodopseudomonas palustris and Rhodospirillum molischianum from sugar refinery wastes and straw paper mill effluents as electron donors has been investigated. On average, more than 30% of the initial chemical oxygen demand (COD) of the wastes was converted into H2. Hence the process attains a partial reclamation of the effluents. Under the experimental conditions used, molecular hydrogen was produced for over a month at a rate ranging from 50 to 139 μl H2 per mg cells dry wt per h depending on the organism and on the substrate. The amount of hydrogen evolved from 1l. of straw paper mill effluent surpassed 2l.