“Green microalgae biohydrogen production”

Nowadays, renewable energy is one of the most discussed issues by the international scientific community. The unrestrained use of fossil fuels has raised relevant questions about sustainability and effects on the environment. Hydrogen (H2) is considered a promising fuel due to its thermodynamical properties and CO2-free combustion. Nevertheless, environmental problems arise when H2 is produced using energy deriving from fossil sources: only “green hydrogen” identifies a production 100% based on renewable energy. Currently, several microorganisms are known for their ability to produce H2 as a metabolic feature. Regarding microalgae, most of the information comes from Chlamydomonas reinhardtii. In this green microalgae, two different photosynthetic production pathways and one fermentative-like metabolism have been described concerning transitory H2 production. To extend H2 production one requirement is the creation of a hypoxic environment. This occurs when photosynthetic activity slows down or if there is an increase in mitochondrial respiration rates. Moreover, the electron flow should be directed preferentially towards the H2 evolution enzyme, hydrogenase. Concerning physiological conditioning, one of the most promising strategies for H2 production is sulphur deprivation from cultivation medium: within three days, anaerobiosis develops under saturated light. Chlamydomonas has also represented an excellent example for the development of different molecular strategies, that allow to overcome some limitations and extend the H2 production. One of the limitations is related to sunlight saturation and dissipation that also affect H2 production. Mutants with the truncated light-harvesting antenna (tla) in the chloroplast are subject to fewer phenomena of photoinhibition and light saturation. Another limitation is linked to the competitive pathways that remove electrons from hydrogenase. Pgrl1 (protein gradient regulation like 1) mutant showed improved H2 production reducing this phenomenon. Another relevant issue is the oxygen sensitivity of the hydrogenase enzyme. The use of an O2-tolerant clostridial [FeFe]-hydrogenase, expressed in C. reinhardtii, showed better enzymatic rate, as the bacterial hydrogenase had a lower inactivation rate in aerobiosis. The work carried out over the past three years was aimed primarily at the isolation and characterization of microalgal species in the Basilicata region for the identification of new biohydrogen producers. Particular attention has been given to the search for strains with good growth rates and able to use different carbon sources. Secondly, the physiological behaviour of single and double mutants of Chlamydomonas was analyzed concerning H2 production by modulating light condition without resorting to stress application, such as sulphur deprivation. Freshwater samples collected in different villages of the Basilicata region were used to isolate microalgae with different morphologies. Microscopical observations and molecular identification made it possible to identify the genus of the isolated pure colonies. The growth of the various strains was followed by different methods: absorbance and chlorophyll content proved to be effective and fast for monitoring cell growth over the days. This made it possible to evaluate the growth rates of the species under examination. Various tests were carried out to detect the production of H2. Bioreactors were kept in dark, limited light (12 PAR) or sulfur deprivation (with intense light, 100 PAR). All the experiments considered different carbon source too. The levels of H2 gas produced were daily assessed by gas chromatography by taking a sample of the airspace in contact with the liquid culture in the airtight bioreactors. Desmodesmus sp. and Haematococcus sp. strains demonstrated production of H2 similar to wild type Chlamydomonas (5-10 ml/litre of culture). Furthermore, the same production occurred similarly using acetate or glucose. For Chlamydomonas mutants, the experiments were conducted in collaboration with the University of Córdoba (Spain). Investigated mutants were tla3, pgrl1, and one engineered with Clostridium bacterial hydrogenase (clostr) and the relative combinations tla3 + pgrl1 and clostr + pgrl1 from genetic cross. In this case, the wild type, single and double mutant strains were subjected to different lighting conditions (12, 50, 100, 450 PAR). In particular, the combination tla3 + pgrl proved to be the best as it is capable of producing H2 even at light intensities that are generally less tolerated, opening up new application scenarios. The single mutant Clostr showed instead a fast hydrogenase activity in a replete media also proportionally with the increase of light. In conclusion, the algae isolated during the PhD project have shown interesting implications for the production of H2 such as the metabolic versatility regarding the use of the different carbon sources. This leads to the need to carry out a more in-depth investigation of the mechanisms underlying the metabolism of these microalgae both from a physiological and a molecular point of view. Regarding single and double Chlamydomonas mutants, knowledge about their behaviour in different light conditions and the feasibility of H2 production has been expande

Biohydrogen from microalgae: Production and applications

The need to safeguard our planet by reducing carbon dioxide emissions has led to a significant development of research in the field of alternative energy sources. Hydrogen has proved to be the most promising molecule, as a fuel, due to its low environmental impact. Even if various methods already exist for producing hydrogen, most of them are not sustainable. Thus, research focuses on the biological sector, studying microalgae, and other microorganisms’ ability to produce this precious molecule in a natural way. In this review, we provide a description of the biochemical and molecular processes for the production of biohydrogen and give a general overview of one of the most interesting technologies in which hydrogen finds application for electricity production: fuel cells

Anaerobic gaseous biofuel production using microalgal biomass – A review

Most photosynthetic organisms store and convert solar energy in an aerobic process and produce biomass for various uses. Utilization of biomass for the production of renewable energy carriers employs anaerobic conditions. This review focuses on microalgal biomass and its use for biological hydrogen and methane production. Microalgae offer several advantages compared to terrestrial plants. Strategies to maintain anaerobic environment for biohydrogen production are summarized. Efficient biogas production via anaerobic digestion is significantly affected by the biomass composition, pretreatment strategies and the parameters of the digestion process. Coupled biohydrogen and biogas production increases the efficiency and sustainability of renewable energy production. (C) 2018 Published by Elsevier Ltd

PHB-rich biomass and BioH2 production by means of photosynthetic microorganisms

Polyhydroxyalkanoates (PHAs) are a family of biopolyesters produced by many bacteria as intracellular storage carbon and energy source. Poly-β-hydroxybutyrate (PHB) is probably the most common type of PHA. It is biodegradable and renewable, with relevant thermoplastic properties along with adjustable thermal and mechanical properties. The thermoplastic properties of PHB and its biodegradability make it a potential alternative to petroleum-based plastics. Several microorganisms growing in the dark and/or in the light produce PHB. The polymer is mainly accumulated in the cytoplasm of cells when microorganisms are growing under conditions of stress. If purple non-sulfur photosynthetic bacteria (PNSB) are grown under nitrogen starvation conditions, a photoevolution of molecular hydrogen occurs as well. The PHB amount increases when carbon and energy sources are in excess, but the growth is limited, for example, by the lack of a nitrogen, phosphorous or sulfur source. This work deals the possibility of producing PHAs by photosynthetic microorganisms belonging to cyanobacteria and PNSB. Different culture broths, with and without organic carbon sources, were investigated to maximize PHA production by photosynthetic microorganisms. An unbalanced agro-industrial wastewater has been also investigated in the present study. It concerns the olive mill wastewater (OMW) containing significant reusable carbon fractions suitable for an eco-efficient valorization by feeding photosynthetic processes. The maximum PHA concentration in a cyanobacterium drybiomass was 317 mg/L, when growing cells in a medium with a low content of acetic acid (LAC). In PNSB drybiomass the maximum PHB content was 215 mg/L, when growing PNSB in a synthetic medium. A simultaneous H2 co-production (1,295 mL/L of culture) was cumulated as well, at the end of the process

Use of Microalgae for Advanced Wastewater Treatment and Sustainable Bioenergy Generation

Given that sustainable energy production and advanced wastewater treatment for producing clean water are two major challenges faced by modern society, microalgae make a desirable treatment alternative by providing a renewable biomass feedstock for biofuel production, while treating wastewater as a growth medium. Microalgae have been known to be resilient to the toxic contaminants of highly concentrated organic wastewater (e.g., organic nitrogen, phosphorus, and salinity) and are excellent at sorbing heavy metals and emerging contaminants. Economic and environmental advantages associated with massive algae culturing in wastewater constitute a driving force to promote its utilization as a feedstock for biofuels. However, there are still many challenges to be resolved which have impeded the development of algal biofuel technology at a commercial scale. This review provides an overview of an integrated approach using microalgae for wastewater treatment, CO2 utilization, and biofuel production. The main goal of this article is to promote research in algae technologies by outlining critical needs along the integrated process train, including cultivation, harvesting, and biofuel production. Various aspects associated with design challenges of microalgae production are described and current developments in algae cultivation and pretreatment of algal biomass for biofuel production are also discussed. Furthermore, synergistic coupling of the use of microalgae for advanced wastewater treatment and biofuel production is highlighted in a sustainability context using life cycle analysis.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140370/1/ees.2016.0132.pd

Algae-Bacteria Consortia as a Strategy to Enhance H2 Production

Biological hydrogen production by microalgae is a potential sustainable, renewable and clean source of energy. However, many barriers limiting photohydrogen production in these microorganisms remain unsolved. In order to explore this potential and make biohydrogen industrially affordable, the unicellular microalga Chlamydomonas reinhardtii is used as a model system to solve barriers and identify new approaches that can improve hydrogen production. Recently, Chlamydomonas–bacteria consortia have opened a new window to improve biohydrogen production. In this study, we review the different consortia that have been successfully employed and analyze the factors that could be behind the improved H2 production

Photosynthetic hydrogen production: Novel protocols, promising engineering approaches and application of semi-synthetic hydrogenases

Photosynthetic production of molecular hydrogen (H-2) by cyanobacteria and green algae is a potential source of renewable energy. These organisms are capable of water biophotolysis by taking advantage of photosynthetic apparatus that links water oxidation at Photosystem II and reduction of protons to H-2 downstream of Photosystem I. Although the process has a theoretical potential to displace fossil fuels, photosynthetic H-2 production in its current state is not yet efficient enough for industrial applications due to a number of physiological, biochemical, and engineering barriers. This article presents a short overview of the metabolic pathways and enzymes involved in H-2 photoproduction in cyanobacteria and green algae and our present understanding of the mechanisms of this process. We also summarize recent advances in engineering photosynthetic cell factories capable of overcoming the major barriers to efficient and sustainable H-2 production

BIOMASS AND LIPID PRODUCTION FROM HETEROTROPHIC AND MIXOTROPHIC FED-BATCH CULTIVATIONS OF MICROALGAE \u3ci\u3eChlorella\u3c/i\u3e \u3ci\u3eprotothecoides\u3c/i\u3e USING GLYCEROL

Chlorella protothecoides is a microalga that can grow both photo-autotrophically and/or heterotrophically under different culture and environmental conditions. In this study both the heterotrophic growth and mixotrophic growth have been conducted. The heterotrophic experiments were conducted completely in the dark while the mixotrophic experiments had the dark cycles with periodic light exposure. The aim of the study was to independently understand the effect of each mode on biomass and lipid yields. For the heterotrophic experiments, glycerol was used as an external organic carbon source while yeast extract was used as the nitrogen source. The carbon and nitrogen source were added to a defined culture medium. Three different grades of glycerol were evaluated for their effect on the biomass and lipid yields in the heterotrophic experiments, with the 65% crude glycerol proving best giving an average biomass concentration of 22.13 ± 0.17 g/L and average lipid concentration of 9.75 ± 0.02 g/L at the end of an eight-day fed-batch fermentation. The average biomass concentrations did not increase after the eighth day of fermentation. The pH was maintained at a constant value of 6.8 and temperature at 280C. As the experiments were carried out in fed-batch mode, addition of the culture medium was done every 24 hours to maintain the carbon and nitrogen sources at 30g/L and 4g/L respectively till the eighth day. Yeast extract was found to be a good nitrogen source, as it also provides vitamins, amino acids and important growth factors as oppose to some other sources like ammonia and urea (Shi et al., 2000; Gonzalez-Bashan et al., 2000; Illman et al., 2000; Chen et al., 2006). The mixotrophic experiments were aimed to expose the algae to alternating light and dark cycles to enhance biomass accumulation during light cycle and lipid accumulation during dark cycles. The light cycle help to assimilate CO2 and produce energy via photosynthesis, which comprises the catabolic reaction, while the switch to the dark cycle allows anabolic reactions where accumulation of lipids and production of other compounds occur. Here, the algae were exposed to light for 8 hours and dark for 16 hours each day for eight days. The 65% crude glycerol was supplemented as the external carbon source to be utilized by the algae during the dark cycles while yeast extract was used as the nitrogen source. Here the average maximum biomass concentration of 28.95 ± 0.26 g/L and the average lipid concentration of 13.14 ± 0.01 g/L were obtained which were found to be higher than the heterotrophic results. With intermittent light exposure, the lipid yields were found to increase from a maximum of 0.44 ± 0.004 gram lipid/gram biomass for heterotrophic experiments to 0.46 ± 0.004 gram lipid/gram biomass for mixotrophic experiments. The mixotrophic experiments also provided an increase in the average maximum overall biomass concentration from 22.13 ± 0.17 g/L in heterotrophic to 28.95 ± 0.26 g/L in mixotrophic experiments