Giorgos Markou

Microalgae for high-value compounds and biofuels production: A review with focus on cultivation under stress conditions

Microalgal biomass as feedstock for biofuel production is an attracting alternative to terrestrial plant utilization for biofuels production. However, today the microalgal cultivation systems for energy production purposes seem not yet to be economically feasible. Microalgae, though cultivated under stress conditions, such as nutrient starvation, high salinity, high temperature etc. accumulate considerable amounts (up to 60-65% of dry weight) of lipids or carbohydrates along with several secondary metabolites. Especially some of the latter are valuable compounds with an enormous range of industrial applications. The simultaneous production of lipids or carbohydrates for biofuel production and of secondary metabolites in a biorefinery concept might allow the microalgal production to be economically feasible. This paper aims to provide a review on the available literature about the cultivation of microalgae for the accumulation of high-value compounds along with lipids or carbohydrates focusing on stress cultivation conditions.

Microalgal carbohydrates: an overview of the factors influencing carbohydrates production, and of main bioconversion technologies for production of biofuels

Microalgal biomass seems to be a promising feedstock for biofuel generation. Microalgae have relative high photosynthetic efficiencies, high growth rates, and some species can thrive in brackish water or seawater and wastewater from the food- and agro-industrial sector. Today, the main interest in research is the cultivation of microalgae for lipids production to generate biodiesel. However, there are several other biological or thermochemical conversion technologies, in which microalgal biomass could be used as substrate. However, the high protein content or the low carbohydrate content of the majority of the microalgal species might be a constraint for their possible use in these technologies. Moreover, in the majority of biomass conversion technologies, carbohydrates are the main substrate for production of biofuels. Nevertheless, microalgae biomass composition could be manipulated by several cultivation techniques, such as nutrient starvation or other stressed environmental conditions, which cause the microalgae to accumulate carbohydrates. This paper attempts to give a general overview of techniques that can be used for increasing the microalgal biomass carbohydrate content. In addition, biomass conversion technologies, related to the conversion of carbohydrates into biofuels are discussed.

Microalgal and cyanobacterial cultivation: the supply of nutrients.

Microalgae and cyanobacteria are a promising new source of biomass that may complement agricultural crops to meet the increasing global demand for food, feed, biofuels and chemical production. Microalgae and cyanobacteria cultivation does not interfere directly with food production, but care should be taken to avoid indirect competition for nutrient (fertilizer) supply. Microalgae and cyanobacteria production requires high concentrations of essential nutrients (C,N,P,S,K,Fe, etc.). In the present paper the application of nutrients and their uptake by microalgae and cyanobacteria is reviewed. The main focus is on the three most significant nutrients, i.e. carbon, nitrogen and phosphorus; however other nutrients are also reviewed. Nutrients are generally taken up in the inorganic form, but several organic forms of them are also assimilable. Some nutrients do not display any inhibition effect on microalgal or cyanobacterial growth, while others, such as NO2 or NH3 have detrimental effects when present in high concentrations. Nutrients in the gaseous form, such as CO2 and NO face a major limitation which is related mainly to their mass transfer from the gaseous to the liquid state. Since the cultivation of microalgae and cyanobacteria consumes considerable quantities of nutrients, strategies to improve the nutrient application efficiency are needed. Additionally, a promising strategy to improve microalgal and cyanobacterial production sustainability is the utilization of waste streams by recycling of waste nutrients. However, major constraints of using waste streams are the reduction of the range of the biomass applications due to production of contaminated biomass and the possible low bio-availability of some nutrients.

Cultivation of Arthrospira (Spirulina) platensis in olive-oil mill wastewater treated with sodium hypochlorite.

The subject of this paper is the cultivation of the cyanobacterium Arthrospira (Sprirulina) platensis in olive-oil mill wastewater (OMWW) treated with sodium hypochlorite (NaOCl). The main positive effect of NaOCl on the OMWW characteristics is the decrease of the phenol concentration and turbidity, rendering the OMWW suitable for A. platensis growth. Maximum biomass production (1696 mg/l) was obtained when the concentration of OMWW in the cultivation medium was 10% with the supplementation of 1g/l NaNO(3) and 5 g/l NaHCO(3). However, the addition of NaHCO(3) has no significant effect, indicating that the only limited nutrient in this wastewater is nitrogen, while carbon is provided by the organic compounds of the wastewater. The maximum of the removals of chemical oxygen demand (COD) and carbohydrates was 73.18% and 91.19%, respectively, while phenols, phosphorus and nitrates in some runs was completely removed.

Using natural zeolite for ammonia sorption from wastewater and as nitrogen releaser for the cultivation of Arthrospira platensis

Herein a new approach for the application of wastewater nutrients for the cultivation of cyanobacteria or microalgae is described. Natural zeolite was used as medium for the sorption of ammonia from wastewater and subsequently as nitrogen releaser in cultures of Arthrospira platensis. The main scope of the present approach was to isolate ammonia from the wastewater and to transfer it into the culture medium excluding thus the suspended solids, the dissolved colored compounds or any other possible contaminant of the wastewater. The results demonstrate that the indirect use of ammonia derived from wastewater using zeolite as sorption and releasing medium for the cultivation of A. platensis is promising. This is the first time that a medium was used for indirect application of wastewater nutrient for the production of cyanobacterial or microalgal biomass.

Alteration of the biomass composition of Arthrospira (Spirulina) platensis under various amounts of limited phosphorus

In this study the biomass composition alteration of the cyanobacterium Arthrospira (Spirulina) platensis under various amounts of limited phosphorus is studied. It was observed that the alteration of the compounds of the biomass occurred gradually as the phosphorus became limited. Carbohydrates and lipids increased from about 9% up to 65% and from about 4.9% up to 7.5%, respectively, while proteins decreased from about 46.5% to 25% as the phosphorus became limited. The increasing of carbohydrates and lipids in addition to the decrease of proteins resulted to an increase of the carbon to nitrogen (C/N) ratio from about 4.6 to 12.2.

Ammonia inhibition on Arthrospira platensis in relation to the initial biomass density and pH

In this study the combined effect of total ammoniacal nitrogen (TAN) concentration, initial biomass density and initial pH of the cultivation medium on growth of Arthrospira platensis was studied. The results indicate that TAN inhibition in relation to the initial biomass in unregulated pH cultures is neither a clearly biomass-independent nor biomass-dependent phenomenon. However, low biomass densities are more susceptible to ammonia inhibition than higher biomass densities. Higher biomass densities seems to mitigate ammonia inhibition through rapider assimilation of TAN. In all cases studied the growth rates were lower compared to the cultures with nitrate as nitrogen source. It was observed that at low TAN concentration, although no ammonia inhibition occured the growth rates were decreased due to nitrogen limitation. Low TAN concentration triggered the accumulation of carbohydrates affecting thus significantly the biomass composition. Ammonia losses from the cultivation system were also determined. Ammonia losses ranged between 17% and 80%.

Harvesting carbohydrate-rich Arthrospira platensis by spontaneous settling

The filamentous cyanobacterium Arthrospira platensis is an attractive feedstock for carbohydrate-based biofuels because it accumulated up to 74% of carbohydrates when nitrogen stressed. Nitrogen stressed A. platensis also settled spontaneously, and this occurred simultaneously with carbohydrates accumulation, suggesting a link between both phenomena. The increased settling velocity was neither due to production of extracellular carbohydrates, nor due to degradation of gas vacuoles, but was caused by an increase in the specific density of the filaments as a result of accumulation of carbohydrates under the form of glycogen. Settling velocities of carbohydrate-rich A. platensis reached 0.64mh(-1), which allowed the biomass to be harvested using a lamella separator. The biomass could be concentrated at least 15 times, allowing removal of 94% of the water using gravity settling, thus offering a potential application as a low-cost and high-throughput method for primary dewatering of carbohydrate-rich A. platensis.

Improved anaerobic digestion performance and biogas production from poultry litter after lowering its nitrogen content.

Poultry litter (PL) was pre-treated in order to reduce its nitrogen content and to increase the C/N ratio. The pre-treatment consisted of a first anaerobiosis phase of about 60days in order to accumulate ammonia nitrogen, followed by an ammonia stripping phase by heating the substrate at 80°C for 24h. The digestion was performed with PL and pre-treated PL (TPL) after ammonia stripping as mono-substrate under four total solids loads, i.e. 5%, 10%, 15% and 20%. The TPL after ammonia stripping displayed lower ammonia (62-73%) and VFA (41-65%) concentrations compared to digesters with raw PL, while bio-methane yield increased about 8-124%. Bio-methane yields in the series with TPL after ammonia stripping were about 193, 196, 215 and 147 [Formula: see text] /kgCOD, based on the COD added, for 5%, 10%, 15% and 20% TS load, respectively. The results indicate that lowering nitrogen content using the suggested process improves bio-methane yields significantly.

Fed-batch cultivation of Arthrospira and Chlorella in ammonia-rich wastewater: Optimization of nutrient removal and biomass production.

In the present work the cyanobacterium Arthrospira platensis and the microalga Chlorella vulgaris were fed-batch cultivated in ammonia-rich wastewater derived from the anaerobic digestion of poultry litter. Aim of the study was to maximize the biomass production along with the nutrient removal aiming to wastewater treatment. Ammonia and phosphorus removals were very high (>95%) for all cultures investigated. Both microorganisms were able to remove volatile fatty acids to an extent of >90%, indicating that they were capable of mixotrophic growth. Chemical oxygen demand and proteins were also removed in various degrees. In contrast, in all cultures carbohydrate concentration was increased. The biochemical composition of the microorganisms varied greatly and was influenced by the indicate that the nutrient availability. A. platensis accumulated carbohydrates (≈ 40%), while C. vulgaris accumulated lipids (≈ 50%), rendering them interesting for biofuel production.