Impact of organic carbon acquisition on growth and functional biomolecule production in diatoms

Diatoms are unicellular photosynthetic protists which constitute one of the most successful microalgae contributing enormously to global primary productivity and nutrient cycles in marine and freshwater habitats. Though they possess the ability to biosynthesize high value compounds like eicosatetraenoic acid (EPA), fucoxanthin (Fx) and chrysolaminarin (Chrl) the major bottle neck in commercialization is their inability to attain high density growth. However, their unique potential of acquiring diverse carbon sources via varied mechanisms enables them to adapt and grow under phototrophic, mixotrophic as well as heterotrophic modes. Growth on organic carbon substrates promotes higher biomass, lipid, and carbohydrate productivity, which further triggers the yield of various biomolecules. Since, the current mass culture practices primarily employ open pond and tubular photobioreactors for phototrophic growth, they become cost intensive and economically non-viable. Therefore, in this review we attempt to explore and compare the mechanisms involved in organic carbon acquisition in diatoms and its implications on mixotrophic and heterotrophic growth and biomolecule production and validate how these strategies could pave a way for future exploration and establishment of sustainable diatom biorefineries for novel biomolecules.

Production of Microalgal Biomass in Photobioreactors as Feedstock for Bioenergy and Other Uses: A Techno-Economic Study of Harvesting Stage

The cultivation of microalgae has become a viable option to mitigate increase in CO2 in the atmosphere generated by industrial activities since they can capture CO2 as a carbon source for growth. Besides, they produce significant amounts of oils, carbohydrates, proteins, and other compounds of economic interest. There are several investigations related to the process, however, there is still no optimal scenario, since may depend on the final use of the biomass. The objective of this work was to develop a techno-economic evaluation of various technologies in harvesting and drying stages. The techno-economic estimation of these technologies provides a variety of production scenarios. Photobioreactors were used considering 1 ha as a cultivation area and a biomass production of 22.66 g/m2/day and a CO2 capture of 148.4 tons/ha/year was estimated. The production scenarios considered in this study have high energy demand and high operating costs (12.09–12.51 kWh/kg and US $210.05–214.59/kg). These results are mainly a consequence of the use of tubular photobioreactors as a biomass culture system. However, the use of photobioreactors in the production of microalgal biomass allows it to be obtained in optimal conditions for its use in the food or pharmaceutical industry.

Difference in Time of Audible Sound Exposure to Chlorella DPK-01 in Tubular Photobioreactors: A Strategy to Improve Photobioreactor System

The cultivation of Chlorella DPK-01 in tubular photobioreactors (PBRs) with difference in time of audible sound exposure was done. The study aims to evaluate the effect of difference in time of audible sound exposure in tubular PBRs to the growth and lipid percentage of microalgae Chlorella DPK-01. This study was using three groups of Chlorella DPK-01 PBRs. One group was control group and not exposed to any sound (Control-PBR), one group was exposed to audible sound in the light (PBR-A), and another group was exposed to audible sound in the dark (PBR-B). Each group consists of three units of PBR. The audible sound (279.9 Hz sine wave) was played in the light for PBR-A and in the dark for PBR-B. The observation period was 14 days. The growth rate of Chlorella DPK-01 was 47.6% per day (Control-PBR), 0.44 per day (PBR-A), and 0.55 per day (PBR-B) respectively. Meanwhile, the lipid percentage of Chlorella DPK-01 was 16% (Control-PBR), 31% (PBR-A), and 11% (PBR-B) respectively. Therefore, exposing audible sound in the light and in the dark may differently affect the growth and lipid production of Chlorella DPK-01.

Operation Regimes: A Comparison Based on Nannochloropsis oceanica Biomass and Lipid Productivity

Microalgae are currently considered to be a promising feedstock for biodiesel production. However, significant research efforts are crucial to improve the current biomass and lipid productivities under real outdoor production conditions. In this context, batch, continuous and semi-continuous operation regimes were compared during the Spring/Summer seasons in 2.6 m3 tubular photobioreactors to select the most suitable one for the production of the oleaginous microalga Nannochloropsis oceanica. Results obtained revealed that N. oceanica grown using the semi-continuous and continuous operation regimes enabled a 1.5-fold increase in biomass volumetric productivity compared to that cultivated in batch. The lipid productivity was 1.7-fold higher under semi-continuous cultivation than that under a batch operation regime. On the other hand, the semi-continuous and continuous operation regimes spent nearly the double amount of water compared to that of the batch regime. Interestingly, the biochemical profile of produced biomass using the different operation regimes was not affected regarding the contents of proteins, lipids and fatty acids. Overall, these results show that the semi-continuous operation regime is more suitable for the outdoor production of N. oceanica, significantly improving the biomass and lipid productivities at large-scale, which is a crucial factor for biodiesel production.

Third generation biofuels: Cultivation methods and technologies for processing of microalgal biofuels

Energy production from biomass is gaining a lot of attention. Algal oil (microand macroalgae) can be used for biofuel production. Biofuels from this type of feedstock are called third generation biofuels or advanced biofuels. Focus of this paper is on the microalgal biofuels and on the available process technologies. Very important advantage of microalgal biofuels is that microalgae can be cultivated on any type of land, with the possibility of using wastewater streams. Microalgae can be cultivated in open systems, so called "raceway ponds" or in closed systems - photobioreactors: flat panel photobioreactors, horizontal tubular, vertical tubular photobioreactors with or without airlift. Also, basic information on cultivation conditions (photoautotrophic, heterotrophic, mixotrophic and photoheterotrophic) are presented. Available technologies for microalgal biofuels production are: transesterification, fermentation, pyrolysis, hydrothermal liquefaction, anaerobic digestion and biomass to liquids (BtL). Additionally, basic information on life cycle assessment of microalgae cultivation and CO2 sequestration potential is given in the final chapter of this work.