Marine microorganisms as a promising and sustainable source of bioactive molecules.

A biorefinery for valorization of industrial waste-water and flue gas by microalgae for waste mitigation, carbon-dioxide sequestration and algal biomass production

https://repositorium.sdum.uminho.pt/bitstream/1822/37216/1/document_19860_1.pdf

Continuous cultivation of photosynthetic microorganisms: Approaches, applications and future trends.

Northern green algae have the capacity to remove active pharmaceutical ingredients

The reliable and efficient production of microalgae in large-scale commercial culture, whether in open ponds or in closed photobioreactors, requires a good understanding of the physiology of the alga being cultured. Since large-scale cultivation is almost always carried out outdoors, the algae experience an environment which changes during the day, from day-to-day, and also seasonally. Key environmental factors are light and temperature, and efficient use of the available light is critical for obtaining maximum productivity. How algal cell metabolism is regulated under such conditions is important as is the ability of the algae to acclimate to changed environmental conditions. Algal productivity and the formation of desired cell products such as carotenoids (e.g. β-carotene and astaxanthin) or long-chain polyunsaturated fatty acids is further affected by nutrient sources and availability and the nutrient requirements of the algae under intensive large-scale culture need to be understood. The availability of inorganic carbon either as carbon dioxide or bicarbonate is especially important. Understanding of the physiology and biochemistry of the algae also enables manipulation of the culture conditions to optimise the productivity of the desired algal metabolite (product). Understanding diurnal and circadian rhythms may also aid in optimising culture conditions and maximising yields by adjusting the timing of harvesting. Monitoring of large-scale cultures is difficult and also requires a good understanding of the physiology of the algae.

Algal Physiology and Large-Scale Outdoor Cultures of Microalgae

Lack of accounting for proton uptake and secretion has confounded interpretation of the stoichiometry of photosynthetic growth of algae. This is also problematic for achieving growth of microalgae to high cell concentrations which is necessary to improve productivity and the economic feasibility of commercial-scale chemical production systems. Since microalgae are capable of consuming both nitrate and ammonium, this represents an opportunity to balance culture pH based on a nitrogen feeding strategy that does not utilize gas-phase CO2 buffering. Stoichiometry suggests that approximately 36 weight%N-NH4+ (balance nitrogen as NO3-) would minimize the proton imbalance and permit high-density photoautotrophic growth as it does in higher plant tissue culture. However, algal media almost exclusively utilize nitrate, and ammonium is often viewed as ‘toxic’ to algae. The microalgae Chlorella vulgaris and Chlamydomonas reinhardtii exclusively utilize ammonium when both ammonium and nitrate are provided during growth on excess CO2. The resulting proton imbalance from preferential ammonium utilization causes the pH to drop too low to sustain further growth when ammonium was only 9% of the total nitrogen (0.027 gN-NH4+/L). However, providing smaller amounts of ammonium sequentially in the presence of nitrate maintained the pH of a Chlorella vulgaris culture for improved growth on 0.3 gN/L to 5 gDW/L under 5% CO2 gas-phase supplementation. Bioreactor pH dynamics are shown to be predictable based on simple nitrogen assimilation as long as there is sufficient CO2 availability. This work provides both a media formulation and a feeding strategy with a focus on nitrogen metabolism and regulation to support high-density algal culture without buffering. The instability in culture pH that is observed in microalgal cultures in the absence of buffers can be overcome through alternating utilization of ammonium and nitrate. Despite the highly regulated array of nitrogen transporters, providing a nitrogen source with a balanced degree of reduction minimizes pH fluctuations. Understanding and accommodating the behavior of nitrogen utilization in microalgae is key to avoiding ‘culture crash’ and reliance on gas phase CO2 buffering, which becomes both ineffective and cost-prohibitive for commercial-scale algal culture.

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Megerle L Scherholz

Papers

Achieving pH control in microalgal cultures through fed-batch addition of stoichiometrically-balanced growth media