A critical factor in implementing microalgal
biofuels for mass production is the nutrient requirements.
The current study investigated the fate of macro- and
micronutrients and their availability in a sequential
phototrophic-heterotrophic production process for the lipid
rich microalga Auxenochlorella protothecoides. More than
99 % (by weight) of overall process nutrients were supplied
during the initial photoautotrophic stage reflecting its significantly
larger volume. Under photoautotrophic growth conditions
only 9–35 % of supplied Mn, S, Fe, N, Mg, and Cu and
less than 5 % of P, Mo, Co, B, Zn, and Ca were consumed by
the algae. The rest of these nutrients remain in the spent
growth media during the culture concentration-down from
an 800 L phototrophic pond to a 5 L heterotrophic fermenter.
In contrast, Zn, Mo, Mn, Mg, Ca, and N were exhausted
(90–99 % removal) during the first 25 h of the heterotrophic
growth stage. The depletion of these key nutrients may have
ultimately limited the final biomass density and/or lipid productivity
achieved. Approximately 10–20 % of the total supplied
S, Mn, Fe, N, and Cu and 5 % of Ca and Zn were
assimilated into algal biomass. Several elements including N,
P, Mn, B, Cu, Ca, Mg, S, and Fe were released back into the
liquid phase by anaerobic digestion (AD) of the residual
biomass after lipid extraction. The nutrients recovered from
the AD effluent and remaining in the spent medium should be
recycled or their initial concentration to the phototrophic stage
decreased to enhance process economics and sustainability for
future commercialization of algal-derived biofuels.
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Appl Microbiol Biotechnol (2014) 98:5261–5273
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