Introduction
In recent decades, biofuels have
drawn considerable attention due to fossil fuel depletion and adverse
climate effects of fossil fuel burning (Martins, et al., 2019; Kung,
2019; Kumar and Singh, 2020). Microalgae are introduced as a unique
biomass feedstock for biofuels since they have several advantages
including fast growth rates, resistance to extreme environmental
conditions, feasible and
eco-friendly large-scale
production, and simple life cycle. Microalgae-derived biofuels
production can be coupled with
flue gas CO2 mitigation, wastewater treatment, and
high-value chemical compounds extraction. However, their production is
not yet financially competitive with fossil fuels and conventional
biofuels . Microalgal storage compounds which are the prime biofuel
precursors, starch, and lipids, are used as the substrates for
bioethanol and biodiesel production, respectively. Since these storage
compounds’ accumulation is not linearly delineated to the growth rate,
one of the technical challenges in making biofuels cost-effective is to
increase starch and lipids productivity. The most frequently reported
approaches are nutrient starvation/limitation including nitrogen ,
phosphorus , and sulfur , salt stress (Takagi and Yoshida, 2006; Pancha
et al., 2015) and light intensity stress . Nitrogen starvation is
reported as the most convenient technique to enhance energy storage
components . Most studies to date have been focused on two-stage
production. In the first stage, microalgae are grown in the non-limiting
growth medium. In the second stage, microalgae are triggered by one of
the above-mentioned nutrient limitations and/or stress approaches to
induce storage polymer production . However, under stress, nutrient or
light limitation conditions, along with the accumulation of storage
compounds, the system is exposed to extended unfavorable environmental
conditions, which lead to high expenses of metabolic energy and
decreasing productivity . To overcome this problem, a single-stage
continuous system (chemostat) with a nutrient-limited medium can be
explored. The main advantage of using chemostat is that the liquid
dilution rate controls the growth rate of the biomass with a defined
limiting substrate. When steady-state conditions are reached at specific
dilution rates, biomass productivity, medium concentrations, and
intracellular biochemical composition remain constant . This permits the
optimization of the dilution rate to a determined value for maximal
storage compound productivity in microalgae .
Another major obstacle in the profitable production of biofuels and
scale-up is to maintain the microalgae cultivation systems monoculture,
due to high-priced sterilization of inlet streams and reactor system. On
the other hand, the use of environmental water bodies as inoculum has
this opportunity to introduce new suitable microalgae species for
biodiesel production which are offered by nature’s microbial diversity.
The natural selection and competition is introduced by Environmental
Biotechnology which is targeted at enrichment and maintaining a
characteristic or function as an alternative to a specific species in a
system in order to engineer the ecosystem rather than the organisms .
Furthermore, several studies claimed that biomass yield improved in
mixed-species growth systems relative to algal monocultures grown under
the same resource supply conditions. Previous related studies have shown
that the strategy of chemostat selection could be successfully applied
to obtain a stable enrichment of a polyhydroxyalkanoates (PHA) storing
microbial community .
The main objectives of this study were to investigate the impacts of
different nitrogen concentrations in feeding medium on mixed microalgae
communities in a chemostat reactor. Especially the starch and lipids
productivities were monitored as they have the potential for the
production of biofuels. Furthermore, the nitrogen loading rates (NLR)
were designed to direct the culture to be under light- or
nitrogen-limited conditions that elucidate the microalgae tendency to
accumulate lipids or starch as a dominant intracellular energy reservoir
compound. For all NLRs, the CO2 supply was unlimited.