Introduction
The filamentous cyanobacterium Arthrospira platensis (Spirulina)is an oxygenic photosynthetic organism able to grow in tropical and
subtropical environments, and one of only a few microalgal systems that
has been successfully commercialized and approved by United States Food
and Drug Administration (FDA) as a food supplement (Trabelsi et al.,
2009). Arthrospira cultivation and processing yields valuable
biochemical components including protein, carbohydrates, fatty acids and
pigments such as phycocyanin (PC), which can be used in nutritional,
pharmacological, and cosmetic products. Due to these high value
applications, as well as relatively easy harvesting and extraction
processes, Arthrospira cultivation has been deployed commercially
at moderate scale (10 – 100 acre open ponds) for many decades (Lu et
al., 2011). It is important to note that Arthrospira is an
extremophile, in that it can maintain high productivity under high
alkalinity, high pH conditions; this limits predation and competition
sufficiently to allow commercial production in open pond systems. Algal
cultures are influenced by various abiotic variables such as
temperature, irradiance levels, and nutrient availability, all of which
play a significant role in regulating photosynthetic activity, biomass
composition and overall productivity. Under outdoor cultivation
conditions, temperature and light intensity are the two key external
factors that determine photosynthetic activity and biomass growth rates.
Obviously, both factors are highly variable on a daily and seasonal
basis in the natural environment, and spatially within the culture as
well (Chaiklahan et al., 2007; Vonshak and Novoplansky, 2008).
Typically, Arthrospira is cultivated outdoors for mass production
in raceway ponds, where cells encounter fluctuating environments in
terms of irradiance, temperature, and nutrient supply. Though the PBR
environment tends to be more homogeneous, similar fluctuations are
present and temperatures are generally higher due to absorptive heating
and the absence of evaporative cooling. Outdoor algal cultures are
subjected to high light intensity as well as possible high temperature
stress that can negatively impact photosynthetic activity (Torzillo et
al., 1991b). These factors can change both the photosynthesis and
respiration rates, thereby directly influencing the growth and the
chemical composition of the biomass produced (Trabelsi et al., 2009).
Overall, the existing literature is consistent with an optimal
temperature range for stable production of roughly 20-35 °C. Our
screening studies are consistent with that range and also consistent
with an activation energy of about 60 kJ mole-1 (Q10
~2) under saturating light conditions over that
temperature range. It is well-known that productivity is enhanced in
semi-continuous operation where the impact of photosaturation effects
are lessened. We know of no detailed studies dealing with the effect of
temperature and acclimation response on growth and pigment content ofArthrospira in a semi-continuous production mode for extended
time scales under tightly controlled (laboratory) conditions. The
intention here is to determine what portion of previous learnings
translate to semi-continuous operation and the dynamic
(light/temperature) conditions experienced outdoors. Therefore, in the
present work we will examine temperature effects at a longer time scale,
and carry out the experiments in semi-continuous operation mode in PBRs
at 20 °C, 30 °C and 35 °C. In subtropical conditions, the outdoor
culture temperature in the summer months can be very high in PBRs,
reaching up to 35-45 °C for several hours. We have only a limited
understanding of temperature impacts on photosynthetic parameters, and
pigment accumulation in that outdoor environment. Thus, the scope of
this work includes Arthrospira growth under a variety of
temperature conditions with a work plan that includes assessment of
temperature response and recovery, and quantification of the dynamic
change in biomass and pigment content of Arthrospira during the
experiments. The work was performed in three phases (Figure 1a): Phase I
employs constant temperature conditions (same for day and night cycles),
Phase II shifts the Phase 1 cultures to opposing temperature conditions,
and Phase III continues the examination under dynamic summer temperature
profiles (hourly variations) in a semi-continuous operation mode in
PBRs. The experimental setup, shown in Figure 1b, involves vertically
oriented tubular photobioreactors, designed to be predictive of outdoor
performance in large PBR arrays.