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 is
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, and examine the
responses to abrupt changes in temperature. 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. There is no doubt that high irradiance levels can be a
confounding factor both at low and high temperatures. We limit the
current study to “average” irradiance conditions in Fort Myers,
Florida. The combination of vertical PBR arrays, which dilute the
average irradiance levels from about 800 μE/m2-s to about 200 μE/m2-s,
and the rapid mixing, which distributes the heat from light absorption
more evenly within the PBR volume, lessen the potential for extreme
effects due to high irradiance. That is born out by cultivation field
observations of Arthrospira growth and laboratory studies of
irradiance effects with the same approach used here. Regarding lower
temperatures than the 20°C included here, our screening studies do not
suggest any issues down to 10°C and field experience in the environment
of interest shows that the concerns lie at high temperatures.
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 (low
to high, and high to low), and Phase III continues the examination under
dynamic summer temperature profiles (hourly variations) in a
semi-continuous operation mode. The experimental setup, shown in Figure
1b, involves vertically oriented tubular photobioreactors, designed to
be predictive of outdoor performance in large PBR arrays.