Strain Characterization: The Effects of Temperature and Salinity on Growth Rates
The PNNL Thermal Gradient and Salinity Gradient Incubators (TGI and SGI) are used to determine the influence of the critical environmental factors, temperature and salt content, on the maximum specific growth rate of algae within the Algae DISCOVR project. This is the first stage of strain screening (TIER I) and characterizes the ability of algae to tolerate the highly dynamic daily and seasonal environment encountered in outdoor cultivation conditions. Algae are categorized according to their potential as summer, winter, or transitional season crops for the testbed location of interest. Algae with robust growth rates (no crashes, exponential growth rates above 1 day-1) are ranked by an algorithm developed within the DISCOVR project. The most promising algae are then tested in the Laboratory Environmental Algal Pond Simulator (LEAPS) photobioreactors under climate conditions simulating 20 cm deep outdoor ponds in Mesa, Arizona (location of the Arizona Center for Algae Technology and Innovation- AzCATI) using winter and/or summer season light and temperature scripts for January 31st and July 1st, respectively.
The PNNL Thermal Gradient Incubator (TGI) is used to determine the maximum specific growth rate under tightly controlled environmental conditions, where the only manipulated variable is temperature. Cultures at up to 8 different temperatures are run simultaneously under identical saturating illumination (450 µmol photons/m2/sec) provided by custom built neutral white (4000 ˚K) LED lights (Figure 1). Temperatures spanning an order of magnitude from approximately 4 to 45 °C are used to encompass most temperatures encountered during year-round cultivation in outdoor ponds in geographic areas of interest. All cultures are grown in a nutrient replete DISCOVR medium and continuously sparged with CO2-enriched air to ensure that no nutrients (including carbon) limit growth. Maximum specific growth rates are determined repeatedly in consecutive culture experiments to allow for temperature adaption and ensure reproducible results. Using a similar protocol, the effect of medium salinity on the maximum specific growth rate is determined using the PNNL Salinity Gradient Incubator (SGI), where cultures are grown at room temperature but at different salinities, such as 5, 15, and 35 ppt (parts per thousand).
The Thermal Gradient Incubator (TGI) running three distinct species of algae (from top to bottom), each at 8 different temperatures spanning 4 to 45 °C (from left to right).
Average maximum specific growth rates of Picochlorum oklahomensis CCMP 2329, Micractinium sp. NREL 14-F2, and Chlorella sp. NREL 4-C12 as a function of temperature. Error bars denote the standard error of the mean, where n=5 for P. oklahomensis CCMP 2329 and M. sp. NREL 14-F2, n=9 for C. sp. NREL 4-C12.
The Algae DISCOVR project taps into the vast diversity of the photosynthetic microbial world (i.e., algae) to find top candidates for biomass production using non-potable waters (brackish to marine) under outdoor cultivation conditions (daily and seasonal temperature fluctuations). Algae are sourced from culture collections as well as academic, industrial, and government research groups. Algae included in the first three rounds of screening include organisms isolated from several continents and many different bioregions (alpine, oceanic, coastal mudflats, deserts, inland seas, salt ponds, anthropogenically-impacted areas, Arctic, and Antarctic locations) and represent an astounding taxonomic diversity from five divisions of eukaryotic algae and cyanobacteria.
Huesemann, M.H., B. Crowe, P. Waller, A. Chavis, S. Hobbs, S. Edmundson, and M. Wigmosta, “A validated model to predict microalgae growth in outdoor pond cultures subjected to fluctuating light intensities and water temperatures”, Algal Research, 13:195-206, 2016.
Strain characterization, salinity screening, temperature tolerance, maximum specific growth rate