Spectroradiometric Monitoring of Algal Culturing
The hyperspectral spectroradiometric monitoring approach developed by SNL provides the potential for continuous, real-time assessment of the growth rate (µg/ml-day) and stage of growth (e.g., exponential versus stationary) at algal production facilities. By applying numerically invertible reflectance models to the data acquired from commercial fieldable spectroradiometers, we can rapidly (< 10 min) resolve changes in the culture optical depth and algal pigment optical activity. Initially demonstrated at the laboratory scale – as reported in the inaugural issue of Algal Research – our spectroradiometric monitoring approach has since been applied to outdoor raceways and deployed at several sites. (Figure 1) The method is currently being extended to the detection of predators, pathogens, and competitors.
Figure 1: SNL simultaneously monitored six mini-raceway ponds at AzCATI
SNL’s development of spectroradiometric monitoring has been guided by its intended future application to algal factory farms. Our pursued embodiment of the approach has thus incorporated inexpensive single-field-of-view sensors that can serve long-term in variable field conditions. Throughout the development stage, we have deployed such sensors in a fixed-staring configuration, but similar sensors have already been deployed on UAVs toward the monitoring of traditional farming. The method utilizes measurement and interpretation of pond reflectance spectra spanning from the visible into the near-infrared and is depicted in Figure 2.
Reflectance spectra are acquired every 5 minutes with a multi-channel, fiber-coupled spectroradiometer, providing monitoring of algal pond conditions with high temporal frequency. The spectra are interpreted via numerical inversion of a reflectance model, in which the above-water reflectance is expressed in terms of the absorption and backscatter coefficients of the cultured species, with additional terms accounting for the pigment fluorescence features and for the water-surface reflection of sunlight and skylight.
Description of technology. A.Fiber-coupled spectrometers allow the near-temporally coincident collection of upwelling radiance and downwelling irradiance in a fixed-staring configuration. B. Because our analysis is based upon the physics of light transport, we can rigorously account for variability in water-surface glint, bottom-surface albedo, etc.
Current contamination monitoring techniques (including optical microscopy, flow cytometry, and qPCR) are all offline methods, requiring laboratory access and the participation of skilled support staff for routine operation. Moreover, these techniques are not extensible to the rapid, broad area coverage sought by the agricultural community. To fill this need, the traditional agricultural community is turning to the use of remote sensing data acquired by multispectral and hyperspectral instruments deployed on airborne platforms, and we anticipate that such platforms will eventually be applied to the commercial-scale culturing of algae. The spectroradiometric monitoring methods have many benefits (Green inset panel) and are compatible with these platforms
Reichardt, T., A. Collins, R. McBride, C. Behnke, and J. Timlin, “Spectroradiometric Monitoring for Open Outdoor Culturing of Algae and Cyanobacteria”, Applied Optics, 53:F31-F45, 2014.
Reichardt, T., A. Collins, O. Garcia, A. Ruffing, H. Jones, and J. Timlin, “Spectroradiometric Monitoring of Nannochloropsis salina Growth”, Algal Research, 1:22-31, 2012.
Spectroradiometric monitoring, reflectance measurements, early crash detection, contamination detection