Rapid Quantification of Microalgae Growth with Hyperspectral Camera and Vegetation Indices

Abstract

Spectral cameras are traditionally used in remote sensing of microalgae, but increasingly also in laboratory-scale applications, to study and monitor algae biomass in cultures. Practical and cost-efficient protocols for collecting and analyzing hyperspectral data are currently needed. The purpose of this study was to test a commercial, easy-to-use hyperspectral camera to monitor the growth of different algae strains in liquid samples. Indices calculated from wavebands from transmission imaging were compared against algae abundance and wet biomass obtained from an electronic cell counter, chlorophyll a concentration, and chlorophyll fluorescence. A ratio of selected wavebands containing near-infrared and red turned out to be a powerful index because it was simple to calculate and interpret, yet it yielded strong correlations to abundances strain-specifically (0.85 < r < 0.96, p < 0.001). When all the indices formulated as A/B, A/(A + B) or (A − B)/(A + B), where A and B were wavebands of the spectral camera, were scrutinized, good correlations were found amongst them for biomass of each strain (0.66 < r < 0.98, p < 0.001). Comparison of near-infrared/red index to chlorophyll a concentration demonstrated that small-celled strains had higher chlorophyll absorbance compared to strains with larger cells. The comparison of spectral imaging to chlorophyll fluorescence was done for one strain of green algae and yielded strong correlations (near-infrared/red, r = 0.97, p < 0.001). Consequently, we described a simple imaging setup and information extraction based on vegetation indices that could be used to monitor algae cultures.

Light and Autofluorescence, Multitasking Features in Living Organisms

“…Chlorophyll is the intrinsic biomarker suitable to carefully monitor growing and quality of algae, which are very sensitive to changes in culture medium depending on the increase in their own growing biomass or on external factors. Devices based on cell counters detecting directly chlorophyll fluorescence together with additional parameters have been proposed to support and facilitate the industrial cultivation of microalgae [48,49]. This could help to adapt the production response to the increasing worldwide commercial interest in algae, because of their remarkable value as food and source of products valuable for dietary, nutraceutical and pharmacology purposes.…”

On-Line Monitoring of Biological Parameters in Microalgal Bioprocesses Using Optical Methods

“…[28] [29] (r,g,b)/DWC: R 2 = 0.97-0.99 [28] Open container/biofilm, suspension [28] (r,b)/DWC: R 2 = 0.90-0.96 [29] Open container/suspension [29] Biomass Transmittance spectrum DWC/T 751 /T 676 : r = 0.51-0.93 [30] Microwells [30] Biomass Chlorophyll fluorometry DWC/Chl a fluorescence: r = 0.95 [35] Fiber probe in PBR bypass [35] [39] CC/T751/T676: r = 0.85-0.96 [30] Microwells [30] [30] [25] [56] [35] CC/HR,EC: R 2 = 0.99 [25] Open container [25] CC/OD560: R 2 = 0.992-0.999 [56] Flask bypass [56] CC/Chl a fluorescence: r = 0.92 [35] Fiber probe in PBR bypass [35] Viable Algal lipids/NMR signal: R 2 = 0.99 [52] Bleed [52] Lipids as %DW/QY(∆F /Fm ): r = −0.96 [46] In-situ fiber [46] Lipids predicted/observed: R 2 = 0.94 [31] Sampling [31] Fatty acids ISM/Image recognition DHA/cell diameter: R 2 = 0.98 (calibration) PBR in-situ probe [34] Protein Chlorophyll fluorometry Protein/Chl a fluorescence: r = 0.92 Fiber probe in PBR bypass [35] Carotenoids (C) VIS/NIR spectrum Predicted/observed C: r = 0.96 Fiber in sample [32] 2.1. Biomass Concentration Biomass concentration (BC, mostly expressed as dry weight concentration, DWC) is the basic biological variable in any microbial cultivation.…”

“…Salmi et al [30] estimated BC in an off-line mode by a commercial hyperspectral camera for imaging suspension samples (24-or 96-well plates) of five microalgal strains in the exponential growth phase. From the transmittance spectra, ratios of transmittance at 751/676 nm wavelengths (width 7 nm) were successfully correlated with the BC expressed as DWC or cell count (measured by an electronic cell counter or single-channel fluorometry) in a BC range differing by the strain.…”