Non-destructive monitoring tools for biomass and astaxanthin production in Haematococcus pluvialis rotating biofilms: a spectroscopy approach
Résumé
Biofilm-based systems have gained increasing attention for microalgae cultivation due
to their potential to improve productivity, reduce water and energy demands and
eventually operating costs. However, effective operation of these systems requires
process control, which relies on the development of on-line monitoring of process
variables such as biomass, pigments and lipids contents. While there are various
methods and technologies for monitoring these biological parameters in microalgae
suspensions, at present, there is a lack of tools for non-destructively monitoring biofilmbased systems.
In this study, we developed a biofilm-based rotating system for cultivating
Haematococcus pluvialis biofilms for astaxanthin production (high-value carotenoid).
The dynamics of biomass and astaxanthin production were afterwards determined
under several light and nutrient conditions. The data collected were then used to
develop regression models based on Fourier-transform infrared (FTIR) and reflectance
spectroscopy (Vis-NIR; 380-1000 nm) to non-destructively monitor biofilm growth and
astaxanthin content.
FTIR spectroscopy is an effective technique for high-throughput screening in biology
as it can distinguish and quantify various components such as proteins, lipids, nucleic
acids, and carbohydrates. In H. pluvialis, the biosynthesis of astaxanthin matches the
accumulation of triacylglycerols (TAGs) leading to a complex macromolecular
reorganization (Figure 1), suggesting that FTIR spectra could be used to quantify the
astaxanthin content. Indeed, cell populations rich in astaxanthin could be easily
identified by the ratio between macromolecules, and a linear regression between
astaxanthin content and the lipids-to-proteins ratio was obtained (Figure 2). This
method is non-invasive, does not require a chemical extraction step, and can be used
to monitor and predict astaxanthin production in microalgal biofilm systems. However,
the strong absorption of water between 1700 and 900 cm-1 may limit its application for
real-time monitoring. To address this, we propose the use of reflectance spectroscopy,
which stands on the light reflected at specific wavelengths to calculate indexes that are
widely applied to remotely characterize plants and microphytobenthos communities.
These indexes strongly reflect specific physiological mechanisms and biochemical
composition, making them effective in estimating biomass, pigments, and identifying
stress changes. To demonstrate this, we applied the same principle to our biofilmbased system and develop indexes for estimating biomass and astaxanthin content.
Reflectance measurements were performed at the biofilm surface over time (Figure 3).
The most influential wavelengths were afterwards determined for these variables as
λ525, λ637, λ563, λ678 nm and near-infrared (NIR λ750-900 nm). A strong correlation
between biomass and an index based on NIR and λ525 nm was found (R2 = 0.938);
another was established between an index using λ563 and λ637 and astaxanthin
content (R2= 0.944) (Figure 4).
In summary, we developed, for the first time, non-destructive and in situ monitoring
tools for microalgae biofilm characterization. Our findings address one of the limitations
in process control, and have the potential to improve the operation of biofilm-based
systems, making them a promising technology for microalgal cultivation and the
production of high-value compounds