How do microalgae perceive light in a high-rate pond? Towards more realistic Lagrangian experiments

Abstract

Hydrodynamics in a high-rate production reactor for microalgae cultivation affects the light history perceived by cells. The interplay between cell movement and medium turbidity leads to a complex light pattern, whose forcing effects on photosynthesis and photoacclimation dynamics are non-trivial. Hydrodynamics of high density algal ponds mixed by a paddle wheel has been studied recently, although the focus has never been on describing its impact on photosynthetic growth efficiency. In this multidisciplinary downscaling study, we first reconstructed single cell trajectories in an open raceway using an original hydrodynamical model offering a powerful discretization of the Navier–Stokes equations tailored to systems with free surfaces. The trajectory of a particular cell was selected and the associated high-frequency light pattern was computed. This light pattern was then experimentally reproduced in an Arduino-driven computer controlled cultivation system with a low density Dunaliella salina culture. The effect on growth and pigment content was recorded for various frequencies of the light pattern, by setting different paddle wheel velocities. Results show that the frequency of this realistic signal plays a decisive role in the dynamics of photosynthesis, thus revealing an unexpected photosynthetic response compared to that recorded under the on/off signals usually used in the literature. Indeed, the light received by a single cell contains signals from low to high frequencies that nonlinearly interact with the photosynthesis process and differentially stimulate the various time scales associated with photoacclimation and energy dissipation. This study highlights the need for experiments with more realistic light stimuli to better understand microalgal growth at high cell densities. An experimental protocol is also proposed, with simple, yet more realistic, step functions for light fluctuations.

Enhanced phycocyanin and protein content of Arthrospira by applying neutral density and red light shading filters: a small‐scale pilot experiment

“…In open ponds operated under natural conditions, even cells are subjected in photoinhibitory light levels, during agitation, cells are propelled towards the dark layers of the ponds (depths more than 5 cm), where photosystem II activity recovers rapidly in the dark . Ponds have generally longer dark periods than photobioreactors leading thus higher levels of recovery . This conclusion is supported as well from the results of the quantum yields of primary photochemistry.…”

“…42 Ponds have generally longer dark periods than photobioreactors leading thus higher levels of recovery. 43 This conclusion is supported as well from the results of the quantum yields of primary photochemistry.…”

Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: influence of grazing-mediated total ammonia-nitrogen

“…Changes in the TAN and NPQ levels, especially in outdoor conditions, can also occur due to abiotic stress conditions such as high light, temperature and nutrient load, relevant to wastewater-based cultivation. In addition, NPQ levels and photosynthetic processes are generally subjected to adverse effects across a light:dark cycle as cells move between intense light and darkness (Demory et al, 2018). Therefore, estimation of NPQ levels alone could indicate a potential false positive outcome.…”

Evaluation of photosynthetic light integration by microalgae in a pilot-scale raceway reactor

“…Recently, the utilization of Computational Fluid Dynamics (CFD) tools, such as ANSYS Fluent, have allowed us to analyse the hydrodynamics in real-scale reactors (Bitog et al, 2011;Fernández-Del Olmo et al, 2017;García et al, 2012;Soman and Shastri, 2015). The studies performed to date are mainly focused on the reactor's energy consumption and on how to minimize the pressure drop caused by the liquid circulation.…”