Elsevier

Algal Research

Volume 54, April 2021, 102176
Algal Research

Photosynthetic performance of Chlorella vulgaris R117 mass culture is moderated by diurnal oxygen gradients in an outdoor thin layer cascade

https://doi.org/10.1016/j.algal.2020.102176Get rights and content

Highlights

  • Outdoor culture of Chlorella R-117 produced high dissolved O2 gradients.

  • In-situ/ex-situ monitoring of O2 production and Chl fluorescence were correlated.

  • Photosynthetic performance was evaluated by various techniques during diurnal cycle.

  • Photosynthetic activity was reduced as dissolved oxygen concentration increased.

  • No photoinhibition effects, but down-regulation of PSII activity was observed.

Abstract

Dissolved oxygen concentration is a critical point for microalgae in large scale cultivation systems. Highly productive cultures inevitably generate a build-up of oxygen gradients along the reactor which can affect photosynthetic performance. In this study, a fast-growing strain of Chlorella vulgaris R117 was cultured outdoors in a thin-layer cascade during a one-week trial reaching a biomass productivity of 4 g DW L−1 d−1 (27 g DW m−2 d−1). High photosynthetic activity led to oxygen oversaturation of up to 400% in some parts along the culture units at midday. The aim was to examine the effect of high dissolved oxygen concentration on diurnal changes in the photosynthetic performance and growth of the Chlorella culture using multi-technique approach. Photosynthetic activity of Chlorella R117 culture was estimated in situ and ex situ using oxygen production and in vivo Chl a fluorescence measurements, which showed good correlation. The rates of electron transport and of oxygen production were related, but the values of the μmolelectrons/μmolO2 ratio was higher than predicted, suggesting the probable involvement of electron and oxygen consuming processes such as photorespiration and Mehler reaction. These processes probably function as photoprotective mechanisms, since no photodamage was observed in the Chlorella R-117 cultures. Depression (down-regulation) of photosynthetic activity due to the exposition to high dissolved oxygen concentration along the cascade area over time was observed. The usefulness of on-line measurements was demonstrated to obtain immediate and in-situ information on the physiological status of the culture. This data can be used in models of operation control for large-scale microalgae production units.

Introduction

Shallow outdoor units for microalgae culturing were designed in the former Czechoslovakia during the 1950–1960s and used as one of the first large-scale microalgal culture systems in Europe [1]. The so-called thin-layer cascades (TLCs) benefits from the advantages of open systems – direct sun irradiance, cooling by evaporation, simple cleaning and maintenance, and efficient degassing. They also profit from several features usually associated with closed systems, such as the operation at high cell densities combined with high biomass productivity and harvesting efficiency. Nevertheless, large-scale cultivation in open units can suffer from some natural and technological perturbations [2]. As the production is scaled up, the control of algal growth becomes more complex and requires the regulation of several interconnected culture variables to achieve optimum values. Among them, irradiance, temperature, supply of nutrients, pH, and dissolved oxygen concentration (DO concentration), particularly when acting in synergy, determine the productivity of the culture [3].

The high ratio of exposed surface to total volume S/V (about 100 m−1 and higher) in TLCs results in a very high light supply per unit of culture volume, a feature that increases their volumetric biomass productivity [4], resulting in a high dissolved oxygen accumulation in the culture which can modulate photosynthetic performance of microalgal cultures [5]. Indeed, if the exchange rate of DO with the atmosphere cannot match photosynthetic production, high gradients can develop and have adverse effects on growth/productivity of microalgae cultures due to photoinhibition under high light [6], photorespiration, associated with the oxygenase activity of the Rubisco [7], and oxidative stress [8]. High gradients along the cultivation unit during the day in synergy with high irradiance, can damage the photosynthetic apparatus due to oxidative stress. While there are several reports concerning the effect of high concentrations of oxygen on culture productivity and biomass composition [6], information on the effect of oxygen gradients on photosynthesis in outdoor TLCs is not so common, most likely this is due to the necessity to perform fast measurements to catch warning signals of changes in photosynthesis rates which are not possible using measurements of DO, or chlorophyll. In addition, the variation of dissolved oxygen during cultivation and its relationship with the different steps and pathways of electron transport rate is still unclear [9,10].

Under laboratory conditions the most commonly used measurement of photosynthetic activity in microalgae cultures is the rate of oxygen production [11], this is less so in outdoor conditions [12]. Starting in the 1990s, chlorophyll (Chl) a fluorescence techniques were introduced to monitor microalgae mass cultures [13,14]. These techniques assess the photosynthetic efficiency estimating the energy distribution between the photochemical and non-photochemical processes [13,15]. The simultaneous in-situ measurement of DO concentration and Chl fluorescence can provide a complete and fast estimate of the physiological status of microalgae at various locations of the cultivation unit. Mathematical modelling can be employed in both in-situ/ex-situ measurements of photosynthesis (oxygen production, carbon fixation or electron transport) to predict the growth and productivity of phytoplankton populations as well as microalgae cultures [16]. This idea initiated the establishment of the Group for Aquatic Primary Productivity (GAP). As mentioned in the first GAP publication [17], these workshops emphasize a multi-method approach for monitoring algal photosynthesis which can provide better comparability and reliability of obtained data and its interpretation.

In the presented study, the primary production by the fast-growing Chlorella vulgaris strain R-117 was measured outdoors in a thin layer cascade and was characterized by its high growth rate, producing increased DO gradients. The aim of this work was to study the effect of high DO concentration along the TLC on diurnal changes of the photosynthetic performance of the Chlorella culture using a multi-technique approach. Photosynthetic activity measurements using in vivo Chl a fluorescence techniques and oxygen evolution were compared and discussed. A complex set of information has been provided to clarify the response of very dense outdoor cultures to extreme DO gradients.

Section snippets

Experimental design and culture conditions

Experiments were carried out during the 10th International GAP workshop held at Centre Algatech (GPS coordinates – 48°59′15” N; 14°46′40.630″ E), Institute of Microbiology of the Czech Academy of Science (Třeboň, Czech Republic) in August 2017. A fast-growing strain of Chlorella vulgaris strain R117 (CCALA 1107, Culture Collection of Autotrophic Organisms, Institute of Botany, Třeboň, Czech Republic) (further as Chlorella R117) was used in a one-week trial starting on the 21st of August 2017

Results and discussion

The effect of DO concentration on the photosynthetic performance was evaluated by monitoring oxygen production and in vivo Chl a fluorescence measurements. One approach was to monitor in vivo Chl a fluorescence on-line/in-situ during the diel cycle to estimate the actual photosynthetic activity and the other was to measure ex-situ using dark-adapted samples collected at certain time periods from the outdoor culture unit [34]. Then, both approaches can be correlated to get the complex set of

Conclusions

A spectrum of in-situ and ex-situ photosynthesis monitoring techniques was applied simultaneously to examine the photosynthetic performance of the highly productive culture of Chlorella R117 grown outdoors in thin-layer cultivation cascades. During the diel cycle the culture experienced high temporal and spatial gradients in DO concentration along the cultivation area. A certain depression (down-regulation) of photosynthetic activity was observed in some periods of the day (midday) due to the

CRediT authorship contribution statement

Tomás Agustín Rearte: Conceptualization, measurements, data analysis, figure and manuscript preparation. Amir Neori, Paula S.M. Celis-Plá and Félix Álvarez Gómez: Conceptualization, measurements, data analysis, figure and manuscript editing. Jiří Masojídek, Giuseppe Torzillo, Félix López Figueroa and Karolína Ranglová: Team management, planning of experiments, measurements, figure and manuscript editing. Cintia Gómez, Ana Margarita Silva Benavides, Roberto Abdala, Martín Caporgno, Thaís Fávero

Declaration of competing interest

The authors declare that they do not have any competing financial interests or personal relationships that could influence this manuscript. No conflicts, informed consent, or human or animal rights are applicable to this study.

Acknowledgements

This work was funded by National Sustainability Programme I of The Ministry of Education, Youth and Sports of the Czech Republic (project Algatech Plus LO1416) and in part by the EU programme Horizon 2020 (project SABANA, grant no. 727874) and Interreg Czech Republic-Austria programme (project Algenetics ATCZ15). TAR was supported by the Universidad de Buenos Aires, Argentina. FLF thanks Junta de Andalucía for financial support of the research group “Photobiology and biotechnology of aquatic

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