An LED-based multi-actinic illumination system for the high throughput study of photosynthetic light responses

An automated high-throughput lighting system for screening photosynthetic microorganisms in plate-based formats

The capacity of photosynthetic microorganisms to fix carbon dioxide into biomass positions them as promising cell factories for sustainable biomanufacturing. However, limitations in screening throughput hinder the identification of enzymes, strains, and growth conditions needed to realize this potential. Here we present a microplate-based high-throughput cultivation system that can be integrated into existing automation infrastructure and supports growth of both prokaryotic and eukaryotic photosynthetic microorganisms. We validate this system by optimizing BG-11 medium compositions for Synechococcus elongatus UTEX 2973, Chlamydomonas reinhardtii UTEX 90 and Nostoc hatei CUBC1040, resulting in growth rates increases of 38.4% to 61.6%. We also identify small molecules that influence growth rates in Synechococcus elongatus UTEX 2973, including candidate compounds for growth rate increase and dozens that prevent growth. The sensitivity, throughput, and extensibility of this system support screening, strain isolation, and growth optimization needed for the development of photosynthetic microbial cell factories.

The Photoprotective Behavior of a Motile Benthic Diatom as Elucidated from the Interplay Between Cell Motility and Physiological Responses to a Light Microgradient Using a Novel Experimental Setup

It has long been hypothesized that benthic motile pennate diatoms use phototaxis to optimize photosynthesis and minimize photoinhibitory damage by adjusting their position within vertical light gradients in coastal benthic sediments. However, experimental evidence to test this hypothesis remains inconclusive, mainly due to methodological difficulties in studying cell behavior and photosynthesis over realistic spatial microscale gradients of irradiance and cell position. In this study, a novel experimental approach was developed and used to test the hypothesis of photosynthesis optimization through motility, based on the combination of single-cell in vivo chlorophyll fluorometry and microfluidic chips. The approach allows the concurrent study of behavior and photosynthetic activity of individual cells of the epipelic diatom species Craspedostauros britannicus exposed to a light microgradient of realistic dimensions, simulating the irradiance and distance scales of light microgradients in benthic sediments. Following exposure to light, (i) cells explored their light environment before initiating light-directed motility; (ii) cells used motility to lower their light dose, when exposed to the highest light intensities; and (iii) motility was combined with reversible non-photochemical quenching, to allow cells to avoid photoinhibition. The results of this proof-of-concept study not only strongly support the photoprotective nature of photobehavior in the studied species but also revealed considerable variability in how individual cells reacted to a light microgradient. The experimental setup can be readily applied to study motility and photosynthetic light responses of other diatom species or natural assemblages, as well as other photoautotrophic motile microorganisms, broadening the toolset for experimental microbial ecology research.

Photoinactivation vs repair of photosystem II as target of thermal stress in epipelic and epipsammic microphytobenthos communities

Microphytobenthos (MPB) inhabiting intertidal flats are exposed to large and sudden changes in temperature, often simultaneously with exposure to direct sunlight. These conditions are expected to negatively impact photosynthesis by exacerbating the photoinhibition under high light. This study addressed the photoinhibitory effects of short-term exposure to cold (5°C) and moderate heat (35°C) on MPB dominated by motile epipelic (EPL) and immotile epipsammic (EPM) diatom species, by evaluating the seasonal variation of photoinactivation and repair of photosystem II (PSII). The susceptibility to PSII photoinactivation and the counteracting repair capacity were measured by the constant rates kPI and kREC, respectively. The photoacclimation state was characterized by hysteresis light-response curves (HLC) of the relative electron transport rate, rETR, and of the nonphotochemical quenching index Y(NPQ). Under non-stress conditions (20°C), kREC was on average almost 10x higher than the corresponding kPI (20.4 vs 2.70 × 10-4 s-1, respectively), indicating the operation of efficient repair mechanisms. Overall, the exposure to low and high temperatures affected both PSII photoinactivation and repair but causing smaller impacts in the former than in the latter. Also, cold stress caused larger effects on repair (decrease of kREC) than on photoinactivation (increase of kPI), but heat stress affected similarly the two processes. These effects varied seasonally, suggesting a role of thermal acclimation, as heat stress had stronger effects in cold-acclimated samples and cold stress resulted in stronger effects in heat-acclimated samples. The changes in kPI and kREC occurred despite the high light-acclimated phenotype found all year round, indicating that these processes vary independently from the photoacclimation state. The results also showed that photoprotection processes, as measured by energy-dependent non-photochemical index qE, appear to have an important role, both by preventing PSII photoinactivation and by alleviating the impacts on PSII repair under acute thermal stress.

Nanocosm: a well plate photobioreactor for environmental and biotechnological studies

Phytoplankton are key primary producers at the bottom of the aquatic food chain. They are a highly diverse group of organisms essential for the functioning of our ecosystems and because of their characteristics, their biomass is considered for various commercial applications. A full appreciation of their abundance, diversity and potential is only feasible by using systems that enable simultaneous testing of strains and/or variables in a fast and easy way. A major bottleneck is the lack of a cost-effective method with the capacity for complex experimental set-ups that enable fast and reproducible screening and analysis. In this study, we present nanocosm, a versatile LED-based micro-scale photobioreactor (PBR) that allows simultaneous testing of multiple variables such as temperature and light within the same plate. Every well can be independently controlled for intensity, temporal variation and light type (RGB, white, UV). We show that our systems guarantee homogeneous conditions because of controlled temperature and evaporation and adjustments for light crosstalk. By ensuring controlled environmental conditions the nanocosm is suitable for running factorial experimental designs where each well can be used as an independent micro-PBR. To validate culture performances, we assess well-to-well reproducibility and our results show minimal well-to-well variability for all the conditions tested. Possible modes of operation and application are discussed together with future development of the system.

Photoinhibition in optically thick samples: Effects of light attenuation on chlorophyll fluorescence-based parameters

Oxygenic photoautotrophs are, paradoxically, subject to photoinhibition of their photosynthetic apparatus, in particular one of its major components, the Photosystem II (PSII). Photoinhibition is generalized across species, light conditions and habitats, imposing substantial metabolic costs that lower photosynthetic productivity and constrain the niches of photoautotrophy. As a process driven by light reaching PSII, light attenuation in optically thick samples influences both the actual extent, and the detection, of photoinhibition. Chlorophyll fluorescence is widely used to measure photoinhibition, but fluorescence-based parameters are affected by light attenuation of both downwelling incident radiation traversing the sample to reach PSII, and emitted fluorescence upwelling through the sample. We used modelling, experimental manipulation of within-sample light attenuation, and meta-analysis of published data, to show substantial, differential effects of light attenuation and depth-integration of emitted fluorescence upon measurements of photoinhibition. Numerical simulations and experimental manipulation of light attenuation indicated that PSII photoinactivation tracked using chlorophyll fluorescence can appear to be over three times lower than the inherent cellular susceptibility to photoinactivation, in optically-dense samples such as leaves or biofilms. The meta-analysis of published data showed that this general trend was unknowingly present in the literature, revealing an overall difference of more than five times between optically thick leaves and optically thin cell suspensions. Although fluorescence-based parameters may provide ecophysiologically relevant information for characterizing the sample as a whole, light attenuation and depth integration can vary between samples independently of their intrinsic physiology. They should be used with caution when aiming to quantify in absolute terms inherent photoinhibition-related parameters in optically thick samples.