Interplay between LHCSR proteins and state transitions governs the NPQ response in intact cells of Chlamydomonas during light fluctuations.

Photosynthetic organisms use sunlight as the primary energy source to fix CO2. However, in the environment, light energy fluctuates rapidly and often exceeds saturating levels for periods ranging from seconds to hours, which can lead to detrimental effects for cells. Safe dissipation of excess light energy occurs primarily by non-photochemical quenching (NPQ) processes. In the model green microalga Chlamydomonas reinhardtii, photoprotective NPQ is mostly mediated by pH-sensing light-harvesting complex stress-related (LHCSR) proteins and the redistribution of light-harvesting antenna proteins between the photosystems (state transition). Although each component underlying NPQ has been documented, their relative contributions to the dynamic functioning of NPQ under fluctuating light conditions remains unknown. Here, by monitoring NPQ throughout multiple high light-dark cycles with fluctuation periods ranging from 1 to 10 minutes, we show that the dynamics of NPQ depend on the frequency of light fluctuations. Mutants impaired in the accumulation of LHCSRs (npq4, lhcsr1, and npq4lhcsr1) showed significantly less quenching during illumination, demonstrating that LHCSR proteins are responsible for the majority of NPQ during repetitive exposure to high light fluctuations. Activation of NPQ was also observed during the dark phases of light fluctuations, and this was exacerbated in mutants lacking LHCSRs. By analyzing 77K chlorophyll fluorescence spectra and chlorophyll fluorescence lifetimes and yields in a mutant impaired in state transition, we show that this phenomenon arises from state transition. Finally, we quantified the contributions of LHCSRs and state transition to the overall NPQ amplitude and dynamics for all light periods tested and compared those with cell growth under various periods of fluctuating light. These results highlight the dynamic functioning of photoprotection under light fluctuations and open a new way to systematically characterize the photosynthetic response to an ever-changing light environment.

Cultivating Microalgae in Desert Conditions: Evaluation of the Effect of Light-Temperature Summer Conditions on the Growth and Metabolism of Nannochloropsis QU130

Temperature and light are two of the most crucial factors for microalgae production. Variations in these factors alter their growth kinetics, macromolecular composition and physiological properties, including cell membrane permeability and fluidity. The variations define the adaptation mechanisms adopted by the microalgae to withstand changes in these environmental factors. In the Qatar desert the temperature varies widely, typically between 10° and 45 °C There are also wide variations in light intensity, with values of over 1500 μmolhν.m−2s−1 in summer. A study of the effects of these thermal and light fluctuations is therefore essential for large-scale outdoor production systems, especially during the summer when temperature and light fluctuations are at their highest. The aim of this work is to study the impact of temperature and light intensity variations as encountered in summer period on the Nannochloropsis QU130 strain, which was selected for its suitability for outdoor cultivation in the harsh conditions of the Qatar desert. It was carried out using lab-scale photobioreactors enabling simulation of both constant and dynamic temperature and light regimes. Biomass productivity, cell morphology and biochemical compositions were examined first in constant conditions, then in typical outdoor cultivation conditions to elucidate the adjustments in cell function in respect of fluctuations. The dynamic light and temperature were shown to have interactive effects. The application of temperature cycles under constant light led to a 13.6% increase in biomass productivity, while a 45% decrease was observed under light and temperature regimes due to the combined stress. In all cases, the results proved that N. sp. QU130 has a high level of adaptation to the wide fluctuations in light and temperature stress. This was shown through its ability to easily change its physiology (cell size) and metabolic process in response to different cultivation conditions.

Bi-directional electron transfer between H2 and NADPH mitigates the response to light fluctuations in green algae

The metabolism of green algae has been the focus of much research over the last century. These photosynthetic organisms can thrive under various conditions and adapt quickly to changing environments by concomitant usage of several metabolic apparatuses. The main electron coordinator in their chloroplasts, nicotinamide adenine dinucleotide phosphate (NADPH), participates in many enzymatic activities and is also responsible for interorganelle communication. Under anaerobic conditions, green algae also accumulate molecular hydrogen (H2), a promising alternative for fossil fuels. However, in order to scale-up its accumulation, a firm understanding of its integration in the photosynthetic apparatus is still lacking. While it is generally accepted that NADPH metabolism correlates to H2 accumulation, the mechanism of this collaboration is still vague and rely on indirect measurements. Here, we investigated this connection using simultaneous measurements of both dissolved gases concentration, NADPH fluorescence and electrochromic shifts at 520-546 nm. Our results indicate that energy transfer between H2 and NADPH is bi-directional and crucial for the maintenance of redox balance under light fluctuations. At light onset, NADPH consumption is initially eventuated in H2 evolution, which initiate the photosynthetic electron flow. Later on, as illumination continues the majority of NADPH is recycled by Nda2 rather than consumed by terminal sinks such as CBB cycle and H2 production. Dark onset triggers re-assimilation of H2, which produces NADPH and so, enables initiation of dark fermentative metabolism.One sentence summaryEnergy transfer between H2 and NADPH is bi-directional and crucial for the maintenance of redox balance under light fluctuations.

Photosynthesis Regulation in Response to Fluctuating Light in the Secondary Endosymbiont Alga Nannochloropsis gaditana

In nature, photosynthetic organisms are exposed to highly dynamic environmental conditions where the excitation energy and electron flow in the photosynthetic apparatus need to be continuously modulated. Fluctuations in incident light are particularly challenging because they drive oversaturation of photosynthesis with consequent oxidative stress and photoinhibition. Plants and algae have evolved several mechanisms to modulate their photosynthetic machinery to cope with light dynamics, such as thermal dissipation of excited chlorophyll states (non-photochemical quenching, NPQ) and regulation of electron transport. The regulatory mechanisms involved in the response to light dynamics have adapted during evolution, and exploring biodiversity is a valuable strategy for expanding our understanding of their biological roles. In this work, we investigated the response to fluctuating light in Nannochloropsis gaditana, a eukaryotic microalga of the phylum Heterokonta originating from a secondary endosymbiotic event. Nannochloropsis gaditana is negatively affected by light fluctuations, leading to large reductions in growth and photosynthetic electron transport. Exposure to light fluctuations specifically damages photosystem I, likely because of the ineffective regulation of electron transport in this species. The role of NPQ, also assessed using a mutant strain specifically depleted of this response, was instead found to be minor, especially in responding to the fastest light fluctuations.