Biology subjects

Madireddi, S. K.

Publications and source records attributed to Madireddi, S. K..

3 recordsLinked to original sources

bioRxiv2025

Sustaining the cell energy in dynamic environments requires photosynthetic electron flows with diverse bandwidths

All living cells depend on the dynamic balance between their energy supply and demand to survive and thrive in dynamic environments. In extreme cases, like for photosynthetic organisms, their energy source, light, can fluctuate dramatically in intensity over timescales of seconds to hours. While various photosynthetic electron flows (EF) are crucial for maintaining bioenergetic homeostasis, how EFs are modulated to respond to dynamic energy intake remains unclear. Here, we show in the model green alga Chlamydomonas reinhardtii that each EF is best suited to a specific domain of energetic fluctuation periodicity for which it can support the cells energetic needs, which we term bandwidth. By systematically exposing cells to a range of light periodicities, we show that while cyclic EF has a large bandwidth, pseudo-cyclic EF (PCEF) can only sustain the cells energetic needs for fast light fluctuations, and that the interplay between the chloroplast and the mitochondria (CMEF) has a limited bandwidth. We further show that the bandwidths of PCEF and CMEF, specialized for dynamic lights, are related to their capacity to generate ATP and protect the photosynthetic apparatus. Finally, we show that in wild-type cells, the activity level of PCEF matches its bandwidth, and we propose that cells tune the relative activity of AEFs depending on the light fluctuation frequency. Our work opens an avenue of research to characterize the molecular mechanisms that can sustain phototrophic growth in complex and dynamic energetic landscapes. It further provides a generalizable framework for understanding the physiological importance of molecular mechanisms in a dynamic environment.

plant biology↗

bioRxiv2023

Algal CO2 capture is powered by alternative electron pathways of photosynthesis

On Earth, microalgae contribute to about half of global net photosynthesis. During photosynthesis, sunlight is converted into chemical energy (ATP and NADPH) used by metabolism to convert CO2 into biomass. Alternative electron pathways of photosynthesis have been proposed to generate additional ATP that is required for sustaining CO2 fixation, but the relative importance of each pathway remains elusive. Here, we dissect and quantify the contribution of cyclic, pseudo-cyclic and chloroplast to mitochondria electron flows for their ability to sustain net photosynthesis in the microalga Chlamydomonas reinhardtii. We show that each pathway has the potential to energize substantial CO2 fixation, can compensate each other, and that the additional energy requirement to fix CO2 is more than 3 times higher than previous estimations. We further show that all pathways have very different efficiencies at energizing CO2 fixation, with the chloroplast-mitochondria interaction being the most efficient, thus laying bioenergetic foundations for biotechnological improvement of CO2 capture.

plant biology↗

bioRxiv2023

Distinctive mechanism of LHCSR3 expression and function under osmotic stress in Chlamydomonas reinhardtii

Light-harvesting complex stress-related protein 3 (LHCSR3) expression is observed in various protoxidizing conditions like high light and nutrient starvation. LHCSR3 expression is essential for energy-dependent quenching (qE), whereas its role under nutrient starvation is elusive. It is also unclear how nutrient starvation can induce LHCSR3 expression under subsaturating light intensities. To study the role of LHCSR3 under nutrient starvation, the C. reinhardtii cells are grown under osmotic stress that would prevent water uptake; therefore same holds true for soluble nutrients in the medium. In this work, we have shown that LHCSR3 expression can occur under osmotic stress and subsaturating light intensities, whereas it does not elicit qE. Further examination of thylakoid membrane architecture from wild-type and npq4 mutant grown under nutrient starvation revealed that LHCSR3 expression affects the interaction between the PSII core with its peripheral LHCII antenna and possibly can prevent excitation energy transfer. Thylakoid lumen acidification is essential for the expression and function of LHCSR3. Under saturating light intensities, this is achieved by the increased rate of photosynthetic electron flow coupled with proton translocation into the thylakoid lumen. Whereas, under nutrient starvation, the reports of LHCSR3 expression also showed reduced photosynthetic electron flow. Therefore, an alternative mechanism should exist for developing the proton gradient. We observed the downregulation of chloroplast (cp) ATP synthase activity and its abundance under osmotic stress, suggesting the role of (cp) ATP synthase in thylakoid lumen acidification under reduced photosynthetic electron flow. This observation is supported by the expression of LHCSR3 in (cp) ATP-synthase mutant atpF upon exposure to moderate light intensity. This study proposes that the mechanism of LHCSR3 expression and its functionality can vary with the type of photooxidizing stress.

plant biology↗