The Water to Water Cycles in Microalgae.

Area of Science:

• Photosynthesis research
• Plant physiology
• Microalgal metabolism

Background:

• Oxygenic photosynthesis generates ATP and NADPH via linear electron transfer (LET) involving Photosystem I (PSI) and Photosystem II (PSII).
• LET may not supply sufficient ATP for the Calvin-Benson-Bassham cycle, necessitating alternative ATP-generating pathways.
• Alternative electron flow (AEF) pathways create proton gradients for ATP synthesis without producing NADPH.

Purpose of the Study:

• To review the current understanding of water-to-water cycles in photosynthesis.
• To focus on the occurrence and physiological roles of these cycles in microalgae.
• To highlight the importance of AEF in optimizing carbon assimilation.

Main Methods:

• Literature review of existing research on photosynthesis, electron transfer pathways, and microalgal physiology.
• Synthesis of knowledge regarding cyclic electron flow around PSI and water-to-water cycles.
• Analysis of components of water-to-water cycles: Mehler reaction, plastoquinone terminal oxidase, photorespiration, and malate shuttle.

Main Results:

• Water-to-water cycles, alongside cyclic electron flow, are recognized as key AEF pathways in Viridiplantae.
• These cycles involve photosynthetic O2 reduction (Mehler reaction), plastoquinone terminal oxidase activity, photorespiration (Rubisco oxygenase activity), and malate shuttle.
• AEF pathways, though often a small fraction of LET, significantly contribute to optimizing CO2 assimilation by balancing ATP/NADPH ratios.

Conclusions:

• Water-to-water cycles play a vital role in supplementing ATP production for efficient carbon assimilation in photosynthesis.
• Understanding these alternative pathways is crucial for comprehending the overall efficiency and regulation of photosynthesis, particularly in microalgae.
• Further research into the specific mechanisms and physiological significance of water-to-water cycles in microalgae is warranted.