Algae after dark: mechanisms to cope with anoxic/hypoxic conditions

Introduction

Chlamydomonas is a powerful model for dissecting aspects of dark, oxic metabolism.

Dark metabolism in photosynthetic organisms

General aspects

Many algae not only have extensive fermentation networks available to generate ATP when O2 is not available, but are also able to respire intracellular energy stores (e.g. starch), as well as assimilate extracellular organic substrates (e.g. acetate and glucose) for growth/ATP generation when O2 becomes
available.
Dark, anoxic metabolism in photosynthetic microbes has important ecological consequences, as many algae and cyanobacteria excrete reduced energy carriers (e.g. organic acids/alcohols and H2) during the night when the environment becomes hypoxic or anoxic.

Mitochondrial mutants defective for heterotrophic growth

QQ图片20180620155951.png

The glyoxylate cycle

The glyoxylate cycle plays an essential role in heterotrophic growth by converting acetate to acetyl CoA, which then fuels gluconeogenesis and other anabolic pathways.

Dark, heterotrophic growth and metabolism

Heterotrophically grown microalgae often grow to higher cell densities and produce lipids, polyunsaturated fatty acids, carotenoids, tocopherol, pigments and other highvalue bioproducts at higher rates.

Interaction between mitochondria and chloroplasts

Communication between organelles is critical for survival of photosynthetic organisms.

Oxyhydrogen reaction

123.png

Disruption of chlamydomonas fermentation pathways

Anoxia and fermentation

Algal cells experiencing hypoxic/anoxic conditions typically generate energy by substrate-level phosphorylation;

11.png

Two major pyruvate-metabolizing enzymes of Chlamydomonas include the pyruvate formate lyase PFL1 and
the pyruvate:ferredoxin oxidoreductase PFR1 (the latter is sometimes designated PFOR).

Mutants affected in fermentation metabolism

The hydEF mutant
pfl1 mutants
The adh1 mutant
The stm6 mutant
The 2–on–2 hemoglobin mutant
pat/ack mutants

Acetate metabolism/fermentation

General aspects

Acetate may be used as the sole energy source for growth of Chlamydomonas when O2 is used as the terminal electron acceptor；
Under anoxic/hypoxic conditions, photophosphorylation appears to be necessary for sustained acetate assimilation