Inorganic carbon acquisition and its energization in eustigmatophyte algae

Area of Science:

• * Eukaryotic cell biology
• * Algal physiology
• * Photosynthetic carbon assimilation

Background:

• * Eustigmatophyceans are basal ochrophytes, primarily photoautotrophic algae.
• * They inhabit diverse environments, including freshwater, soil, and marine ecosystems.
• * Understanding their inorganic carbon (Ci) uptake is key to their ecological roles.

Purpose of the Study:

• * Investigate inorganic carbon (Ci) uptake mechanisms in representative eustigmatophyceans.
• * Determine the role of carbonic anhydrase (CA) and active transport in Ci acquisition.
• * Elucidate the dependence of carbon fixation on mitochondrial activity.

Main Methods:

• * Mass spectrometry was employed to analyze Ci uptake characteristics.
• * Experiments involved freshwater (Monodus subterraneus, Vischeria stellata) and marine (Nannochloropsis gaditana) species.
• * Inhibitors and anoxia were used to probe transport dependencies.

Main Results:

• * All studied species exhibited a CO2 concentrating mechanism but lacked external CA.
• * Ci acquisition relied on active transport of bicarbonate (HCO3-), CO2, or both.
• * Nannochloropsis gaditana exclusively used HCO3-, even in the dark, while Monodus subterraneus used CO2, and Vischeria stellata used both.

Conclusions:

• * Eustigmatophyceans employ diverse active transport strategies for Ci uptake.
• * Intracellular CA is crucial for converting transported HCO3- to CO2 for fixation.
• * Mitochondrial respiration significantly supports photosynthetic CO2 fixation in these primitive algae.