The first eukaryotic cells — Acid hot-spring algae

TLDR: In this article, the authors suggested placing these algae in progressively evolutionary steps: (Cyanidioschyzon → Cyanidium → Galdieria) based on various areas of research like biochemistry, amount of nuclear genome and shape of chloroplast nucleoid, ultrastructure and ecological aspects.

Abstract: The Cyanidiophyceae members (PreRhodophyta) may serve as a transitional algal group bridging the cyanobacteria and the unicellular Rhodophyta. This thermoacidic algal group is composed of three genera containing several species. We suggested placing these algae in progressively evolutionary steps: (Cyanidioschyzon → Cyanidium → Galdieria). This evolutional ladder is based upon various areas of research like biochemistry, amount of nuclear genome and shape of chloroplast nucleoid, ultrastructure and ecological aspects. The first alga —Cyanidioschyzon — is the cornerstone of this succession; it shows mixed features between cyanobacterium and archaebacteria(Thermoplasma-like cell). It demonstrates simple eukaryotic cellular features and has the smallest amount of nuclear and chloroplast DNA. The intermediate alga in this line,Cyanidium, is also a simple cell, but shows more progressive characterizations than theCyanidioschyzon. The third taxon,Galdieria, is already very close to the unicellular rhodophytes (red algae) and indicates typical advanced eukaryotic characterization. We propose thatCyanidioschyzon (considered to be the simplest eukaryote) may have evolved from an association betweenThermoplasma-like archaebacterium and a thermophilic cyanobacterium. Autogenous (non-symbiotic) compartmental steps may have taken place fromCyanidioschyzon toCyanidium and then toGaldieria, and from this alga (group) towards the other unicellular red algae.