Plastids are the photosynthetically active organelle found in all recent photoautotrophic eukaryotes. All but one subfamily, ranging from red and green algae to higher plants can trace the origin of their plastids back to a single event of primary endosymbiosis 1.5 billion years ago. Here, a heterotrophic eukaryote engulfed a cyanobacterium, which was not digested but instead evolved into the ancestor of the plastids in the Archaeplastida.

We want to investigate the early stages of this endosymbiosis, where the endosymbiont still resembles a free-living cyanobacterium. For this relationship to be selected for, the relationship has to be beneficial for both partners even in these early stages. A likely factor that stabilized this interaction was metabolic connectivity, where the cyanobacterial cell efficiently exported fixed carbon to provide a benefit to the host cell. Additionally, there is evidence to suggest that Chlamydia-like, prokaryotic endoparasites aided the metabolic linking of the cyanobacterial and eukaryotic partners. Using cyanobacteria with an altered central carbon metabolism and chlamydial sugar-phosphate- and ATP-transporters we want to generate a minimal endosymbiotic metabolism in model cyanobacteria (Synechocystis PCC 6803 and Synechococcus UTEX 2973). The metabolic effects will be evaluated in detail to investigate the modified cyanobacteria’s potential to enable endosymbiosis.

Unicellular algae are a highly diverse group of photosynthetic microorganisms that inhabit a wide range of diverse habitats. As the only photosynthetic eukaryotes, red algae of the class Cyanidiales thrive in extreme environments with low pH 0-4, high temperatures (up to 56°C) and high salinity making them ideal candidates for large-scale cultivation and industrial applications. 

All three genera of Cyanidiales (Cyanidioschyzon, Cyanidium and Galdieria) grow photoautotrophically using inorganic CO₂ as a carbon source. However, Galdieria has evolved the ability to import and metabolize a wide range of organic carbon sources enabling heterotrophic and mixotrophic lifestyles. We want to understand (i) how inorganic carbon is efficiently fixed under low dissolved CO₂ conditions (low pH), (ii) if and how simultaneous carbon acquisition through photosynthetic CO₂ fixation and respiration of organic carbon facilitate mixotrophy in Galdieria, and (iii) how the mechanisms investigated in (i) and (ii) can be utilized or manipulated to increase growth rates and algal biomass. For this purpose, we perform comparative genomics of different algal strains, transcriptomic analyses (RNA-Seq) under different growth conditions, as well as analysis of steady-state metabolite levels and metabolic fluxes using isotope labeled metabolites by GC-MS and IC-MS.