With her Vici grant, Anja Spang will shed light on the evolutionary history of symbionts

Without symbiosis, none of us would exist. Once upon a time, a bacterium lost its independence by merging with an archaeal cell. Now that bacterium is represented as mitochondria in all our cells. The mitochondrion is an organelle that powers eukaryotic cells – cells with a cell nucleus, such as those found in humans. Chloroplasts in green plants and algae were also once independent organisms.

Branches of the tree of life

Symbiosis is very common in the family tree of life and symbionts are integral for the functioning of natural ecosystems ranging from corals to forests and our human bodies. NIOZ scientist Anja Spang focuses on single-celled bacteria and archaea, which make up much of life’s biodiversity and include many symbionts with various different hosts ranging from other single-celled organisms to humans. While some of these symbionts are well known, others have only recently been discovered. ‘Within this grant we will be able to compare the origin, evolutionary history and global abundance of these various symbionts and thereby gain a deeper understanding of what it means to be a symbiont and what role they play in the context of our planet.’

Common denominators

Spang furthermore hopes to bridge symbiont research and identify common denominators. ‘In my project Symbioses across time and space I am going to focus on archaea and bacteria that are obligately dependent on another organism to be able to grow and thrive.’ She hopes to discover whether there are common features that characterizes the evolution of these symbionts, identify who their hosts are and determine when they evolved in geologic time. To uncover evolutionary history, she cannot rely on fossils, for example. She reconstructs ancestral protein content using phylogenetic models and machine learning approaches. ‘And we also cultivate symbionts and their hosts in the lab to study real-time evolution.’ 

See Figure 1: The DPANN symbiont Candidatus Nanohaloarchaeum antarcticus with its host .One of the obligate symbionts that will be studied within the frame of this grant and the model system we maintain in the laboratory at NIOZ. (Credit: Hamm et al., 2019, PNAS, 2019 Jul 16;116(29):14661-14670)