Stochastic Character Mapping, Bayesian Model Selection, and Biosynthetic Pathways Shed New Light on the Evolution of Habitat Preference in Cyanobacteria

Structure and evolution of photosystem I in the early-branching cyanobacterium Anthocerotibacter panamensis

Thylakoid-free cyanobacteria are thought to preserve ancestral traits of early-evolving organisms capable of oxygenic photosynthesis. However, and until recently, photosynthesis studies in thylakoid-free cyanobacteria were only possible in the model strain Gloeobacter violaceus, limiting our understanding of photosynthesis evolution. Here, we report the isolation, biochemical characterization, cryo-EM structure, and phylogenetic analysis of photosystem I (PSI) from a recently discovered thylakoid-free cyanobacterium, Anthocerotibacter panamensis, a distant relative of the genus Gloeobacter. We find that A. panamensis PSI exhibits a distinct carotenoid composition and has one conserved low-energy chlorophyll site, which was lost in G. violaceus. Furthermore, PSI in thylakoid-free cyanobacteria has changed at the sequence level to a degree comparable to that of other strains, yet its subunit composition and oligomeric form might be identical to that of the most recent common ancestor of cyanobacteria. This study therefore provides a glimpse into the ancient evolution of photosynthesis.

Ecosystem relocation on Snowball Earth: Polar-alpine ancestry of the extant surface biosphere?

Geological observations informed by climate dynamics imply that the oceans were 99.9% covered by light-blocking ice shelves during two discrete, self-reversing Snowball Earth epochs spanning a combined 60 to 70 Myr of the Cryogenian Period (720 to 635 Ma). The timescale for initial ice advances across the tropical oceans is ~300 y in an ice-atmosphere-ocean general circulation model in Cryogenian paleogeography. Areas of optically thin oceanic ice are usually invoked to account for fossil marine phototrophs, including macroscopic multicellular eukaryotes, before and after each Snowball, but different taxa. Ecosystem relocation is a scenario that does not require thin marine ice. Assume that long before Cryogenian Snowballs, diverse supra- and periglacial biomes were established in polar-alpine regions. When the Snowball onsets occurred, those biomes migrated in step with their ice margins to the equatorial zone of net sublimation. There, they prospered and evolved, their habitat areas expanded, and the cruelty of winter reduced. Nutrients were supplied by dust (loess) derived from cozonal ablative lands where surface winds were strong. When each Snowball finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops while others restocked the oceans. This ecosystem relocation scenario makes testable predictions. The lineages required for post-Cryogenian biotic radiations should be present in modern polar-alpine biomes. Legacies of polar-alpine ancestry should be found in the genomes of living organisms. Examples of such tests are highlighted herein.

Wide-ranging organic nitrogen diets of freshwater picocyanobacteria

Freshwater picocyanobacteria (Syn/Pro clade) contribute substantially to the primary production of inland waters, especially when nitrogen is limiting or co-limiting. Nevertheless, they remain poorly understood ecologically and genomically, with research on their nitrogen acquisition mainly focused on inorganic sources. However, dissolved organic nitrogen is often a major component of the freshwater nitrogen pool and it is increasingly evident that many forms are bioavailable. Comparative genomic analyses, axenic growth assays, and proteomic analyses were used here to investigate organic nitrogen acquisition mechanisms in the Syn/Pro clade. Comparative analysis of the genomes of 295 freshwater and marine strains of picocyanobacteria identified a large diversity of amino acid transporters, the absence of degradation pathways for five amino acids (asparagine, phenylalanine, serine, tryptophan, and tyrosine), and alternative mechanisms for chitin assimilation (direct chitin catabolism vs initial acetylation to chitosan and subsequent degradation). Growth assays demonstrated the widespread bioavailability of amino acids, including basic amino acids though the known basic amino acid transporter is not encoded. This suggests further genetic components are involved, either through extracellular catabolism or the presence of novel transporters. Proteomic analysis demonstrates the dual utilization of nitrogen and carbon from the amino acid substrate and provides evidence for a mild stress response through the up-regulation of lysine biosynthesis and FtsH1, potentially caused by accumulation of secondary metabolites. Our results are relevant to understanding how picocyanobacteria have come to thrive in dissolved organic nitrogen-rich oligotrophic environments and explores how their different molecular capabilities may influence communities between habitats.

Bayesian tip-dated timeline for diversification and major biogeographic events in Muroidea (Rodentia), the largest mammalian radiation

BACKGROUND

Extinct organisms provide vital information about the time of origination and biogeography of extant groups. The development of phylogenetic methods to study evolutionary processes through time has revolutionized the field of evolutionary biology and led to an unprecedented expansion of our knowledge of the tree of life. Recent developments applying Bayesian approaches, using fossil taxa as tips to be included alongside their living relatives, have revitalized the use of morphological data in evolutionary tree inferences. Eumuroida rodents represent the largest group of mammals including more than a quarter of all extant mammals and have a rich fossil record spanning the last ~ 45 million years. Despite this wealth of data, our current understanding of the classification, major biogeographic patterns, and divergence times for this group comes from molecular phylogenies that use fossils only as a source of node calibrations. However, node calibrations impose several constraints on tree topology and must necessarily make a priori assumptions about the placement of fossil taxa without testing their placement in the tree.

RESULTS

We present the first morphological dataset with extensive fossil sampling for Muroidea. By applying Bayesian morphological clocks with tip dating and process-based biogeographic models, we provide a novel hypothesis for muroid relationships and revised divergence times for the clade that incorporates uncertainty in the placement of all fossil species. Even under strong violation of the clock model, we found strong congruence between results for divergence times, providing a robust timeline for muroid diversification. This new timeline was used for biogeographic analyses, which revealed a dynamic scenario mostly explained by dispersal events between and within the Palearctic and North African regions.

CONCLUSIONS

Our results provide important insights into the evolution of Muroidea rodents and clarify the evolutionary pathways of their main lineages. We exploited the advantage of tip dating Bayesian approaches in morphology-based datasets and provided a classification of the largest superfamily of mammals resulting from robust phylogenetic inference, inferring the biogeographical history, diversification, and divergence times of its major lineages.

Structure and evolution of Photosystem I in the early-branching cyanobacterium Anthocerotibacter panamensis

Thylakoid-free cyanobacteria are thought to preserve ancestral traits of early-evolving organisms capable of oxygenic photosynthesis. However, and until recently, photosynthesis studies in thylakoid-free cyanobacteria were only possible in the model strain Gloeobacter violaceus. Here, we report the isolation, biochemical characterization, cryo-EM structure, and phylogenetic analysis of photosystem I from a newly-discovered thylakoid-free cyanobacterium, Anthocerotibacter panamensis, a distant relative of the genus Gloeobacter. We find that A. panamensis photosystem I exhibits a distinct carotenoid composition and has one conserved low-energy chlorophyll site, which was lost in G. violaceus. These features explain the capacity of A. panamensis to grow under high light intensity, unlike other Gloeobacteria. Furthermore, we find that, while at the sequence level photosystem I in thylakoid-free cyanobacteria has changed to a degree comparable to that of other strains, its subunit composition and oligomeric form might be identical to that of the most recent common ancestor of cyanobacteria.

Early-Branching Cyanobacteria Grow Faster and Upregulate Superoxide Dismutase Activity Under a Simulated Early Earth Anoxic Atmosphere

The evolution of oxygenic photosynthesis during the Archean (4-2.5 Ga) required the presence of complementary reducing pathways to maintain the cellular redox balance. While the timing of the evolution of superoxide dismutases (SODs), enzymes that convert superoxide to hydrogen peroxide and O2, within bacteria and archaea is not resolved, the first SODs appearing in cyanobacteria contained copper and zinc in the reaction center (CuZnSOD). Here, we analyse growth characteristics, SOD gene expression (qRT-PCR) and cellular enzyme activity in the deep branching strain, Pseudanabaena sp. PCC7367, previously demonstrated to release significantly more O2 under anoxic conditions. The observed significantly higher growth rates (p < 0.001) and protein and glycogen contents (p < 0.05) in anoxically cultured Pseudanabaena PCC7367 compared to control cultures grown under present-day oxygen-rich conditions prompted the following question: Is the growth of Pseudanabaena sp. PCC7367 correlated to atmospheric pO2 and cellular SOD activity? Expression of sodB (encoding FeSOD) and sodC (encoding CuZnSOD) strongly correlated with medium O2 levels (p < 0.001). Expression of sodA (encoding MnSOD) correlated significantly to SOD activity during the day (p = 0.019) when medium O2 concentrations were the highest. The cellular SOD enzyme activity of anoxically grown cultures was significantly higher (p < 0.001) 2 h before the onset of the dark phase compared to O2-rich growth conditions. The expression of SOD encoding genes was significantly reduced (p < 0.05) under anoxic conditions in stirred cultures, as were medium O2 levels (p ≤ 0.001), compared to oxic-grown cultures, whereas total cellular SOD activity remained comparable. Our data suggest that increasing pO2 negatively impacts the viability of early cyanobacteria, possibly by increasing photorespiration. Additionally, the increased expression of superoxide-inactivating genes during the dark phase suggests the increased replacement rates of SODs under modern-day conditions compared to those on early Earth.