Lopez-Alforja, X. | Search bioRxiv

bioRxiv2026

Single-cell sequencing reveals genome streamlining and functional diversity in ecologically dominant marine protists

A large fraction of marine protists, particularly the smallest ones, belong to uncultured lineages that lack genomic data, limiting insights into their ecological roles and evolutionary strategies. Here, we generated 325 single-cell amplified genomes (SAGs) from 2-5 {micro}m planktonic protists, which resulted in 147 genomes from dominant marine species at varying levels of completeness (40 of them >50%). These genomes matched the in situ community, with Prymnesiophyceae, Mamiellophyceae and Chrysophyceae dominating pigmented cells and MAST, Choanoflagellata and Picozoa prevailing among heterotrophic colourless cells. This resource allowed us to describe the genomic architecture of marine protist species, and revealed a pronounced genome streamlining in ecologically successful lineages. Comparative analyses highlighted unique functions enriched in photosynthetic and heterotrophic taxa (including motility, signal transduction, digestion and secondary metabolism), and revealed a broad distribution of gene families with adaptive traits such as polyketide synthases and rhodopsins. This large-scale single-cell genomics dataset provides a mechanistic foundation for investigating functional diversity, ecological strategies and genome evolution in the ocean.

ecology↗

bioRxiv2020

"The good, the bad and the double-sword" effects of exposure to MPs and their organic additives on N2-fixing bacteria

The accumulation of microplastics (MPs) pollution at depths suggests the susceptibility of benthic organisms (e.g. seagrasses and their associated macro- and micro-organisms) to the effects of these pollutants. Little is known about the direct effects of MPs and their organic additives on marine bacteria, e.g. in one of the most ecologically significant groups, the diazotrophs or N2-fixing bacteria. To fill this gap of knowledge, we exposed marine diazotrophs found in association with the endemic Mediterranean seagrass Posidonia oceanica to pure MPs which differ in physical properties (e.g. density, hydrophobicity and/or size), namely, polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS) and to their most abundant associated organic additives (e.g. fluoranthene, 1,2,5,6,9,10-hexabromocyclododecane [HBCD] and dioctyl-phthalate [DEHP]). Growth, protein overexpression, direct physical interactions between MPs and bacteria, phosphorus (P) acquisition mechanisms and N2-fixation rates were evaluated. Our results show species-specific responses of the autotrophic and heterotrophic N2-fixing bacteria tested and the responses were dependent on the type and concentration of MPs and additives. N2-fixing cyanobacteria were positively affected by environmental and high concentrations of MPs (e.g. PVC), as opposed to heterotrophic strains, that were only positively affected with high concentrations of [~]120 {micro}m-size MPs (detecting the overexpression of proteins related to plastic degradation and C-transport), and negatively affected by 1 {micro}m-size PS beads. Generally, the organic additives (e.g. fluoranthene) had a deleterious effect in both autotrophic and heterotrophic N2-fixing bacteria and the magnitude of the effect is suggested to be dependent on bacterial size. We did not find evidences that specific N2-fixation rates were significantly affected by exposure to MPs, albeit changes in bacterial abundance can affect the bulk N2-fixation rates. In summary, we reported for the first time, the beneficial (the "good"), deleterious (the "bad") and/or both (the "double-sword") effects of exposure to MPs and their organic additives on diazotrophs found in association with seagrasses.

microbiology↗