3D intrusions transport active surface microbial assemblages to the dark ocean

Macroecological patterns of planktonic unicellular eukaryotes richness in the Southeast Pacific Ocean

In recent years, studies focusing on microbial biogeography have been developed, but macroecological processes in marine microorganisms remain unclear, especially in seemingly continuous environments such as the Southeast Pacific Ocean (SPO), where information on microbial distribution patterns is limited, and they may vary depending on the habitat and lifestyle. We used unicellular planktonic eukaryotes as model organisms to determine their biogeographic patterns in the SPO, identify the underlying ecological and historical-evolutionary processes and compare with other microorganism groups. Our analyses were based on the Niche Theory to model species diversity distribution using large open-access ecological and physical-biogeochemical databases based on Bayesian approaches, an integrated nested Laplace approximation (INLA), and Generalized Additive Models (GAM). As a result, two richness hotspots were observed, which are associated with coastal and offshore regions in the central southern areas of SPO. The richness hotspots were associated mainly with nutrients (N/Si ratio) and Mixed Layer Depth (MLD), which could be explained by highly productive upwelling events in the SPO. In contrast, the negative correlation of predicted richness with low pH is strongly related to the effect of calcareous shells (tests), as lower pH levels hinder the formation and stability of calcium carbonate shells in protists like foraminifera and radiolaria, thereby affecting overall unicellular planktonic eukaryote diversity. Our results support the role of ecological processes related to productivity, energy dynamics, and ecological limits in shaping broad-scale diversity patterns of unicellular planktonic eukaryotes in the SPO. The results show colonization and extinction dynamics through species replacement (i.e. High Turnover) along the Chilean and Equatorial coasts associated mainly with the Hotspots of their biodiversity, but also a gradual species loss (i.e. High Nestedness) along the Peruvian Coast associated mainly with the Coldspots of their biodiversity; highlighting how local environmental fluctuations can shape these planktonic microorganisms' behavior, ecology and distribution. The distribution patterns of planktonic unicellular eukaryotes show little evidence of the effects of historical and evolutionary processes. This is because the high dispersal capacity of planktonic microbes probably dilutes the influence of these processes in environments lacking clear barriers to species dispersal. Additionally, the effect of historical events could be highlighted in specific taxonomic groups at the kingdom, phylum level or habitat type and addressing gaps about latitudinal richness in the SPO. This provides insight into the spatial distribution of marine microbes and contributes to conservation efforts, as these organisms are an essential foundation of the upper levels of the food web.

Symbionts of predatory protists are widespread in the oceans and related to animal pathogens

Protists are major predators of ocean microbial life, with an ancient history of entanglements with prokaryotes, but their delicate cell structures and recalcitrance to culturing hinder exploration of marine symbioses. We report that tiny oceanic protistan predators, specifically choanoflagellates-the closest living unicellular relatives of animals-and uncultivated MAST-3 form symbioses with four bacterial lineages related to animal symbionts. By targeting living phagotrophs on ship expeditions, we recovered genomes from physically associated uncultivated Legionellales and Rickettsiales. The evolutionary trajectories of Marinicoxiellaceae, Cosmosymbacterales, Simplirickettsiaceae, and previously named Gamibacteraceae vary, including host-engagement mechanisms unknown in marine bacteria, horizontally transferred genes that mediate pathogen-microbiome interactions, and nutritional pathways. These symbionts and hosts occur throughout subtropical and tropical oceans. Related bacteria were detected in public data from freshwater, fish, and human samples. Symbiont associations with animal-related protists, alongside relationships to animal pathogens, suggest an unexpectedly long history of shifting associations and possibilities for host expansion as environments change.