👤 Alexandra Z Worden

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. Show less

Photosynthetic picoeukaryotes contribute a significant fraction of primary production in the upper ocean. Micromonas pusilla is an ecologically relevant photosynthetic picoeukaryote, abundantly and widely distributed in marine waters. Grazing by protists may control the abundance of picoeukaryotes such as M. pusilla, but the diversity of the responsible grazers is poorly understood. To identify protists consuming photosynthetic picoeukaryotes in a productive North Pacific Ocean region, we amended seawater with living Show less

Marine stramenopiles (MASTs) are a diverse suite of eukaryotic microbes found in marine environments. Several MAST lineages are thought to contain heterotrophic nanoflagellates. However, MASTs remain uncultured and data on distributions and trophic modes are limited. We investigated MASTs in provinces on the west and east sides of the North Pacific Subtropical Gyre, specifically the East China Sea (ECS) and the California Current system (CALC). For each province, DNA was sampled from three zones: coastal, mesotrophic transitional, and more oligotrophic euphotic waters. Along with diatoms, chrysophytes, and other stramenopiles, sequences were recovered from nine MAST lineages in the six ECS and four CALC 18S rRNA gene clone libraries. All but one of these libraries were from surface samples. MAST clusters 1, 3, 7, 8, and 11 were identified in both provinces, with MAST cluster 3 (MAST-3) being found the most frequently. Additionally, MAST-2 was detected in the ECS and MAST-4, -9, and -12 were detected in the CALC. Phylogenetic analysis indicated that some subclades within these lineages differ along latitudinal gradients. MAST-1A, -1B, and -1C and MAST-4 size and abundance estimates obtained using fluorescence in situ hybridization on 79 spring and summer ECS samples showed a negative correlation between size of MAST-1B and MAST-4 cells and temperature. MAST-1A was rarely detected, but MAST-1B and -1C and MAST-4 were abundant in summer and MAST-1C and MAST-4 were more so at the coast, with maximum abundances of 543 and 1,896 cells ml(-1), respectively. MAST-4 and Synechococcus abundances were correlated, and experimental work showed that MAST-4 ingests Synechococcus. Together with previous studies, this study helps refine hypotheses on distribution and trophic modes of MAST lineages. Show less