Marine Group II Archaea: A Review with a Focus on the Arctic Ocean

Marine Group II (MGII) archaea are among the most abundant planktonic archaea in the global ocean, yet no cultured representative exists, and most of what is known about them comes from 16S rRNA gene surveys and metagenome-assembled genomes (MAGs). Here we review the taxonomy, biogeography, physiology, and metabolism of MGII archaea, now proposed as the order Candidatus Poseidoniales, with its two main subgroups MGIIa (Ca. Poseidoniaceae) and MGIIb (Ca. Thalassoarchaeaceae). MGIIa tend to dominate surface waters and are often described as motile and particle-associated, whereas MGIIb are more common at depth. Both subgroups encode enzymes for degrading high-molecular-weight organic matter, and many surface populations carry proteorhodopsin, suggesting a photoheterotrophic lifestyle. MGII archaea co-occur with heterotrophic bacteria such as SAR11 and SAR86 and respond to phytoplankton blooms, although their seasonal patterns differ among ocean regions. We then summarize what is known about MGII archaea in the Arctic Ocean, where they are widespread but comparatively little studied, covering their distribution across water masses, associations with particles, seasonal dynamics, and the metabolic potential of Arctic MAGs. Finally, we discuss how sea ice loss and increasing stratification may affect MGII archaea and outline priorities for future work in this rapidly changing ocean.