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Abstract The Great AtlanticSargassumBelt first appeared in 2011 and quickly became the largest interconnected floating biome on Earth. In recent years,Sargassumstranding events have caused substantial ecological and socio-economic impacts in coastal communities.Sargassumrequires both phosphorus (P) and nitrogen (N) for growth, yet the primary sources of these nutrients fuelling the extensiveSargassumblooms remain unclear. Here we use coral-bound N isotopes to reconstruct N₂fixation, the ultimate source of the ocean’s bioavailable N, across the Caribbean over the past 120 years. Our data indicate that changes in N₂fixation were primarily controlled by multidecadal and interannual changes in equatorial Atlantic upwelling of ‘excess P’, that is, P in stoichiometric excess relative to fixed N. We show that the supply of excess P from equatorial upwelling and N from the N₂fixation response can account for the majority ofSargassumvariability since 2011.Sargassumdynamics are best explained by their symbiosis with N₂-fixing epiphytes, which render the macroalgae highly competitive during strong equatorial upwelling of excess P. Thus, the future ofSargassumin the tropical Atlantic will depend on how global warming affects equatorial Atlantic upwelling and the climatic modes that control it. 

Subsurface environments are among Earth’s largest habitats for microbial life. Yet, until recently, we lacked adequate data to accurately differentiate between globally distributed marine and terrestrial surface and subsurface microbiomes. Here, we analyzed 478 archaeal and 964 bacterial metabarcoding datasets and 147 metagenomes from diverse and widely distributed environments. Microbial diversity is similar in marine and terrestrial microbiomes at local to global scales. However, community composition greatly differs between sea and land, corroborating a phylogenetic divide that mirrors patterns in plant and animal diversity. In contrast, community composition overlaps between surface to subsurface environments supporting a diversity continuum rather than a discrete subsurface biosphere. Differences in microbial life thus seem greater between land and sea than between surface and subsurface. Diversity of terrestrial microbiomes decreases with depth, while marine subsurface diversity and phylogenetic distance to cultured isolates rivals or exceeds that of surface environments. We identify distinct microbial community compositions but similar microbial diversity for Earth’s subsurface and surface environments.