# Ecological Processes Shaping Marine Microbial Assemblages Diverge Between Equatorial and Temperate Time‐Series

## TL;DR

Marine microbes in tropical and temperate regions are shaped by different ecological processes, with temperate microbes showing more environmental influence and tropical microbes being more stable and biologically driven.

## Contribution

This study compares microbial assemblages across equatorial and temperate regions using standardized protocols, revealing divergent ecological processes.

## Key findings

- Temperate microbial communities show strong seasonal turnover and tighter environmental coupling.
- Tropical communities are more stable and influenced by biotic factors and stochastic processes.
- Ecological patterns are consistent across domains and size fractions, with stronger selection in prokaryotes.

## Abstract

Marine microbial communities are structured by a complex interplay of deterministic and stochastic processes, yet how these vary across latitudes remains poorly understood. Most long‐term microbial observatories are restricted to temperate regions, limiting our ability to assess latitudinal contrasts in microbial dynamics. Here, we compare coastal microbial communities from two contrasting marine time‐series stations using standardised molecular protocols: a new tropical site in the Equatorial Atlantic (EAMO, 6° S) and a well‐studied temperate site in the Mediterranean Sea (BBMO, 41° N). Monthly 16S and 18S rRNA gene sequencing of two size‐fractions (0.22–3 μm and > 3 μm) over 41 months (from April 2013 to August 2016) revealed marked differences in taxonomic composition, temporal variability and ecological assembly processes. Temperate communities exhibited strong seasonal turnover, higher beta‐diversity and tighter coupling with environmental variables such as temperature and daylength. In contrast, tropical communities were compositionally more stable and more governed by biotic factors and stochastic processes such as historical contingency and ecological drift. These patterns were consistent across taxonomic domains and size‐fractions, though selection was generally stronger in prokaryotes and the smallest size‐fraction. Co‐occurrence networks at the temperate site were more densely connected and environmentally responsive compared to tropical networks, where stochastic processes and putative biological interactions gain prominence. This study highlights the importance of integrating observatories from underrepresented latitudes into global microbial monitoring efforts, particularly as climate change alters the amplitude and frequency of environmental drivers across the ocean.