# Unravelling the molecular diversity of marine cyanobacterial communities in the lagoon of Nouméa (New Caledonia): impact of a cyclonic event

## TL;DR

This study explores how cyclones and environmental factors shape cyanobacterial communities in a tropical lagoon, revealing shifts in species diversity and distribution.

## Contribution

The study identifies a 'minor cyanosphere' and links cyclonic events to changes in cyanobacterial community structure in a subtropical lagoon.

## Key findings

- A single Synechococcus ASV dominated nutrient-rich coastal waters, while diverse subclusters and Prochlorococcus ecotypes were found in other zones.
- Cyclone Uesi caused a surge of estuarine Synechococcus strains transported to offshore reefs, altering community composition.
- Cyanobacterial assemblages showed spatial structuring, indicating potential as bioindicators of water masses in lagoon systems.

## Abstract

In tropical marine ecosystems, cyanobacteria are key components of phytoplankton communities, yet their diversity and spatio-temporal dynamics in tropical lagoons remain poorly documented. Here, we use the Nouméa lagoon (New Caledonia) as a model system to genetically characterize cyanobacterial communities in a subtropical lagoon subjected to both natural and anthropogenic pressures. Using metabarcoding, we investigated seasonal and spatial variations along environmental gradients, spanning from estuarine zones to oligotrophic reef areas. A single Synechococcus ASV dominated most communities, particularly in nutrient-enriched coastal waters. Beyond this dominant lineage, we uncovered a diverse “minor cyanosphere,” which comprised Synechococcus subclusters 5.1, 5.2, and 5.3, several Prochlorococcus ecotypes, an as-yet-uncultivated group of picocyanobacteria, and various diazotrophic cyanobacteria. This assemblage exhibited spatial structuring, suggesting environmental filtering and thereby underscoring its potential as a bioindicator of water masses in lagoon systems. We also assessed the impact of Cyclone Uesi (February 2020), which triggered shifts in community composition. Floodwaters resulted in an atypical surge of fast-growing estuarine Synechococcus strains, which were subsequently transported to offshore reef zones, as confirmed by buoy tracking. These findings suggest that increasingly frequent cyclones could influence plankton dynamics and, consequently, reef health under climate change scenarios. Overall, this study highlights the complex structuring of cyanobacterial communities in the Nouméa lagoon, shaped by spatial gradients, seasonal changes, and episodic extreme disturbances, and emphasizes the ecological significance of reef passages, which help maintain the trophic gradient within the lagoon following cyclonic events.