Corals form characteristic associations with symbiotic nitrogen-fixing bacteria

Area of Science:

• Marine microbiology
• Symbiotic relationships
• Coral reef ecology

Background:

• Coral holobionts involve symbiotic dinoflagellates (Symbiodinium) and associated bacteria.
• The role of nitrogen-fixing bacteria (diazotrophs) in coral symbiosis is poorly understood.
• Diazotrophs may provide essential nutrients to corals and their dinoflagellate partners.

Purpose of the Study:

• Investigate the diversity of diazotrophic bacterial communities in three coral species.
• Compare diazotroph communities in coral tissue, mucus, and surrounding seawater.
• Determine if coral-diazotroph associations are species-specific.

Main Methods:

• Targeted the nifH gene (encoding dinitrogenase iron protein) to profile diazotrophs.
• Analyzed bacterial communities from Acropora millepora, Acropora muricata, and Pocillopora damicornis.
• Collected samples from coral tissue, mucus, and seawater across three Great Barrier Reef locations.

Main Results:

• Coral tissue and mucus harbor distinct diazotrophic communities.
• Mucus diazotrophs were diverse, non-specific to coral species or location, and similar to seawater communities.
• Tissue diazotrophs were species-specific across all sampled reefs, dominated by rhizobia (71% of sequences).

Conclusions:

• Coral-associated diazotroph communities are structured by microhabitat (tissue vs. mucus).
• Species-specific associations between corals and dominant tissue diazotrophs (rhizobia) were identified.
• Rhizobia may form mutualistic relationships with corals, contributing fixed nitrogen to the holobiont.