Metabolic streamlining in an open-ocean nitrogen-fixing cyanobacterium

Area of Science:

• Marine microbiology
• Biogeochemistry
• Genomics

Background:

• Nitrogen (N2)-fixing marine cyanobacteria are crucial for oceanic primary productivity and atmospheric carbon dioxide removal.
• The globally distributed UCYN-A cyanobacterium lacks photosystem II, suggesting a novel metabolism, but remains uncultivated.

Purpose of the Study:

• To elucidate the metabolic capabilities and nutritional dependencies of the uncultivated UCYN-A cyanobacterium.
• To understand the evolutionary adaptations and ecological role of UCYN-A in marine ecosystems.

Main Methods:

• Assembly of the complete UCYN-A genome using massively parallel pyrosequencing.
• Metabolic reconstructions based on genomic data.
• Analysis of UCYN-A's reduced genome structure and gene content.

Main Results:

• UCYN-A possesses a photofermentative metabolism, generating energy and reducing power from light.
• The organism lacks major metabolic pathways like the tricarboxylic acid cycle and essential biosynthetic pathways for amino acids and purines.
• UCYN-A exhibits a reduced genome (1.44 Mb) with structural similarities to chloroplasts and bacteria, including inverted rRNA operons.

Conclusions:

• UCYN-A is metabolically dependent on other organisms for essential nutrients, likely through close association or symbiosis.
• The findings reveal UCYN-A as a paradox in microbial evolution and adaptation, demonstrating tight metabolic coupling in oligotrophic environments.