Polyhydroxybutyrate production by freshwater SAR11 (LD12)

Area of Science:

• Microbiology
• Environmental Science
• Biochemistry

Background:

• SAR11 bacteria are abundant aquatic microbes.
• Some SAR11 genomes possess genes for polyhydroxyalkanoate (PHA) synthesis.
• The function of PHA production in SAR11 biology was previously unclear.

Purpose of the Study:

• To investigate the relevance and mechanism of PHA production in SAR11 bacteria.
• To characterize polyhydroxyalkanoate synthesis in "Candidatus Fonsibacter ubiquis" (LD12 subclade).
• To determine the evolutionary origin and regulation of PHA synthesis in SAR11.

Main Methods:

• Phylogenetic analysis of PHA genes (pha) within SAR11.
• Microscopy and Nile red staining to visualize PHA granules in "Ca. F. ubiquis" LSUCC0530.
• Heterologous expression of the phaCAB locus in Escherichia coli.
• Analysis of PHA production under varying nutrient conditions.

Main Results:

• Phylogenetics suggest PHA genes originated in a common ancestor of brackish IIIa and freshwater LD12 SAR11 subclades.
• "Ca. F. ubiquis" LSUCC0530 produces polyhydroxybutyrate (a type of PHA), visualized as single granules per cell.
• The LSUCC0530 phaCAB locus is functional and likely responsible for polyhydroxybutyrate synthesis.
• PHA production and phaCAB expression remained constant across different nutrient concentrations.

Conclusions:

• Polyhydroxyalkanoate synthesis is a characterized trait in SAR11 bacteria, specifically in the LD12 subclade.
• Polyhydroxyalkanoate synthesis in "Ca. F. ubiquis" may be constitutively active, linked to slow growth and minimal regulation.
• This metabolism likely provides a fitness advantage to SAR11 bacteria in oligotrophic environments.