Hawaiian coastline at sunrise

Alternative oxidase (AOX) in bacteria

Bacteria have an amazing diversity of energy conservation pathways for generating the ATP they need to live and function.  This diversity contributes to the wide range of habitats they can colonize.  Not only are bacteria versatile, but they are also found in large numbers in the environment and play essential roles in ecosystem function.  Despite this importance, our knowledge of the diversity and impact of bacteria on the environment is far from complete.  This is especially true in the ocean, where genome and transcriptome sequencing efforts have focused on learning more about the genetic potential of the resident microbes.  The influx of sequencing information brings questions of how this functional potential relates to processes in the marine environment. 

One exciting outcome of early environmental sequencing efforts was discovering that genes encoding the respiratory protein, alternative oxidase (AOX), are abundant in bacteria in ocean surface waters.  AOX is a quinol oxidase found as a component of the electron transport chain in all plants, and certain members of other groups, including fungi, protists, animals, and bacteria.  AOX is unusual in that, unlike other characterized quinol and cytochrome oxidases, its activity does not directly contribute to the generation of the proton motive force.  In contrast, the energy produced by AOX during the reduction of oxygen to water is released as heat.  While this activity could seem wasteful, AOX plays an essential role in heat production in thermogenic plant species.  In non-thermogenic plants, AOX activity results in lower energy yield and carbon use efficiency than electron flow to a cytochrome oxidase. However, it can positively affect growth and stress tolerance under abiotic stress conditions. 

AOX was initially thought to be absent in bacteria until the discovery of an aox gene in the bacterium Novosphingobium aromaticivorans.  We discovered an aox gene in the genome sequence of Vibrio fischeri ES114 and have characterized the regulation AOX in V. fischeri during growth with nitric oxide (NO) in laboratory culture.  Our studies revealed that V. fischeri AOX functions as a NO-resistant respiratory oxidase that can reduce signs of cellular stress in a parental strain lacking Hmp (NO detoxifying enzyme) under aerobic growth conditions.  

Since this initial report, we have identified that members of numerically dominant and ecologically relevant bacterial groups such as SAR11 (~25% of all plankton), SAR86, SAR116, and the Roseobacter clade (~20% of coastal bacterial communities) have aox-like genes that are predicted to encode proteins with the conserved amino acid residues known to be necessary for AOX function.   Current research in our lab integrates research approaches in marine microbial physiology, genetics, and biochemistry with bioinformatics to gain a deeper and broader understanding of the expression of AOX and its functional benefit in marine bacteria. We currently focus on Vibrio spp. and Roseobacter group bacteria in our culture-based studies and utilize available genomic, metagenomic, and transcriptomic datasets to generate testable hypotheses for understanding the regulation, function, distribution, and evolutionary history of AOX in bacteria. 

The outcomes of this research will lay the foundation for a broader exploration of the influence of bacterial AOX on microbial physiology, marine ecosystem function, and biogeochemical cycling while also providing insight into commonalities between AOX regulation and function in different kingdoms of life.

agar plate of bioluminescent bacteria

Bioluminescence in Photobacterium leiognathi

Bioluminescence is a remarkable trait with broad applications for symbiosis, biotechnology, ecology, and evolution. The study of light production in bacteria has mainly focused on two model organisms, Vibrio fischeri and Vibrio harveyi. It has revealed the importance of density-dependent signaling (i.e., quorum sensing or pheromone signaling) in regulating this process.  However, not all bacteria use canonical pathways to induce light production.  An excellent example is Photobacterium leiognathi strain KNH6, an extremely bright isolate commonly used for educational outreach activities.  In contrast to model Vibrios, KNH6 does not increase light production in response to pheromone.

We have developed techniques to genetically manipulate P. leiognathi. We currently use genetic, genomic, and transcriptomic approaches to understand how closely related Photobacterium strains control light production and how light production impacts bacterial fitness and survival. This work will lay a foundation for comparative studies to increase understanding of the diversity, regulation, and benefits of light production in environmental bacteria and contribute to a general understanding of how bacteria coordinate responses to changing environmental conditions.