Bacterial utilization of light: rhodopsins and carotenoids

Date
2016
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Sunlight is the most abundant and sustainable resource of energy available. Organisms that can use sunlight for phototrophy fall into two categories: photoautotrophs and photoheterotrophs. Photoautotrophs are organisms that use light to carry out photosynthesis, utilizing carbon dioxide as their principal carbon source. Photoheterotrophs use light energy to break down organic carbon compounds from the environment to drive ATP synthesis. In particular, rhodopsin-based photoheterotrophic bacteria use a rhodopsin-retinal protein complex to capture light and potentially establish a transmembrane ion gradient, which could drive ATP synthesis. In addition, there are other ways for organisms to use sunlight. Some phototrophic organisms can utilize accessory molecules, such as carotenoids, as light-harvesting antennae for photosynthesis. Here, we discuss two ways bacteria can use light through rhodopsins and carotenoids. Microbial rhodopsins are a family of transmembrane proteins, with photosensitive retinal cofactors, found in every domain of life. Rhodopsins respond to light by transporting ions across the cell membrane or by initiating a signaling cascade that leads to altered gene expression. Rhodopsins are abundant in nature, and recent estimates indicate that up to 70% of microbial cells in some aquatic environments possess rhodopsin genes, suggesting that more bacteria utilize sunlight than previously thought. However, these estimates are based on gene abundances, not direct observation. In order to determine the abundance of functional rhodopsins, visualization of these low-fluorescing proteins is essential. We recently developed a method that uses total internal reflection fluorescence (TIRF) microscopy to identify rhodopsin-containing cells in environmental samples. Here, we use TIRF microscopy to quantify the total number of rhodopsin-containing cells in water samples collected along the Chesapeake Bay, demonstrating that rhodopsin production is correlated with daylight and salinity. Approximately up to 60 percent of cells produce functional rhodopsins; therefore, microbial capture and utilization of sunlight by rhodopsin-type photosystems is likely common throughout this estuary. Carotenoids are pigmented, organic compounds that have been used in industry for years as food colorants, with up to 750 different carotenoids known today. The most abundant form of carotenoids are 40-carbon molecules (C40), but 50 carbon molecules (C50) exist as well. While most C40 carotenoid biosynthetic pathways have been well characterized, C50 pathways have not. Using genomic and phylogenetic analyses, we have identified two potential genes involved in the biosynthesis of a C50 carotenoid, bacterioruberin, in Rhodoluna lacicola. When the pathway is expressed in Escherichia coli , bacterioruberin is produced. Overall, these insights into two different methods by which bacteria use sunlight can enhance our ability to produce alternative energy source technologies focused on biological organisms and their ability to generate power in the form of electron gradients.
Description
Keywords
Citation