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Ecophysiology of membrane lipid remodelling in marine bacteria

Periodic Reporting for period 4 - EcoLipid (Ecophysiology of membrane lipid remodelling in marine bacteria)

Reporting period: 2021-10-01 to 2023-03-31

Membrane lipids form the structural basis of all cells. In bacteria Escherichia coli uses predominantly phospholipids in its cell envelope. However, beyond E. coli a range of lipids are found in bacterial membranes. In the marine environment, it is well established that phosphorus availability significantly affects lipid composition in the phytoplankton, whereby sulfur-containing lipids are used to substitute phospholipids. This remodelling offers a significant competitive advantage for these organisms, allowing them to adapt to oligotrophic environments. Until very recently, abundant marine heterotrophic bacteria were thought to lack the capacity for lipid remodelling in response to phosphorus deficiency. However, recent work by myself and others has now demonstrated that lipid remodelling occurs in many ecologically important marine heterotrophs, which are not only numerically abundant in marine waters but also crucial players in the biogeochemical cycling of key elements. However, the ecological and physiological consequences of lipid remodelling, in response to nutrient limitation, remain unknown.

The overall aim of this project is to use a synthesis of molecular biology, microbial physiology, and "omics" approaches to reveal the fitness trade-offs of lipid remodelling in marine heterotrophic bacteria, providing novel insights into the ecophysiology of lipid remodelling and its consequences for marine nutrient cycling. The project is further divided into three main work packages.

1) Obtaining a fundamental understanding of whole cell responses to lipid remodelling using comparative “omics” approaches.

2) Mechanistic insight into lipid remodelling and its impact on membrane composition and transporter activities.

3) lipid remodelling in marine heterotrophic bacteria and interaction with protists grazers and bacterial phages
The project is further divided into three main work packages. Major achievements in each work packages include

1) Using Ruegeria pomeroyi as the model, we systematically uncovered the cellular response of lipid remodelling using comparative proteomics and lipidomics (Smith et al., 2019, Silvano et al., 2020). We also identified a novel sulfur-containning lipids in this bacterium and studied its role in the adaptation to phosphorus stress (Smith et al., 2023).

2) We successfully generated several mutants, including plcP, olsAB, glsB and systematically characterised their role in membrane lipid remodelling. As we hypothesised, membrane lipid remodelling does impact not only membrane protein composition but also affect transporter activities (Stirrup et al., 2023)

3) We successfully isolated several bacterial phages which infect marine heterotrophs. Two of the phages infecting strain DSS-3 have unexpected DNA substitution from thymidine by deoxyuridine (dU) (Branko et al. 2021). We successfully obtained several eukaryotes and managed to maintain and grow them successfully in the laboratory. We uncovered a trade-off mechanism of lipid remodelling in interaction of lipid remodelled cells with their predators (Guillonneau et al., 2021).
The project aims to bridge a knowledge gap in our understanding of lipid remodelling and its potential biological and ecological consequences. We have made significant progress in all work packages and established methods and protocols in order to test our hypotheses. The project therefore transforms our understanding of the ecophysiology of adaptation to nutrient limitation in model marine bacteria and opens up a whole new horizon for future investigation of their role in nature systems.
Detection of mannose-containing glycoconjugates on cell surface