A layer of lipid, really, although there are certainly proteins in the membrane. I've no idea about pressure adaptation, but a quick google gives this (open) p

A layer of lipid, really, although there are certainly proteins in the membrane. I've no idea about pressure adaptation, but a quick google gives this (open) paper that looks useful: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5487899/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5487899/ From the abstract: >Maintenance of the functionality of the membrane during changing environmental conditions relies on the cell’s potential to rapidly adjust the lipid composition of its membrane So in other words, bacteria at depth seem to use a particular mix of lipids in their membrane. Since things have to cross the membrane it makes sense to me that it has to be stiffer or less so at different pressures.