Low temperature and high hydrostatic pressure have compounding effects on marine microbial motility

Motility is one of the most temperature- and pressure-sensitive cellular processes. Both low temperature and elevated pressure impact rates of various cellular processes - including motor and filament assembly and motion - through their impact on reaction rates and volume changes of activation, respectively. Consequently, motility serves as a proxy for assessing microbial activity in the deep sea. While the impacts of low temperature and high pressure on microbial motility have been assessed individually, their combined effects have yet to be thoroughly investigated. Here we utilize qualitative growth-dependent motility agar assays and quantitative growth-independent microscopy assays to understand how low temperature and high hydrostatic pressure impact motility both individually and in combination. At pressures equivalent to bathyal and abyssal depths, we found that low temperature had a greater impact on motility than pressure. However, the impacts of low temperature and high pressure were compounding. Exposure to high pressure alone was also found to have varying degrees of impact on flagellar function, depending on the strain and the exposure pressure. These impacts ranged from short-term impacts that were quickly reversible to long-term impacts that were detrimental to the function of the flagellum, leading to complete loss of motility. Motility reflects the physiological status of a cell under various physical conditions, and the compounding impact of low temperature and high hydrostatic pressure on motility has implications for microbial activity in the deep sea. This study highlights the importance of performing motility and activity assays under both in situ temperature and pressure conditions.