Low Temperature and High Hydrostatic Pressure Have Compounding Negative Effects on Marine Microbial Motility

Approximately three fourths of all pelagic marine prokaryotes live in the deep-sea, an environment characterized by low temperature and high hydrostatic pressure. Within deep-sea environments labile organic matter is often scarce and motility can serve as a competitive advantage for microorganisms. Experimental work with a handful of species suggests motility is one of the most temperature- and pressure-sensitive cellular processes, however the combined effects of temperature and pressure together have yet to be investigated in detail. Here we employed growth-dependent motility agar assays and growth-independent microscopy assays to assess how changes in these two physical factors impact motility both individually and in combination, using ecologically relevant model organisms from the cosmopolitan genera Halomonas, Alcanivorax, and Marinobacter. At pressures equivalent to bathyal and abyssal depths, changes in temperature from 30°C to 4°C (motility assays) or 23°C to 7°C (microscopy assays) had a greater influence on motility than pressure. In addition, low-temperature and high-pressure impacts were additive. Exposure to high pressure had varying degrees of effect on flagellar function, depending on the strain and the magnitude of the pressure. These 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. These findings highlight the sensitivity of deep-sea bacterial motility systems to combined temperature/pressure conditions, phenotypes that will contribute to the modulation of diverse microbial activities at depth.