A positive pressure effect (1.4 to 3.3x) on the binding of Mn by a natural population of bacteria in a deep-sea hydrothermal plume was discovered over the intermediate pressure range of 1 to 200 atm (1 to 200 bars; ca. 1.01 x 10 to 2.03 x 10 kPa). The data suggest Mn binding is functionally barophilic rather than simply barotolerant.