Use of NMOS in an i2c bus
The mosfet is converting a 3.3V signal from the MS5611 to a 5V signal.  When SDA is low (0v), the N_MOS will be on, pulling H_SDA low.  When SDA is high (3.3v) the N_MOS will be off (Vgs = 0), allowing the H_SDA line to be pulled high (5V) by R14.