To date, few studies have quantified metabolic rates of deep sea microbes or animals. Fewer still have made these measurements at environmentally relevant conditions, with the appropriate in situ chemistry, temperature and pressure. Our high-pressure respirometry system (HPRS) allows the maintenance of microbial communities and animals at in situ temperatures, pressures and chemistries. It also allows the simultaneous measurement of metabolite flux in pressurized flow-through aquaria. The HPRS consists of three sub-systems: a gas and seawater equilibration system to re-create the chemical conditions in situ, three high-pressure pumps and aquaria to maintain the (micro)organisms at in situ pressures and temperatures, and an inline analytical system consisting of a flow-through spectrophotometer, pH meter and membrane inlet mass spectrometer.
To simulate the conditions found in situ, filtered seawater is pumped via metering pump into an acrylic gas equilibration column. Mass flow controllers regulate the flow of CO2, H2S, O2, CH4, N2 or He into the equilibration column to achieve desired in situ concentrations. Seawater pH is maintained by a proportional pH controller controlled two metering pumps which deliver 1 M NaOH and 1 M HCl as needed. The resulting seawater is pumped from the equilibration column into the each aquaria using three high pressure pumps. Aquaria temperatures are maintained by immersing them in a circulating waterbath. The titanium high-pressure aquaria are water jacketed and do not require immersion into a bath. Aquaria pressures are maintained via pneumatically-charged or spring-loaded backpressure valves. Vessel effluents are analyzed for changes in dissolved gas concentrations by a residual gas analyzer mass spectrometer (Hiden Analytical Inc.). Changes in seawater nitrate and pH are determined by inline spectrophotometric and pH measurements respectively.
The high-pressure recovery vessels (HPRVs) are designed to be readily deployed by elevator, DSV or ROV to enable the collection of samples without de-pressurization. To date this system has been used to collect Alviniconcha and Ifremeria snails from hydrothermal vent sites in the Lau Basin.
Figure 1. A schematic for the HPRS system.
Figure 2 (left). The high pressure recovery vessel (HPRV) is shown during a deployment at 2400 meters water depth in the Lau Basin. Samples are placed inside the HPRV by a net on the manipulator of the ROV Jason robotic submersible.