In an underwater-based analogue of space flight, the National Aeronautics and Space Administration Extreme Environment Mission Operations (NEEMO) project, crewmembers live in an underwater laboratory for days to weeks. NEEMO missions are saturation dives, meaning that the diver’s tissue gases reach equilibrium with the pressure environment and therefore divers are allowed to live and work underwater for long periods. Aquarius, the unique laboratory in which the divers stay, provides a remote, confined environment similar to that found aboard the International Space Station (ISS). The habitat is located 19 meters below the ocean surface, 4.8 kilometers off the coast of Key Largo, Florida. The 2.5-atm (253 kPa) pressure and 21% oxygen inside the habitat provide increased oxygen availability, similar to that on ISS EVAs, during which spacewalkers breathe 100% oxygen at reduced pressure (0.3 atm). Because of the increased air pressure in the Aquarius habitat, crewmembers are exposed to higher oxygen pressures, which increase their risk for oxidative damage to DNA, proteins, and lipids in tissues and blood.
All subjects were required to pass a modified Air Force Class III physical examination and were required to have logged a minimum of 25 dives before they were selected to participate in the mission. The combined crew consisted of one female and eleven males, all of whom were subjects in the study. The female was on the NEEMO XII mission. Subjects were trained in all procedures required for successful completion of the in-dive sample and data collections. Their mean body weight before the dive was 85 ± 14 kg.
Sample collection days before the dive were designated as Dive–X days (D-X days), during the mission as Mission Day X (MDX), and after the dive as Return + X days (R+X days). Blood was collected before (D-7 and D-2), during (MD6 or 7 and MD9 or 11), and after the dive (R+10 and R+16 or 7). During the dive, NEEMO XII blood samples were collected one or two days later in the mission than NEEMO XIII blood samples because of the length of the mission. Blood collections (14.2 mL at each session) were performed at the same time each day after an eight hour fast. Pre- and post-dive (D-6, D-2, R+10, and R+16/7) blood samples were collected near the Aquarius habitat (on shore) and centrifuged for processing within 45 minutes of phlebotomy. Aliquots of whole blood were made before centrifugation. Serum aliquots were frozen at -20°C until they were transported to Johnson Space Center (JSC) in Houston. Whole blood was transported to JSC and analyzed for hemoglobin and hematocrit within 48 hours of blood collection. Aliquots used for other tests remained frozen at –20°C until they were transported on dry ice to JSC.
Hemoglobin and hematocrit both decreased during the dive but not consistently in the two NEEMO missions. Immediately after the dive, hemoglobin and hematocrit were lower in NEEMO XII than in NEEMO XIII crewmembers. Body iron stores increased during the dive, as indicated by increased ferritin and decreased transferrin and transferring receptors. Total body iron, estimated from ferritin and transferrin receptors, increased up to 29% during and after the dive. Total body iron was negatively correlated with superoxide dismutase (SOD). Total plasma heme increased 48% during the dive and was generally higher in NEEMO XII crewmembers than in the NEEMO XIII crew. Statistical analyses were not conducted for non-transferrin–bound iron and labile plasma iron, because they were not detectable in most samples; however, when these pro-oxidant forms of iron were detectable, it was always during or immediately after the dive and never before the dive. One-fourth of the subjects had detectable levels of nontransferrin–bound iron at the end of the dive period (MD 9/11) and five of the 12 crewmembers had detectable levels of labile plasma iron during or immediately after the dive. These data indicate that the NEEMO environment increases body iron stores and labile forms of iron, which may contribute to oxidative damage.