The ocean depths are a mysterious realm. We must don SCUBA gear if we want to spend more than a few tens of seconds beneath the waves. But seals are perfectly capable of plumbing the depths for as much as 80 min on a single breath. How diving animals and birds manage their meagre oxygen supply fascinates chemical physicist Thomas Jue from U. C. Davis and marine biologist Paul Ponganis from U. C. San Diego. Jue explains that unlike human bodies,which only carry enough oxygen for a couple of minutes, diving seals' muscles are packed with 20 times more myoglobin (an oxygen-carrying protein) than human muscles, and their red blood cell levels are much higher than ours. This allows seals to carry up to an hour's supply of oxygen in their own personalised `SCUBA tank'. Knowing that seals naturally hold their breath for up to 20 min while sleeping, Ponganis and Jue teamed up with Napapon Sailasuta and Ralph Hurd from GE Medical Systems to use groundbreaking NMR technology to track elephant seals' oxygen levels as they held their breath().
Jue admits that the project was an ambitious three-way collaboration that took several years of planning before the seals even saw the inside of the NMR magnet. Ponganis and Torre Stockard had to train two young elephant seals to climb unrestrained into a fibreglass tube that could be fitted inside the NMR magnet where the animals could slumber while the team made their measurements. Curious to know how the seals managed their blood flow while holding their breath, the duo also measured the animals' blood flow and found that it was reduced to just over 30% by the end of a breath-holding session. Meanwhile,Jue, Ulrike Kreutzer, Ping Chang Lin and Tuan-Khan Tran worked with Sailasuta and Hurd to tune the sensitive NMR magnets to pick up the incredibly weak signal generated by free myoglobin in the seal's muscle to monitor the animal's oxygen levels.
Eventually the team was ready to make its measurements on the sleeping animals. Driving from San Diego to the San Francisco Bay Area with the seals in a truck, Ponganis coaxed one of the seals into the NMR magnet, placed a smaller magnetic coil above the animal's longissimus dorsi muscle group and waited for it to fall asleep and stop breathing.
According to Jue it was a tense moment when the first seal stopped breathing; would the deoxymyoglobin signal increase as the storage protein began losing oxygen? Amazingly, it did. After all the years of preparation the team could clearly see the oxygen leaving the muscle's myoglobin stores for consumption by energy-generating mitochondria. `It was one of those great moments,' says Jue, `we were ecstatic'. The muscle's myoglobin-bound oxygen levels dropped by 20% during the first minute, before stabilising at a constant level until the seal began breathing again. Jue explains that by reducing oxygen levels in the muscle, the seal establishes an oxygen gradient between the muscle and blood, allowing the animal to extract more oxygen from its blood to replenish its muscle supplies while holding its breath.
The team also retuned the magnet to track phosphocreatine and ATP. Jue explains that seals consume oxygen to produce the two energy-rich molecules,so their levels can be tracked as an indicator of the amount of anaerobic energy the seals consume while holding their breath. The team found that the animals were not switching to anaerobic metabolism to supplement their energy demand. Instead they reduced their metabolism to conserve oxygen.
