Metazoans (Animals) Really do Need Oxygen

n 2010, a research team garnered attention when it published evidence of finding the first animals living in permanently anoxic conditions at the bottom of the sea. But a new study, led by scientists at the Woods Hole Oceanographic Institution (WHOI), raises doubts.

One alternative scenario is that cadavers of multicellular organisms were inhabited by bacteria capable of living in anoxic conditions, and these "bodysnatchers" made it seem that the dead animals were living, said Joan Bernhard, a geobiologist with WHOI and the lead author of the new study published in the December 2015 issue of the scientific journal BMC Biology.

Bernhard and Virginia Edgcomb, her colleague at WHOI, led an expedition in 2011 that returned to the site of the initial findings: a deep hypersaline anoxic basin (or DHAB) two miles deep in the Mediterranean Sea. DHABs are curious phenomena. They exist in depressions on the seafloor, where long-buried salt deposits become exposed to seawater and dissolve into the sea. The hypersaline water is extremely dense and remains separated, like oil and water, from surrounding normal seawater. It forms "lakes" on the seafloor, tens to hundreds of meters deep, that are extremely salty and devoid of oxygen.

"We have known for a long time that some metazoans inhabit extreme anoxic habitats on a periodic or even semi-permanent basis," Bernhard said. "But scientists have thought that metazoan's high-energy activities, such as reproduction, would require oxygen. If these loriciferans spend their whole lives and reproduce in a zero-oxygen environment, we would have to reconsider our concepts of animal metabolism. It was important to revisit the DHABs to confirm and understand those previous remarkable findings."

In the 2010 study published in the same journal, researchers from Polytechnic University of Marche and the Natural History Museum of Denmark, led by Roberto Danovaro, analyzed samples collected from a Mediterranean DHAB called L'Atalante. They reported finding multicellular animals (or metazoans), including previously unknown species of a type of tiny animals called loriciferans.

The contrast between conditions at the seafloor and at the surface makes it nearly impossible to recover live specimens, and so in the past, metazoan specimens collected from DHABs have been "interpreted as the result of a rain of cadavers that sunk to the anoxic zone from adjacent oxygenated areas," according to the Danovaro study.

But the scientists conducted experiments with fluorescent tags, taken up only by metabolically active organisms, which gave indications that the loriciferans had been alive. In addition, a few loriciferans appeared to have reproductive structures called oocytes (or eggs), indicating that the organisms were reproducing.

Intrigued by these findings, Bernhard and Edgcomb returned to L'Atalante and other nearby DHABs in 2011 to further investigate aboard the research vessel Atlantis. They collected sediment and water from the edges of three brine pools with different chemical compositions, using WHOI's remotely operated vehicle Jason to visually guide carefully targeted push-core samples. Samples were taken from points in the upper, middle, and lower levels of the layer of water immediately overlying the brine lake. This so-called "interface zone" is where normal seawater at the top transitions to the brine at the bottom, becoming more concentrated and anoxic the closer to the brine. The highly dense, saline, chemical-laden and oxygen-depleted water in all three pools was too dense for Jason to fully penetrate. Control samples from nearby sediment and water of normal oxygen and salinity were also collected.

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Sadly, this seems to be not true.