Alive, dead or just resting?
There may be a state of being between life and death, suggest the authors of a paper who argue that certain organisms, tissues and cells can survive in an in-between, limbo-like state.
"We think that viability, the middle state between life and death, can best be defined as 'resting life'," says Dr Rene Severijnen, a paediatric surgeon at Radboud University Nijmegen Medical Center in the Netherlands.
Microorganisms stored in glycerol mixtures at low temperatures and sperm cells stored in liquid nitrogen can exist in that middle state, he writes with Dr Ger Bongaerts in a paper accepted for publication in the journal Medical Hypotheses.
Bongaerts, a biochemist and microbiologist also at Radboud, further theorises that bacteria growing in very concentrated salt or sugar mixtures, such as a pot of honey, may also wind up in the suspended state.
The lower relative concentration of water outside the bacterial cells creates the force of osmosis, which sucks water from the bacteria, leaving them dehydrated.
Certain brine shrimp can enter into a state of life known as cryptobiosis in response to adverse environmental conditions, such as freezing or drying. The shrimp eggs are sun-dried in salt evaporation flats.
"When released into the aquarium, they leave this state," Bongaerts says. "In fact, this cryptobiosis is also 'resting life'."
How about embryonic stem cells?
The researchers apply their theory to human embryonic stem cells, often derived from embryos discarded after in vitro fertilisation procedures.
They argue that while the life of stem cells cannot be considered independent, isolating the cells for medical purposes prevents the cells from fully dying "and continues [them] in a primitive way of life".
As a result, the researchers say, it would be better for the cells to be used in research, rather than destroyed.
Two professors at Columbia University, Donald Landry and Howard Zucker, have also questioned traditional notions of life and death.
In a recent Journal of Clinical Investigation paper, they point out that residual embryos from in vitro fertilisation are not even capable of producing a foetus, yet they may contain isolated viable cells that could save other lives.
Landry and Zucker hope future regulations may be comparable to those established for organ donation, when viable organs are removed from individuals whose bodies and brains have stopped functioning. The person may be dead, but the isolated organ is not.
Do atoms die?
To further complicate the life versus death debate, Bongaerts and Severijnen point out that atoms, the building blocks of all matter, don't normally die at all.
"The death of an atom means that matter is converted to energy," says Bongaerts. "This happens during an explosion of a nuclear fusion bomb."
Severijnen adds that although growth, metabolism and other bodily processes stop as soon as the human body dies, "the degradation processes go on and may even increase".
And in the end, he says, all the complex compounds are degraded again to the smallest compounds, such as water, carbon dioxide and ammonia. These compounds may then fuel other organisms, and never die at all.