Nucleus, but no mitochondria — scientists probe weird organism

A single-celled organism lacking mitochondria, cellular structures thought to be essential for all complex lifeforms, has been identified by scientists at Charles University, Prague, in the Czech Republic. 

In the study, published in Current Biology, the scientists sequenced the genome of the Monocercomonoides organism, which lives in the gut of vertebrates. They discovered that these cells did not have any of the genes associated with mitochondria, the structures which supply energy for most eukaryotic, or complex, cells.

'We have characterised a eukaryotic microbe which indeed possesses no mitochondrion at all,' explained Dr Anna Karnkowska, a lead author of the study, now based at the University of British Columbia, Canada.

Eukaryotic cells, which are the cells in plants, animals, fungi, and many single-celled organisms, are distinct from simpler prokaryotic cells, such as bacteria, in several ways. Chief among the differences is that eukaryotic cells possess a nucleus where genetic material is kept, and also cell structures for specific functions, called organelles, which include mitochondria.

In most eukaryotic cells, mitochondria provide energy by using oxygen to break down sugars, in a process called aerobic respiration, but some cells do not require this function.

'In low-oxygen environments, eukaryotes often possess a reduced form of the mitochondrion,' said Dr Karnkowska. Previously, it was thought that these reduced forms were essential, as they carried out some secondary functions for the cell, such as the production of iron-sulphur proteins.

However, even these reduced forms were not found in the Monocercomonoides cells. The researchers found that these cells use a different mechanism for carrying out the production of iron-sulphur proteins, independent of mitochondria. They propose that the genes for this mechanism were acquired from bacteria.

It is generally thought that mitochondria themselves originated as bacteria which lived inside the earliest eukaryotic cells. The scientists say that the ancestors of the Monocercomonoides organisms did possess mitochondria, but that they were lost over time after the new mechanism replaced the remaining mitochondrial functions.

The team now thinks that more eukaryotes that lack mitochondria might be found. 'It is very likely that the mitochondrion is absent in the whole group called oxymonads,' said Professor Vladimir Hampl, another lead author of the study. 'We would like to know how long ago the mitochondria were lost.'

Although highly significant for biologists, the implications for our understanding of human cellular function may be limited. Dr Mark Van Der Giezen, a senior lecturer in evolutionary biochemistry at the University of Exeter, who was not involved in the study, told NPR that the organism 'lives in an area without oxygen and therefore can get rid of a lot of biochemistry that you and I would need in our cells to survive'.

However, he added, it managed to adapt to an oxygen-free environment by losing an organelle 'which every textbook will tell you is an essential feature of eukaryotes. That's pretty amazing. It shows you that life is extremely creative in finding a way to eke out an existence.'