A bacterium dubbed the "platypus of microbiology" is even stranger than first thought, with the discovery it contains structures normally only found in more complex cells.

Key points: Bacterium Gemmata obscuriglobus originally found in Queensland in 1984

Bacterium Gemmata obscuriglobus originally found in Queensland in 1984 Closer inspection of its structure reveals it is the most complex bacterium ever discovered

Closer inspection of its structure reveals it is the most complex bacterium ever discovered Finding sheds light on a major problem in evolutionary cell biology

The find, by an international team led by University of Queensland researcher Emeritus Professor John Fuerst, adds to the debate about how complex cells evolved, and casts doubt on long-held theories of evolution.

According to the dominant theory, there are three domains of life: bacteria and archaea, which are single-celled organisms without a nucleus, and eukaryotes, organisms that include everything from yeast to us.

However, the bacterium Gemmata obscuriglobus has been described as the "platypus of microbiology" because it appears to defy this theory by containing features associated with eukaryotes.

These include a membrane-bounded nucleus, the ability to transport molecules such as proteins into the cell, and its ability to reproduce by a unique way of budding.

Now Professor Fuerst and his team have discovered G. obscuriglobus has pore-like structures in its internal membrane that have elements structurally similar to eukaryote nuclear pores, which are found in the membrane surrounding the nucleus.

"This is a remarkable evolutionary finding, since most bacteria do not possess these structures," Professor Fuerst said.

He said nuclear pore complexes were important in transporting molecules between the nucleus containing the DNA and the rest of the cell contents in eukaryote organisms.

"They are dotted over the surface of the membranes separating the nucleus from the rest of the cell and enable communication between the nucleus and other parts of the cell," he said.

"Like the membrane-bounded nucleus, nuclear pore complexes had been thought to be restricted to eukaryotes."

Most complex bacteria yet uncovered

The discovery, published today in PLOS ONE, meant G. obscuriglobus, first found in Maroon Dam in south-east Queensland in 1984, was the most complex bacteria yet uncovered.

Professor Fuerst said the find — made using techniques including advanced electron microscopy, a protein analysis method called proteomics, and bioinformatics genome analysis — was important for understanding how the first complex cells may have evolved.

It suggested the evolution of complex cell structures may not be unique to eukaryotes, which if confirmed, would overturn long-held biological theories.

"The results are of evolutionary significance, since the origin of eukaryotes is a major event in life's history," he said.

"Finding nuclear pore-like structures in the bacterial species G. obscuriglobus is significant for understanding how the cell nucleus and the pores embedded in its membrane envelope could have evolved — a major unsolved problem in evolutionary cell biology."

A major unanswered question of biology

Professor Fuerst said there were two possible interpretations from the discovery.

First that the nuclear-cell structures developed in bacteria and eukaryotes completely independently of each other.

"Evolutionarily it could be that these nuclear pores are a good solution to the problem and maybe there is a reinvention a number of times in nature," Professor Fuerst said.

"The alternative view is that the bacterium share a common ancestor as far back as 3.8 billion years ago.

"It is a major question of biology being investigated right now and it is not easy to resolve."