By Matt Walker
Editor, Earth News
Verdigellas: older than we thought
Ancient seaweed that have been found growing in the deep sea are "living fossils", researchers have reported.
The two types of seaweed, which grow more than 200m underwater, represent previously unrecognised ancient forms of algae, say the scientists.
As such, the algae could belong to the earliest of all known green plants, diverging up to one billion years ago from the ancestor of all such plants.
Details of the discovery are published in the Journal of Phycology.
"The algae occur in relatively deep marine waters - 210m, which is certainly deep for a photosynthetic organism," Professor Frederick Zechman told the BBC.
"They can be found in shallower water but typically under ledges in low light.
"They appear to possess special chlorophyll pigments that allow them to utilise the low intensity blue light found at depth."
Professor Zechman of California State University in Fresno, US, sampled the seaweeds with a team of researchers based across the US and in Belgium.
The algae had previously been identified. They belong to the scientific groups, or genera, called Palmophyllum and Verdigellas.
But Professor Zechman's team is the first to study their genetic make-up, and it is this research that has revealed their startling ancestry.
Green plants in general belong to one of two groups, or clades.
One clade includes all land plants and the green algae with the most complex structures, known as charophytes or more commonly stoneworts.
The other clade, known as the Cholorophyta, comprises all other green algae.
Most studies have sought to determine what ancient plants gave rise to the land plants and stoneworts.
But little research has been done into the origin of the other green algae.
So Professor Zechman's team collected and studied Palmophyllum algae from New Zealand waters and Verdigellas from the western Atlantic Ocean.
These algae are unusual as they are multicellular, but their individual cells do not interact with each other in any meaningful way.
Instead, single cells sit in a gelatinous matrix, which can form complex shapes such as stalks.
The scientists analysed the DNA within the nuclei and chloroplasts in the algae's cells.
Palmophyllum umbracola growing in the waters off New Zealand
Instead of belonging to the Cholorophyta, the scientists discovered that both types of algae actually belong to a distinct new group of green plants, one that is incredibly ancient.
The algae are so different that they should be assigned their own Order, a high level taxonomic group, say the scientists.
What is more, "by comparing those gene sequences to the same genes in other green plants, we have discovered that these green algae are among the earliest diverging green plants... if not the earliest diverging lineage of green plants," Professor Zechman told the BBC.
"That would put them in the ball park of over a billion years old."
The discovery could "vastly change" our view of which green plant was the ancestor to all those we see today, he says.
That progenitor of green plants is currently thought to be a single-celled plant that had a tail-like structure called a flagellum, which allows the cell to move itself in water.
But no single-celled or flagellated algae of the types studied by Professor Zechman's team have been observed, suggesting the earliest green plants may not have had flagella after all.
Professor Zechman said the previously unrecognised ancient algae could be characterised as "living fossils", even though no actual fossils of such algae are known to exist.
The algae's ability to harness low light intensities allows them to grow in deep water habitats - and that may be the key to their incredible longevity.
At such depths, plants face less stress from the actions of waves, variations in temperature and fewer herbivores that might feed on them.