SIMS

Image of Sea Squirt
Photo: Rob Harcourt
© Rob Harcourt

Peter Ralph from the Faculty of Science, University of Technology, Sydney and an international team of researchers recently reported in Nature that a type of cyanobacteria (Acaryochloris marina) that lives under coral reef ascidians (sea squirts) use an unusual variant of chlorophyll to photosynthesise.

Chlorophyll is contained within organisms (plants and cyanobacteria) and absorbs sunlight to power reactions between carbon dioxide and water to make sugar that the organism uses for food. Most cyanobacteria photosynthesise using chlorophyll a – while a few also use chlorophyll b. Chlorophyll a and b use light in the visible spectrum. However this cyanobacteria Acaryochloris marina, uses a rare version called chlorophyll d and utilise light from the near infrared spectrum. Infrared light is familiar to us as heat and is part of the light spectrum that is strongly absorbed by water, so these organisms can only grow in shallow high light conditions.

“This find is exciting because these cyanobacteria use a completely different type of chlorophyll to photosynthesise and have found a niche which is not being used by any other organism. They are able to live completely under another organism and use the small amount of light (thermal energy) that filters through the ascidian’s bodies,” Ralph said.

“We have also solved a puzzle as to why other cyanobacteria using chlorophyll d have been found under the leaves of red algae, in murky, salty lakes and in shaded areas of coral reefs they too are living in an environment rich in infrared light and low on visible light.”

“The fact that these cyanobacteria have evolved using chlorophyll d shows that there is more than one way to harness energy and that life is possible in environments that do not have visible light such as other planets in the universe.”