Algae understand the language of bacteria

It has hitherto not been known that higher organisms, such as green algae, can communicate with bacteria. But Debra Milton, associate professor at Umeå University in Sweden, shows in the recent issue of the prominent journal Science that bacteria attract green algae with the aid of signal molecules. Surfaces under water are rapidly colonized by bacteria, which cover the surface with a thin film known as biofilm. Within this biofilm bacteria coordinate activities among the cells with the help of chemical signal molecules, such as N-acyl homoserine lactones (AHL). It is well known that bacteria produce and make use of AHL-signal molecules. On the other hand, it has not been known that organisms, such as algae, also have the capacity to make use of these signal molecules.

Enteromorpha is a common green alga that binds to and thereby damages human constructions like oil rigs, pipes, vessels, etc. This has led to many unwanted problems, such as increased friction for ships, which in turn leads to increased fuel costs, deposition of minerals, and degradation of materials, all entailing major economic consequences.

Green algae are spread in water by producing mobile microscopic zoospores that seek out suitable surfaces on which to adhere. Once these spores have found a suitable place, they excrete an adhesive molecule that permanently fastens the zoospore to the surface, and a new alga can develop and grow. Researchers have previously shown that zoospores adhere to bacteria cells and that biofilm increases the number of zoospores that latch on to such surfaces.

But how do the zoospores find the bacteria? Debra Milton, in collaboration with researchers from the UK, has discovered that Enteromorpha zoospores find bacteria by seeking AHL signal molecules. Thus the signal molecules affect what surfaces the algae will bind to, and they only adhere to those surfaces that are covered by biofilm, where the signal molecules are produced.

A new method for preventing the binding of zoospores could be to block the production of signal molecules in bacteria. This would make it possible to control the harmful colonization of underwater constructions via green algae, with substantial economic benefits as a result.