Chemical communication between algae and bacteria
f green algae of the species Chlamydomonas reinhardtii meet Pseudomonas protegens bacteria, their fate is sealed. The bacteria, measuring only some two micrometers, surround the algae, which are around five times larger, and attack them with a deadly toxic cocktail. The algae lose their flagella, which renders them immobile. The green single-celled organisms then become deformed and are no longer able to proliferate.
The chemical communication mechanism underlying this extremely effective attack has now been uncovered by botanists and natural product chemists at Friedrich Schiller University, Jena (FSU) and the Leibniz Institute for Natural Product Research and Infection Biology — Hans Knöll Institute (HKI).
It is a gruesome spectacle that meets the eyes of Prasad Aiyar as he looks down the microscope. The doctoral candidate from India, who came to Jena to do his Master’s degree in Molecular Life Sciences, examines the species Chlamydomonas reinhardtii on a microscope slide. The oval-shaped microalgae, a good 10 micrometers in size, have two flagella with which they busily swim around — that is, until Prasad Aiyar uses a pipette to add a drop of a bacterial solution. The even smaller bacteria gather together into swarms, which surround the algae. Just 90 seconds later, the algae are motionless and when one looks more closely, one can see that their flagella have fallen off.
The Jena researchers have discovered why these bacteria have such a devastating effect on the green algae. It seems that a chemical substance plays a central role in the process, as the teams under Prof. Maria Mittag and Dr. Severin Sasso of the FSU, and Prof. Christian Hertweck of the Leibniz Institute for Natural Product Research and Infection Biology — Hans Knöll Institute (HKI) — report in the scientific journal Nature Communications.
Orfamide A, as the substance is called, is a cyclical lipopeptide which the bacteria release, together with other chemical compounds. “Our results indicate that orfamide A affects channels in the cell membrane, which leads to these channels opening,” explains Dr. Sasso. “This leads to an influx of calcium ions from the environment into the cell interior of the algae.”
A rapid change in the concentration of calcium ions is a common alarm signal for many cell types, which regulates a large number of metabolic pathways. “To be able to observe the change in the level of calcium in the cell, we introduced the gene for a photoprotein into the green algae, which causes bioluminescence if the calcium level increases. This enables us to measure the amount of calcium with the help of the luminescence,” explains Prof. Mittag, Professor for General Botany. In some cases, the changes in the calcium lead to changes in the direction of movement, for example, after light perception. In other cases, for example after the bacterial attack, they cause the loss of the flagella.