Lichtgedanken 03

S C HW E R P U N K T 33 03 | LICHT GEDANKEN Ambush in a petri dish If green algae of the species Chlamydomonas reinhardtii meet Pseudomonas protegens bacteria, their fate is sealed. The bacteria, measuring only some two micrometres, 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 mechanism underlying this extremely effective attack has now been uncovered by botanists and natural product chemists. BY UTE SCHÖNFELDER It is a gruesome spectacle that meets the eyes of Prasad Aiyar as he looks down the microscope. The doctoral candida- te from India, who came to Jena to do his Master’s degree in Molecular Life Sciences, examines the species Chlamy- domonas reinhardtii on a microscope sli- de. The oval-shaped microalgae, a good 10 micrometres 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 mo- tionless and when one looks more clo- sely, one can see that their flagella have fallen off. The Jena researchers have discovered why these bacteria have such a devasta- ting 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 Matthias Schleiden Institute (Uni- versity Jena) and Prof. Christian Hert- weck of the Leibniz Institute for Natural Product Research and Infection Biolo- gy—Hans Knöll Institute (HKI)—report in the journal Nature Communications. Orfamide A, as the substance is called, is a cyclic lipopeptide which the bacte- ria release, together with other chemical compounds. »Our results indicate that orfamide A affects channels in the cell membrane, which leads to these chan- nels opening,« explains Dr Severin Sas- so. »This leads to an influx of calcium ions from the environment into the cell interior of the algae,« adds the head of the Research Group Molecular Botany. 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 in- troduced the gene for a photoprotein into the green algae, which causes lumi- nescence if the calcium level increases. This enables us to measure the amount Agar plate with green algae cocultivated with bacteria (center) and bacterial extracts re- spectively. In the center algal growth arrest is visible as a halo.