Hostile takeover in the ocean

Oomycetes, also known as egg fungi, are known to cause serious diseases in plants and animals. They also affect algae, which tend to succumb to the infection. A team of researchers from Jena and Frankfurt/Main have managed to shed light on the biochemical fungal attack and how exactly the fungi kill the algae.

Whenever the sea suddenly turns blood­red or lights up in bright shades of green or brown, the cause is usual­ly algae. We can often observe a mass proliferation of algae in the oceans, but such algal blooms attract other organ­isms that stop the growth of algae and sometimes even bring about the end of an entire algae population. The exact details of this process were previously u nclear.

Algal metabolism attacked and reprogrammed by fungus

In a collaborative study conducted by researchers at the Max Planck Insti­tute for Chemical Ecology in Jena, the Friedrich Schiller University, and the Goethe University in Frankfurt/Main, the underlying mechanisms of algal death have been investigated. They have shown that a pathogenic fungus alters the metabolism of its host uni­cellular alga and changes it for its own purposes. It makes the algae produce bioactive substances that benefit its own propagation, while preventing algal proliferation. Ultimately, the algae are killed and parasites can thus contribute to the termination of the bloom.

For the study, the team established a laboratory system, in which a marine diatom was infected by the pathogenic oomycete Lagenisma coscinodisci under controlled conditions. This parasite is known to in­fest unicellular algae and prevent them from proliferating.The researchers examined the metab­olism of the algae during the infection process. They found that all infected algae began to generate two chemical substances that they do not normally produce: »carbolines« from the class of substances known as alkaloids, which also include nicotine and caffeine. »We were surprised by the presence of these carbolines. Their formation was com­pletely unknown in diatoms, but they were formed by all algal cells during the oomycete infestation,« explains Ma­rine Vallet, one of the two main authors of the new study. Interestingly, both of these substances only benefited the oomycete; they damaged the algae and ultimately killed it.

This study was challenging because the parasites usually kill their host in a very short time and the substances are produced in very low amount in the small cells. The other main author, Tim Baumeister, describes some of the chal­lenges that had to be overcome at the beginning of the project: »Oomycetes are known to take various forms: They are often only found as tiny spores in their host, and they sometimes do no harm at all and simply ›sleep‹ in their host. At other occasions they can cause massive killing of the algae. These processes re­sult in a highly dynamic fluctuation of dominant species in the ocean«. 

Active chemical compounds identified

Using high­resolution spectrometric methods combined with microscopic techniques, the scientists were able to identify the active chemical compounds produced by a single algal cell. »One has to bear in mind that a single cell is 30 times smaller than a pinhead and that the concentration of all substances is very low. Shedding light on the chem­istry occurring in a single cell is a major technical achievement,« explains Georg Pohnert. The professor of Instrumental Analytics at the University of Jena di­rects the Max Planck Fellow Group »In­teraction in Plankton Communities«.

The scientists would now like to con­duct further studies to find out how the diatoms are able to defend themselves against this pathogenic fungal attack, because they are aware that not all dia­tom species are equally susceptible to hostile infestation by parasitic oomycet­es. However, diatoms and their various interactions with their environment continue to present researchers with numerous questions. »The sea is a treas­ure that needs to be protected. There are still many fantastic discoveries to be made,« concludes Marine Vallet.

By Angela Overmeyer