Some fungal spores can swim

A fungus with a flagellum

A bolete spore is easily imagined as a tiny particle carried by the wind. Chytrids, members of the Chytridiomycota, offer another strategy: a motile zoospore with a flagellum. The cell can actively move through water before settling and entering its next developmental stage. This is not a miniature swimming mushroom but one part of a microscopic fungus's life cycle.

Dominic Laundon and colleagues produced a cellular atlas of Rhizoclosmatium globosum. They combined imaging with gene-expression analysis to compare zoospores, germinating cells and developing thalli. The resulting atlas connects changes in appearance with internal organisation and cellular processes, rather than classifying stages only by their outline under a basic microscope.

Settling changes the cell's task

The motile stage helps the organism reach a suitable location. After settling, the fungus develops a cell wall and structures involved in acquiring nutrients. A mature sporangium eventually produces the next generation of zoospores. Separating dispersal from attached growth explains why two very different microscopic views need not represent two different species.

Chytrids occupy several ecological roles. Some decompose organic material; others are parasites. Research on one cultured species therefore cannot define the lifestyle of the entire group. Nor should chytrids be reduced to the amphibian pathogens that have attracted much of the public attention. Their evolutionary history and cellular variety extend well beyond those particular disease systems.

A route through aquatic food webs

In studies of fungi infecting plankton, zoospores provide another connection: small animals can eat them. Material from a large or otherwise difficult-to-consume host may therefore reach a grazer through a fungal intermediate. This route is often called the mycoloop. Its existence does not mean that fungi always suppress blooms, or that it carries an identical share of food-web production in every lake.

The field-observation lesson is straightforward. An absence of mushrooms along a shore says little about the fungi living in the water. Yet movement itself is not an identification character sufficient to establish fungal identity. Algae, protists and bacteria also move in water samples. A recording of a swimming cell needs additional evidence before it can be labelled a fungus; microscopy, cultivation or sequence data supply different parts of that evidence.