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Their German name ‘Armleuchteralgen’ (lit. ‘chandelier algae’) refers to the morphology of Chara species (and some other relatives within the family Characeae) which has been likened to candleholders: especially at Christmas time one might be tempted to look for seasonal decorations amongst the whorl-like branches of the algae. The English name for the group (‘stoneworts’) is based on the thick calcium carbonate layers that surround the algae when growing in hard waters.

There are about 300 different Chara species worldwide; most of the species found in Germany live in calcium-rich and nutrient poor lakes, whilst some thrive in brackish water at intermediate salinity. Many Chara species are amongst the first colonisers of newly-formed water bodies but others take longer to become established.  ‘Even though the restoration of a water body may be considered successful with regard to other aspects, this does not automatically mean that Chara can become re-established – even if the genus was present there in the past’ explains Chara expert and ecologist Dr Irmgard Blindow who is based at the Biological Research Station Hiddensee which is part of the University of Greifswald.

Keystone organisms

Once they become re-established, some stoneworts can dominate a whole water body; a good example of this is the Coral Stonewort (Chara tomentosa). This species can form dense underwater ’meadows’ which are valuable nursery grounds for fish. The algae are also an important food source for birds and provide habitats for smaller aquatic animals. Stoneworts can shelter and protect juveniles of many animal species from sit-and-wait predators such as the pike. Because of the essential role they play in ecosystems, biologists refer to such species as keystone organisms.

Birds may contribute to the colonisation of new water bodies; Chara subspecies and other stoneworts form robust spores which can survive both freezing and desiccation. These persistent spores, so-called oospores, can also survive the passage through the digestive system of ducks and geese and in this way can be transported to new water bodies.

The spores, which are surrounded by a calcium carbonate layer, allow algal researchers to explore the evolution of the stoneworts. These spores are very persistent compared to the otherwise very soft body of the algae. Their ancestral evolution can be traced back for more than 400 million years to the Devonian period. Some botanists consider the stoneworts the closest relatives of today’s land plants. However recent molecular evidence increasingly suggests that this theory needs to be revised: ‘even if we do not have 100% proof, we now consider the conjugating green algae (Zygnemophyceae) to be the closest relatives, and not the stoneworts’, adds Burkard Becker, Secretary of the Phycology Section.

Threatened species Chara horrida rediscovered

As more and more water bodies are impacted by eutrophication, many Chara species are under threat and included in the Red List of endangered species. In turn, because of the high sensitivity of the stoneworts, biologists can use their presence to assess local water quality. ‘The presence of stoneworts tells us that a water body is more or less healthy’ says Blindow. Over the last few decades the introduction of sewage treatment plants has improved water quality in many places so that stonewort populations can recover. One particular species (Chara horrida) which had been declared extinct in Germany in 1980 has recently been rediscovered off the Baltic island of Hiddensee in the Western Pomerania Lagoon Area National Park.