This week in Science: Seagrasses survive thanks to subterranean symbiosis

It has always been a mystery why seagrasses are the only vascular plants that can survive at the bottom of the sea despite lethal concentrations of sulphides. In a joint study by the University of Groningen, Radboud University Nijmegen, the Royal Netherlands Institute for Sea Research and the University of Florida researchers have concluded that seagrasses cooperate with small clams and bacteria that live in the sea soil and render sulphides harmless. The results will appear this week in the leading scientific journal Science.

Studies of fossils have revealed that these clams and bacteria have been present in beds of seagrasses since these plants evolved in the Cretaceous period. The researchers conclude that this symbiosis must have developed some 100 million years ago, when the first land-based ancestors of the modern seagrasses conquered the sea. Seagrasses have been of great importance for the biodiversity of the world’s oceans ever since. These plants grow in brackish to saline water and occur all over the world in shallow coastal waters.

Ecosystem engineers

Seagrasses are ‘ecosystem engineers’, organisms that can dramatically change their environment. For example, dense seagrass fields reduce tidal currents and wave action, causing suspended silt to sink to the bottom and resulting in clearer water. These environmental changes are beneficial to the fish, turtles, dolphins and birds that live and hunt amongst seagrass fields. But the seagrass itself benefits from the clear water too, because it gets more light and so can grow faster.

Sulphides

However, the silt that settles on the sea floor is bad news for the seagrass. It consists mainly of dead matter from plants and animals that is decomposed in the soil by bacteria. Because sea beds contain little oxygen, this decomposition is carried out by bacteria that use the sulphate in seawater instead of oxygen. But these bacteria produce sulphide as a by-product, a substance that is toxic for most plants and animals, including seagrass.

Farming bacteria

It was unclear before how seagrasses are able to thrive with so much poisonous sulphide being released among their roots. In Science, the researchers describe how large numbers of small clams of the family Lucinidae often occur among the roots of seagrasses. It was already known that these tiny animals ‘breed’ special bacteria in their gills which they live off. Sulphide is not toxic to these clams; in fact, they thrive on it.

Cooperation

In a laboratory experiment, the researchers grew seagrass with and without the clams. They also cultivated the clams without seagrass. The results show that the clams and their bacteria flourish much better among seagrass roots than in bare sand, because the seagrasses provide the clams with oxygen via their roots. At the same time, almost all the sulphide was ‘eaten' by the clams’ accompanying bacteria, so that the seagrass plants grew better in pots with the clams than without. ‘A unique form of cooperation,’ according to the researchers.

Conservation

While seagrasses are important for biodiversity, the areal extent of seagrass beds has fallen sharply in recent decades due to man-made pollution and other environmental disruptions. In many places attempts are made to protect the remaining seagrass beds and restore damaged ones.

‘Our research shows that, if we are to preserve and restore seagrass beds, we need to examine how they cooperate with other organisms as well as studying the plants themselves,’ says Tjisse van der Heide of the University of Groningen. ‘Attempts made to date involved simply transplanting the seagrass plants, but these had little success. We may well significantly increase the chances of success if we transplant complete “seagrass ecosystems” instead, i.e. both the plants and the sulphide-eating soil organisms. This could turn out to be critical, especially in the tropics.’

The research was financed by the Waddenfonds and the Netherlands Organization for Scientific Research (NWO).

Note for the press

More information: Dr. Tjisse van der Heide

Reference: A Three-Stage Symbiosis Forms the Foundation of Seagrass Ecosystems, Tjisse van der Heide, Laura L. Govers, Jimmy de Fouw, Han Olff, Matthijs van der Geest, Marieke M. van Katwijk, Theunis Piersma, Johan van de Koppel, Brian R. Silliman, Alfons J. P. Smolders, Jan A. van Gils. Science, 15 juni 2012, http://dx.doi.org/10.1126/science.1219973