Two's company, three's community ecology

A few days ago I wrote about a review paper in Nature Geoscience, which concluded that seagrass meadows were better at storing carbon than terrestrial forests. One of the reasons for this is that, rather than being broken down to carbon dioxide, the organic matter beneath seagrass meadows is broken down anaerobically to hydrogen sulphide.

Hydrogen sulphide is toxic to the seagrass and should build up as more organic matter accumulates over time. Until a new paper in Science, it was unclear how the plants coped with the gas. Seagrasses do transport oxygen to their roots, which is released into the surrounding soil, mitigating against the toxic effects of sulfide. But, sulfide production can outpace the transport of oxygen, especially at higher temperatures, which should result in reduced productivity and mortality. 

Seagrass meadows are, in contrast, persistent and highly productive, which suggests that there's more than oxygen transport going on. The paper provides data to show that burrowing Lucinid bivalves commonly co-occur in high abundance with seagrass meadows. And furthermore, the Lucinids increasingly diversified as seagrasses evolved and became more common in the oceans.

A map showing the locations where seagrass occur with (green spots) or without (red spots) lucinid bivalves. The bivalves were present in 97% of tropical, 90% of subtropical, and 56% of temperate seagrass meadows (image taken from the paper).

Lucinids are known to form symbiotic relationships with sulfur oxidising bacteria. The bivalves bring oxygen and sulfides to their gills where the bacteria use them to fuel the production of sugars, which feed them and their bivalve hosts. The paper hypothesises that seagrass also participates in the symbiosis through the transport of oxygen to its roots, which provides ideal conditions for the lucinids and their gill bacteria.

The lucinid bivalve Loripes lacetus (photo idscaro seashell directory)

They tested the hypothesis by growing both a lucinid bivalve (Loripes lacteus) and a seagrass (Zostera noltii) in the laboratory. They examined the effect of sulfide on the bivalve and seagrass when they were both present or when the partner was absent. And their results were intriguing, particularly the effects on seagrass.

The seagrass Zostera marina, which is very similar to Z. noltii (image wikipedia)

The seagrass had greater biomass of both leaves and roots when no sulfide was added and the bivalves had higher weights relative to their shell size when it was. But, when sulfide was added both the seagrass and the bivalve did better when they both occurred together. Interestingly, the seagrass did best when the bivalve was present, but no sulfide was added. This seems to be because sulfide built up in the treatments, even when it was not added.

The results largely confirmed the hypothesis that there is a three species symbiotic relationship. Seagrass benefits from the removal of sulfide by the bacteria-bivalve symbiosis and the bivalve benefits from the oxygen provided by the seagrass. Furthermore, in the wild the bivalve may benefit from the presence of seagrass because seagrasses indirectly stimulate sulfide production by accumulating organic matter in the soils beneath the growing meadows.

The results have potentially significant implications for seagrass conservation. Many conservation projects attempt to restore lost seagrass meadows by moving plants from other locations were they're doing well. But, the results of this procedure are mixed. This research suggests that moving the bivalve as well as the seagrasses might lead to greater success.

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 Koppe, Brian R. Silliman, Alfons J. P. Smolders, & Jan A. van Gils (2012). A three-stage symbiosis forms the foundation of seagrass ecosystems Science, 336 (6087), 1432-1434 DOI: 10.1126/science.1219973