How to keep seagrasses as happy as a clam

In a severe hot spell, breaking plant-mollusk partnerships speeds degradation

TOUGHING IT OUT  Researchers investigate what lets certain patches of seagrasses survive hot spells and drying winds when receding tides leave them as a flat green carpet on the Banc d’Arguin mudflats off the West Africa coast.

Jeroen Onrust

Clamming up could help underwater seagrass meadows better withstand drought, heat waves and climate change.

Breakdown of a symbiotic bond between seagrasses off the West Africa coast and tiny lucinid clams can exacerbate damage in hard times, researchers say March 10 in Current Biology. So protecting meadows may mean worrying just as much about the partnership as the seagrasses themselves, says coauthor Tjisse van der Heide of Radboud University Nijmegen in the Netherlands.

Seagrasses aren’t seaweed like kelp. They’re genuine flowering plants whose ancestors over about 100 million years adapted to a salty underwater life that would quickly kill most land plants (SN: 12/5/09, p. 22).

Clams can play a role in easing one risk of this life: debris breakdown that goes toxic. Their own dead leaves plus floating bits of dead stuff and waste, including runoff from nearby land, snag in a seagrass meadow’s expanse of swaying leaves. Sea-bottom microbes that break down the rain of debris release sulfides, which can poison the plants.

In 2012, van der Heide and colleagues proposed that in some seagrass communities, especially in the tropics and subtropics, clams burrowing among the plant roots help detoxify sulfides. The Loripes clams and other species in the lucinid group benefit from oxygen that plant roots give off, and the sulfides from debris breakdown nourish symbiotic bacteria in the clams. “They’re farming bacteria in their gills,” says van der Heide. As the bacteria feast, they leak sugars that nourish their clam, and the toxic sulfides are turned into harmless sulfates, lowering the toxic risks to seagrasses.

MEADOW PALS An underwater meadow of dense seagrasses at Banc d’Arguin (left) trades oxygen for detoxification services from thousands of tiny bacteria-fostering clams (right). Laura L. Govers (both)

Not every seagrass meadow has lucinid clams, but intertidal meadows at Banc d’Arguin in Mauritania do. The clam density there can average 1,500 to 2,000 small clams per square meter in the top 10 centimeters or so of mud. At low tides, mats of seagrasses lie exposed to air for hours at a time. During the severe drought of 2011, the hot, thirsty air desiccated swaths of the meadow. Dying plants faltered in oxygenating the clams and their bacteria, causing a cascade of effects that weakened the detoxification process.

As drought-stricken seagrasses and clams failed to provide each other with benefits, a feedback loop of increasingly worse performance for both partners accelerated their demise, van der Heide and his colleagues propose. That’s the scenario they draw from satellite data on the meadow during the drought. Seeing considerable living patches mix with dead zones at the same elevation and exposed to the same climate conditions points to something else — such as a mutualism breakdown — speeding the change from leafy to bald.

Comparing seagrass patches that died with patches that survived, van der Heide and his colleagues propose that maintaining the clam partnership made a difference. Of 32 plots of meadow, the half where seagrasses survived the drought had nine times as many clams as the dead zones. These survival spots also averaged only about a quarter of the toxic sulfides.

This seagrass-clam mutualism may be important in Banc d’Arguin but has yet to be demonstrated as a more general phenomenon, says marine ecologist Carlos Duarte of King Abdullah University of Science and Technology in Thuwal, Saudi Arabia. 

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