Superpowered seagrasses: the hidden gems of the plant world

Ruby O'Grady examines how these humble plants are helping to thwart disease and combat climate change

Many organisms depend on seagrasses to survive. Here a green sea turtle frolics in the meadow.

If you’ve ever swum in the sea around the UK, you’ve no doubt had to carefully pick your way around those unpleasant, slippery patches of seaweed. Seaweeds seem incredibly plant-like—with long, often green ‘leaves’ connected together on the seafloor—but they are actually a quite disparate group of multicellular red, brown and green algae. The Green Algae are in fact the group which contained the ancestors of all plants, from the tallest of the trees to the miniature succulent sitting in your bedroom. Perfectly adapting to life on land, terrestrial plants have massively diversified, spreading across the continents in the process. However, only a handful of species have returned to the environment of their algal ancestors, living a fully aquatic lifestyle for the duration of their life-cycle. These unique plants are the seagrasses.

Populating only 0.2 percent of the world’s oceans, these seagrasses are of hugely disproportionate ecological importance for the wider ecosystem, with the full extent of their impact only just beginning to be understood.

It is well known that terrestrial ecosystems, such as our rainforests, lock away much of the carbon we produce—a phenomenon known as a carbon sink. But in 2012 it was found that seagrasses store about twice the average amount of carbon per hectare when compared to the land plants, making seagrasses much more effective carbon sinks. Following this revelation, the term ‘blue carbon’ was coined to describe the carbon captured by seagrasses in the world’s oceans, reflecting their growing importance.

A study published in January hinted that this hitherto underdog of the plant world may have even more hidden benefits. They examined the effect of seagrasses on the amount of potentially pathogenic bacteria emanating from sewage pollution in the sea water, as well as on land when exposed during the low tide. They found a clear difference: 50 per cent fewer bacteria were found at sites with seagrass than those without. The mechanism behind this impressive reduction in disease has yet to be established, though there are a number of hypotheses. The authors suggest the blades of the grasses may act to reduce water flow and therefore cause more sediment, containing bacteria, to fall to the sea floor, though this is simply one of many hypotheses. Perhaps the microbiome around the seagrasses outcompetes pathogenic bacteria from the sewage, similar to the bacterial competition seen in the human gut. Or maybe the plants launch an effective immune response, directly removing the bacteria from the sea. Although the study only showed an effect of this difference in bacterial concentrations on coral (which had significantly less disease when located near seagrass), these findings raise the strong possibility that the seagrasses could be highly effective when used to filter out sewage bacteria that cause harm to human health.

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Unfortunately, seagrasses are yet another example of organisms suffering at the hands human activity. 29 per cent of the meadows known to exist at the beginning of the twentieth century are thought to have disappeared after suffering destruction through processes such as seabed dredging. However, hope remains. In the face of climate change, these remarkable plants have displayed yet another valuable characteristic: they show high degrees of phenotypic plasticity, the ability by which an organism can change its physical shape and behaviour in response to rapidly changing environmental conditions.

Studies have shown that these formidable grasses can acclimatise to chronically poor water quality by changing their physiological and morphological characteristics, using mechanisms such as increasing their cellular starch levels. Some species are even able to withstand periods of prolonged light deprivation. Research into these remarkable plants is still undergoing and promises to continue, undoubtedly expanding our understanding of their incredible abilities.

It’s time to acknowledge the underwater underdog of plant world. If we can save and cherish them, seagrasses will prove to be great assets in buffering the human impact on the environment, as well as helping us fight disease and better our everyday lives.