The Secret Life of the Ocean’s Green Giants

By Joel Tester

The title of Earth’s largest living organism may bring to mind classic examples like giant sequoias or blue whales. Yet, in a recently published paper, researchers at the University of Western Australia revealed an unexpected new winner of this accolade. The scientists studied a 180 km2 meadow of the seagrass Posidonia australis in West Australia’s Shark Bay, taking DNA samples to analyse the population’s genetic diversity. When the DNA sequences of shoots throughout the entire meadow came back identical, they made the shocking realisation that they weren’t dealing with a large population of plants at all. This was a single individual, clocking in at three times the size of Manhattan, making it the largest known organism on the planet.

A dive into the strange biology of seagrasses reveals how they grow to be so enormous. Their terrestrial ancestors gave up dry land for a watery existence whilst dinosaurs were still roaming the Earth, and they are the only flowering plants to have evolved a fully aquatic lifestyle, producing delicate and alien-looking underwater blooms which use the ocean’s currents to disperse their pollen and seeds. But life underwater is not without its challenges. Unlike firm soil, the sediments in which seagrasses grow are constantly churned about by the tides – so whilst wind-pollinated grasses on land can generally rely on at least a few of their seeds successfully germinating, the efforts of a seagrass seed to anchor itself down are usually in vain.

This was a single individual, clocking in at three times the size of Manhattan, making it the largest known organism on the planet.

Instead of producing seeds, a better strategy is for seagrasses to grow an underground stem, called a rhizome, which spreads out horizontally and gives rise to new shoots, connected to each other below the sediment. Shoots grown in this way are more likely to survive than those emerging from seeds, as their connection to an already-mature shoot keeps them anchored into the soft sediment whilst they grow their own roots. This strategy works so well that seagrasses generally withhold from reproducing sexually by flowering, only dispersing seeds as a last-ditch response to stresses which threaten the whole colony’s survival.

Seagrass meadows grow year-on-year with the addition of new shoots, with some eventually reaching the gigantic sizes achieved by seagrass beds like that of Shark Bay. Whilst individual shoots are subject to wear-and-tear and eventually die, the clonal replacement of shoots also allows the colony to persist for much longer than any single shoot would otherwise be capable of. The meadow in Shark Bay is estimated to be a whopping 4,500 years old. Nonetheless, even this impressive age pales in comparison to a meadow of the closely-related species Posidonia oceanica in the Mediterranean, which is estimated to have been growing for between 12,000 and 20,000 years.

The vast, long-lived coastal beds formed by these marine giants also provide habitats and food for a multitude of other marine species. In the tropics, they are home to large herbivores like sea turtles and dugongs, which in turn support populations of predators like tiger sharks, to which Australia’s Shark Bay owes its name. Closer to home, seagrasses also provide habitat for the UK’s two native species of seahorse, as well as octopuses, cuttlefish, crabs and seals.

The sheltered coastal areas in which seagrasses thrive coincidentally tend to be heavily urbanized, and are thus subjected to high levels of agricultural runoff, sewage discharge, and other forms of pollution. Whilst this is generally bad news for seagrass meadows, there is evidence that despite this, seagrasses may form a first line of defence which helps to slow down the flow of pollution further out to sea.

Seagrass beds are important nursery grounds for many fish species, which spend their juvenile stages sheltered between seagrass shoots before dispersing out to open water or coral reefs as adults. As a result, they are vital for offshore fisheries: in the Mediterranean, seagrass-associated species constitute up to 40% of fisheries’ total catches, despite seagrasses only occupying only 2% of its area.

But it isn’t just their role as habitats for other marine species which makes seagrass meadows important. The sheltered coastal areas in which seagrasses thrive coincidentally tend to be heavily urbanized, and are thus subjected to high levels of agricultural runoff, sewage discharge, and other forms of pollution. Whilst this is generally bad news for seagrass meadows, there is evidence that despite this, seagrasses may form a first line of defence which helps to slow down the flow of pollution further out to sea. When their shoots die, tidal action aggregates the fibrous matter of seagrass leaves into buoyant clusters referred to as Neptune balls, which are then washed ashore. One study in 2021found that these aggregations trap plastic debris, bundling up microplastics and beaching them before they reach the open ocean.

Seagrasses can also help combat nutrient pollution, another threat to marine life. Inputs of nitrogen and phosphorus, in the form of fertilizer runoff and organic waste, provoke phytoplankton blooms – and when these algae die, they provide abundant food for bacteria which guzzle up dissolved oxygen in the process of decomposing them. The result is ocean dead zones – regions of coastal waters where there is no longer sufficient oxygen to support animal life. This is bad news for marine biodiversity and fisheries, forcing vessels to move further and further offshore in search of fish. However, the swaying leaves of seagrasses intercept and collect suspended particulate matter, like organic waste, as it flows over them. In this way, they can help mop up excess nutrients before they reach open water, protecting offshore marine life from the worst effects of coastal pollution.

Despite occupying only 0.1% of the ocean’s surface, seagrass meadows and their underlying sediments account for up to 9% of the earth’s total carbon sequestration each year, making seagrass meadows one of the most powerful carbon sinks on the planet.

Seagrass meadows are also incredibly important for the mitigation of climate change. On land, only around 2-3% of the carbon dioxide absorbed by plants actually remains locked up in plant biomass, and usually only for short timescales, because most of the carbon fixed through photosynthesis ends up returning to the atmosphere as CO2 through microbial decomposition. But this is an oxygen-demanding process, and water holds a much lower concentration of dissolved oxygen than air, so the submerged sediments in which seagrasses grow only support very slow rates of decomposition. Accordingly, seagrass beds hold onto their stored carbon for much longer than is possible on land.

Additionally, seagrasses dedicate a bigger proportion of their biomass to below-ground root tissue than most land plants, to stabilise themselves in soft sediments: their networks of roots and rhizomes form a peat-like ‘matte’, which can extend for several metres below the visible surface of the meadow. So, when seagrass shoots die, a large proportion of the carbon which they have fixed during their lifetime is already buried deep within the anoxic mud, hidden far beyond the reach of decomposers where it can remain for centuries.

As a result, despite occupying only 0.1% of the ocean’s surface, seagrass meadows and their underlying sediments account for up to 9% of the earth’s total carbon sequestration each year, making seagrass meadows one of the most powerful carbon sinks on the planet. But the flip-side of this is that when seagrass meadows are degraded, the carbon buried within the sediment is re-exposed to oxygen, converting these beds from net sinks to net sources of carbon. Between 1897 and 2009, 29% of known seagrass cover was lost globally – with the decline having been even more drastic in the UK specifically, where an estimated 92% of historic seagrass cover is has been lost. The main drivers behind these losses include coastal pollution, seabed dredging, destructive fishing practices like bottom trawling, and damage caused by the anchors and propellers of boats.

Simply regulating these coastal stressors more effectively through policy, alongside re-planting meadows where they have been lost, is a simple, and effective way to ensure the continued functioning of these vital habitats.

But unlike terrestrial carbon sinks such as forests, whose restoration efforts must grapple with the politics of land-use conflict, most of the stressors facing seagrasses are the unintentional byproducts of other activities – like ineffective wastewater treatment, excessive fertiliser application, destructive fishing, and recreational boating. Simply regulating these coastal stressors more effectively through policy, alongside re-planting meadows where they have been lost, is a simple, and effective way to ensure the continued functioning of these vital habitats.

In the 4,500-year lifetime of Shark Bay’s seagrass meadow, the world has undergone extraordinary change. It has witnessed humans transform from bands of nomadic hunters into a geological superpower, and now watches us grapple with the realisation that our destructive activities are threatening our very own life support system. But there is a lot we can learn from these ancient aquatic giants. They teach us that through the flow of living things and organic matter, global ecosystems are more interconnected than we often realise – and that often, solutions to some of the world’s biggest problems can be found in the places that we least expect.


This article is based on the paper: Edgeloe, J.M et al., “Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment”, Proc. R. Soc. B. Vol 289, Issue 1976, 01 Jun 2022.