The comment about the seagrasses helping combat climate change by globally removing 10% of carbon dioxide emissions annually raises a mass-balance issue. (neat!, the Ctrl-U and Ctrl-B commands work). If every year, seagrasses are sequestering 10% of the carbon from carbon dioxide into new biomass, where is it going? If the quantity of active seagrasses is constant (or decreasing due to environmental degradation), then only three possibilities occur to me.
One is the release of a certain amount of biomass through the water to elsewhere (acting as nursery area), but unless this biomass is increasing in this elsewhere area, it is not causing a net removal of CO² (that's the closest I could come to the proper format, by using Alt-253 as there is no subscript option with ASCII codes. I don't know a keyboard command for subscripts). A mature ecosystem would have a stable biomass quantity (allowing for fluctuations about some constant median) because it would be full. Organisms would be growing and dying in rough equilibrium and so the carbon tied up in active biomass would not change.
The second option is to increase the carbon in the root zone, with possible further transport of non-living carbon deeper if (an important caveat) there were downward water circulation to transport it below the root zone in this part of the sea floor. This implies that as plants replace themselves (natural cycle of aging, predation, dying, replacement), the roots of the dead plants are not recycled (eaten) by burrowing animals, bacteria, and other micro-organisms. This is possible. Conditions at some depth below the sea floor surface may be inhospitable to recycling organisms while allowing roots to grow. Peat forms by this scenario. I am not a biologist and so don't know what the ecological mass balance is for this plant. There may be a net increase in storage of non-living carbon in or below the root zone by these plants. The living plant mass in the roots should be constant in a mature ecosystem if the above-ground plant mass is the same. I presume the roots functions are to retrieve nutrients from the sea-floor soils and to act as hold-fasts rather than to obtain water for the plant. The article mentions the root network is abundant.
A third option is storage of carbon by precipitation of carbonates, such as limestone muds. This is an effect of photosynthesis because it changes the the CO² concentrations of the adjacent seawater between night and day provided the saturation with CO² is sufficient and water alkalinity appropriate. If present, this mechanism would be apparent because the precipitated muds build up the sea floor and bury the older parts of the plants or other organisms. Limestone algal mats grow upward this way. It should be obvious if this is how the grasses store carbon because of this progressive rising of the sea floor. It would also leave the dead root zones behind in this case as an additional contribution to carbon sequestration.
My concern is whether it is really plausible that the mass balance is really that skewed to sequestration of carbon, as opposed to simply making new biomass that replaces an equivalent amount of biomass consumed or otherwise returned to the environment as new CO². Mature woodlands hit a point when the only new carbon being tied up is dead material in and below the root systems because the quantity of living biomass has peaked. The ecosystem is full. Peat bogs, some swamps, and anoxic zones on the sea floor are examples where dead material is not recycled back into CO². I don't know enough about marine ecology to know whether the statement that 10% of our annual human CO² production is being sequestered by these plants (globally) every year, year after year, is really happening. However, I need to see this confirmed by a professional marine biologist familiar with these plants and determining mass budgets of ecosystems. Too often people are confused by the difference between the productivity of an ecosystem in making new biomass and the net removal of carbon from the system once an ecosystem has come into balance with its conditions (it's full).