In the coastal zone, the edge of the world’s oceans, there live various plants. But what exactly are plants? The Oxford English Dictionary tells us that a plant is “a living thing that grows in the earth and usually has a stem, leaves and roots, especially one that is smaller than a tree or bush”. This seems confusing when we know that there are plants in the sea that are not necessarily smaller than a tree or bush! Although the Oxford definition is obviously too specific, many botanists would agree in so far that plants are those organisms that live on land and perform photosynthesis. These definitions ignore the vast diversity of plants in the ocean, such as all algae.

“Plant” is in fact a non-taxonomic term, which means that it does not define how closely the organisms in that group are evolutionarily related. If we look at the eukaryotic tree of life, we can begin to understand that plants are spread across at least four supergroups, namely Archaeplastida, Cryptista, Haptista and SAR, while humans and mushrooms share the same, relatively tiny subgroup, called Opisthokonta. Keeping this frequent misinterpretation of the term “plant” in mind, let’s return to marine plants.

Some marine plants are flowering plants of the Archaeplastida supergroup: seagrasses and mangrove trees that are used to a life in the sea. Others are distantly related, large brown algae, so-called kelps belonging to the SAR supergroup, that never left the ocean. But all of these plants build vast coastal vegetated habitats: the planet’s marine forests and meadows. These forests and meadows are very different from what you are used to seeing on land because the ocean has selected for many fascinating adaptations! In the film you already learned about mangroves, so let’s explore how seagrasses survive in the sea.

Like all flowering plants, seagrasses need to pollinate each other to have sex. Living entirely submerged, where there is no wind and no insects fly busily between flowers, that requires some modification. Seagrasses have either evolved pollen and flowers that float or submarine pollination. Underwater, pollen is usually carried to the female flower by currents. To make sure that it arrives at its destination in this turbulent environment, pollen is either shaped like long sticky noodles that wrap around the stigma or is released in slimy strands. But some seagrass flowers also get visited by little crustaceans and worms, which carry out pollination just like their insect cousins!

Seagrasses have also reengineered their entire cell wall matrix. For instance, they lack stomata, the little pores that allow land plants to exchange gases and water with the atmosphere. Instead, the cell wall is more like that of algae, sporting maximum flexibility to avoid damage in the onrush of waves. This adaptation is what allows seagrasses to dance so hypnotisingly in the swell, a phenomenon referred to as monami. Their specialised cells also allow seagrasses to take up CO₂ from seawater to perform photosynthesis under the dim light conditions near the seabed. Accordingly, seagrass seeds need the blue light of the underwater world to germinate.

But if all these modifications are necessary, why did these plants return to the sea in the first place? To answer this question, we need to return to the very beginning of life on land, long after the first organisms and first plants had evolved in the ocean. Around 500 million years ago, a group of green algae left the sea to explore uncharted territory. These were the first land plants that radiated into an enormous diversity of flowering plants around 100 million years ago. This explosion of life led to intense predation and competition, so some flowering plants colonised less sought-after habitats, such as the tidal zone and beyond. This explanation makes sense if we consider that seagrasses no longer have to produce costly defences against insect predators. Nevertheless, such diverse animals as sea turtles, sea cows and even sea sloths soon followed the seagrasses on their seaward journey to graze on their lush, untouched meadows.

There are a multitude of reasons why we should be protecting marine forests and meadows, which are nowadays threatened by anthropogenic pressures. Much of our carbon emissions are shuttled into the deep sea by kelp forests, which are also important for future food security. Mangroves can teach us how to desalinate enough water to supply a growing human population and seagrasses are surprisingly closely related to rice, which could enable us to grow more food in salty environments. Clearly, our fates are closely tied to those fascinating plants of the watery world. Here are some useful websites addressing the conservation of coastal vegetated habitats:

https://www.projectseagrass.org
https://www.seagrasswatch.org
https://wsa.seagrassonline.org
https://www.mcsuk.org/ocean-recovery/seagrass
https://www.fisheries.noaa.gov/west-coast/ecosystems/kelp-conservation
https://www.thebluecarboninitiative.org
http://www.mangrovealliance.org
https://en.unesco.org/commemorations/mangroveday
https://www.oceandecade.org
https://www.decadeonrestoration.org

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