Plants without roots, others that are carnivorous, some capable of flowering underwater- Aquatic plants have developed spectacular adaptations that challenge our vision of the plant world.
Hidden from view, beneath the surface of the waters, unfolds a plant world of astonishing inventiveness and among the most important from an ecological point of view. As I highlighted in a recent publication , aquatic plants have developed an extraordinary diversity of adaptations to live underwater.
Some flower below the surface, while others capture animals using ingenious traps. Here are seven facts that demonstrate how these remarkable organisms challenge our preconceived notions about what a plant is and the strategies it employs to survive.
The plants keep returning to the water
When we think of plants, we spontaneously picture forests, meadows, or fields. Yet, throughout their evolutionary history, plants have returned to aquatic environments on numerous occasions, the very places where they first appeared.
Around 500 million years ago , they conquered the land. Since then, many have made the return journey. Scientists estimate that aquatic life has emerged independently more than 100 times within different plant groups.
Water lilies float their leaves on the surface, duckweed drifts freely, and seagrass meadows live fully submerged in the ocean. Some of these groups returned to the water more than 100 million years ago .
This repeated reappearance of aquatic plants is one of the most spectacular examples of convergent evolution in nature.
Plants that are not
Among the most visible organisms beneath the water’s surface are algae. They perform photosynthesis and often resemble underwater plants. However, despite appearances, algae are not true plants .
Seaweed actually belongs to several distinct lineages in the tree of life. Giant kelp, which forms veritable underwater forests, is a brown algae. Nori and dulse are red algae, while sea lettuce belongs to the green algae.
Unlike plants, they have neither true roots, nor stems, nor leaves, and produce neither flowers nor seeds. Their resemblance to plants, however, serves as a reminder that evolution can lead to very similar forms in organisms that are otherwise very different, when faced with the same environmental constraints.
Plants that live in the depths
Plants need light to photosynthesise, which generally confines them to terrestrial environments or shallow waters. However, some aquatic mosses survive at astonishing depths.
The curved sickle (Drepanocladus aduncus) has been observed 140 metres below the surface in the exceptionally clear waters of Crater Lake , in the US state of Oregon. It is the known aquatic plant that also grows on land and lives at the greatest depth, roughly the height of Strasbourg Cathedral.
Deep-sea mosses have also been recorded in lakes in New Zealand , Antarctica , and other regions. They thrive in environments so deep that they are almost completely devoid of light and where very few animals can survive.
Rootless plants
Roots are one of the defining characteristics of plants. They anchor them in the soil and draw water and nutrients from it. Yet, many aquatic plants have significantly reduced their root systems, and some even appear to have lost their roots completely .
Life underwater changes the rules of the game. Water and dissolved nutrients directly surround the plant, making extensive root systems far less useful than on land. Many aquatic species therefore absorb nutrients directly through their leaves and stems.
Duckweed offers one of the most extreme examples. Some species have only a single root, unlike relatives such as the giant duckweed, which develops several.
As for species of the genus Wolffia-the smallest flowering plants in the world-they have no roots at all and float freely on the water’s surface. An individual measures barely one millimetre in length, and its flowers do not exceed 0.3 millimetres.
Carnivorous plants underwater
Not all aquatic plants rely solely on sunlight and nutrients dissolved in water. Some supplement their diet by capturing and digesting small animals.
The most spectacular examples are bladderworts (Utricularia), a group of rootless aquatic plants found in freshwater around the world.
Their leaves have transformed into tiny bladder-like traps that create a vacuum by expelling the water contained within their cavity.
When a tiny animal brushes against the sensitive hairs at the entrance of the trap, a hatch opens abruptly and the prey is sucked in in less than a millisecond.
Bladderwrack traps are thus among the fastest movements in the plant kingdom. While they most often capture small aquatic invertebrates, they also sometimes trap fish larvae and tadpoles .
This carnivorous lifestyle allows bladderworts to thrive in nutrient-poor waters, where most other plants struggle to survive.
Pollination carried by currents
When we think of plant pollination, we often picture bees flitting from flower to flower on a beautiful sunny day. But underwater, pollination becomes much more complicated.
Instead of relying on insects or wind , many aquatic plants, such as seagrasses, directly use currents to transport their pollen to its destination. On land, plants attract pollinators by releasing scents into the air. Underwater, however, these volatile signals are ineffective.
This constraint led to an evolutionary shift: fully aquatic plants, such as seagrasses, lost the genes responsible for producing these fragrant compounds. No longer providing any advantage, they gradually disappeared through evolution.
Seagrass meadows and mangroves, powerful carbon sinks
Seagrass meadows and mangroves capture and store carbon in their tissues and the surrounding sediments, making them among the most efficient natural carbon sinks on the planet. Together, they store what scientists call “blue carbon”: carbon trapped in coastal ecosystems, where it can remain stored for centuries, even millennia.
Globally, these ecosystems- seagrass beds and mangroves- store 11.5 billion tonnes of carbon. Mangroves alone represent the largest reservoir of blue carbon, with 6.5 billion tonnes .
Whether they capture their prey in fractions of a millisecond, grow in near darkness or store carbon for centuries, aquatic plants demonstrate the extraordinary ability of living things to adapt.
(The Conversation)
