Only a small group of highly specialized vascular plants, known as halophytes, have evolved the ability to survive and reproduce in saltwater or brackish conditions, which creates a significant osmotic challenge. Halophytes must be distinguished from macroalgae, commonly called seaweed, which lack the complex root systems and internal vascular structures of their land-based relatives. The strategies employed by these plants to manage high salinity and waterlogged, oxygen-poor soils are unique, allowing them to occupy different coastal niches around the world.
Fully Submerged Flora: Seagrasses
Seagrasses are the only known group of flowering plants that live their entire lives fully submerged in marine or estuarine waters. These plants, which are not true grasses but rather monocots, anchor themselves into soft sediment using an extensive network of underground stems called rhizomes. Their long, slender blades absorb nutrients directly from the water column or the sediment. Seagrasses reproduce sexually through underwater pollination, where specialized pollen grains are carried by water currents to fertilize the female flowers. These meadows serve as powerful ecosystem engineers: their dense root system stabilizes the seafloor, slowing water flow, trapping sediments, and functioning as essential nursery grounds for juvenile fish, shellfish, and invertebrates.
Coastal Woody Plants: Mangroves
Mangroves are a diverse group of trees and shrubs that thrive in the intertidal zone of tropical and subtropical coastlines, tolerating daily fluctuations in salinity and water level. They face high salt concentrations and the anaerobic conditions of waterlogged mud. To address the lack of oxygen, many species develop specialized breathing roots called pneumatophores, which grow vertically out of the mud and absorb air directly. These aerial roots contain a spongy tissue that transports oxygen down to the submerged root system. Mangroves manage salt using two strategies: exclusion or secretion; species like the Red Mangrove (Rhizophora) filter salt at the root level, while others, like the Black Mangrove (Avicennia), excrete excess salt through specialized glands on their leaves.
Tidal Flat Vegetation: Salt Marsh Grasses
Salt marsh grasses dominate the temperate intertidal zone, occupying mudflats and estuaries where they are regularly flooded by tides. These herbaceous plants are highly resilient to periodic inundation and shifts in salinity. Common species, such as Cordgrass (Spartina alterniflora), form dense meadows that help buffer inland areas from storm surges. They survive the anoxic soil conditions by utilizing a specialized internal structure called aerenchyma tissue. This spongy tissue creates air pockets that deliver oxygen from the leaves down to the submerged roots, and they also actively secrete excess salt through tiny salt glands located on the surface of their leaves.
Survival Mechanisms in Saline Environments
Halophytes rely on a combination of morphological and physiological strategies to manage the twin threats of osmotic stress and ion toxicity. The presence of high salt outside the roots reduces the water potential, making it difficult for the plant to absorb water, which can lead to physiological drought. To counteract this, many halophytes employ an osmotic adjustment strategy, synthesizing organic compounds known as compatible solutes, such as proline and sugars, to balance the osmotic pressure inside their cells. A fundamental defense is salt exclusion, where root membranes act as ultra-filters to prevent most sodium and chloride ions from entering the plant. For the salt that inevitably gets past the roots, plants utilize salt secretion through specialized leaf glands or salt bladders, or they use succulence, storing water in fleshy leaves or stems to dilute the salt concentration until the leaves drop.