Mangroves can filter pure freshwater straight out of the sea
Thriving along tropical coastlines where other trees would quickly wither and die, red mangroves act as natural desalinators. Their root membranes filter out over 90% of the salt from seawater before drawing it upward. Other species, like black mangroves, take in salty water and pump excess salt crystals out through microscopic glands on their leaves. This leaves their foliage visibly encrusted with white salt crystals while the plant stays hydrated.
The Challenge of the Saline Shore
Coastal wetlands in tropical and subtropical regions present some of the most hostile conditions on Earth for plant life. While terrestrial plants depend on a consistent intake of freshwater from soil, intertidal habitats are repeatedly inundated by the ocean. For ordinary trees, exposure to high levels of sodium and chloride creates intense osmotic pressure. Instead of absorbing water, root systems in saline water have moisture drawn out of them, causing dehydration even while sitting in saturated mud.
Compounding the problem of salinity is the nature of the sediment. Coastal mud is fine-grained, heavily waterlogged, and frequently flooded by the tide. Because water fills the tiny gaps between sediment particles, atmospheric oxygen cannot penetrate the soil. Without oxygen, typical root systems suffocate and rot. Mangroves are among the few woody plants that have evolved specialized morphological and physiological adaptations to survive both saltwater saturation and suffocating soils.
Filtering Salt at the Root Boundary
Different mangrove species inhabit specific zones along the shore, largely dictated by their distinct methods of managing salt. Red mangroves, which typically grow closest to the open water where tides submerge their lower halves daily, employ an exclusion strategy. Rather than taking in salt and dealing with it internally, their roots block the mineral before it enters the vascular system.
The root membranes of red mangroves act as ultra-filtration barriers. Driven by the negative pressure created as water evaporates from leaves during transpiration, the tree pulls water out of the sea while physical and chemical barriers in the roots block more than 90 percent of dissolved salts. Supported above the soft seabed by sprawling arching prop roots, red mangroves absorb water that is virtually fresh, allowing the upper branches to stay hydrated without accumulating toxic mineral concentrations.