Mangrove

Biology

Among recognized mangrove species, about 70 species in 20 genera from 16 families constitute the "true mangroves," occurring almost exclusively in mangrove habitats. Convergent evolution led many of these species to develop similar solutions to variable salinity, tidal inundation, anaerobic soils, and intense sunlight. Plant biodiversity within a given mangrove is generally low, with the greatest biodiversity found in Southeast Asia, particularly the Indonesian archipelago.

The red mangrove (Rhizophora mangle) survives in the most inundated areas, propping itself above water with stilt roots and absorbing air through lenticels. The black mangrove (Avicennia germinans) lives on higher ground and develops pneumatophores—root-like structures up to 30 cm or, in some species, over 3 m tall—that act as breathing tubes. These roots contain wide aerenchyma for gas transport. Waterlogged soil limits free oxygen, and anaerobic bacteria release nitrogen gas, iron, phosphates, sulfides, and methane, reducing soil nutrition.

Pneumatophores allow mangroves to absorb gases directly from the atmosphere and process them even when submerged at high tide.

Species distribution

Mangroves are primarily tropical vegetation, with outliers in subtropical latitudes such as South Florida, southern Japan, South Africa, New Zealand, and Victoria (Australia). These outliers result from unbroken coastlines, island chains, or propagules carried by warm ocean currents. At distribution limits, the formation is often scrubby, monotypic Avicennia-dominated vegetation, as seen at Corner Inlet, Victoria, Australia—the highest latitude (38° 45' S) for natural mangroves.

New Zealand mangroves extend to 37° S, starting as low forest in the northern North Island and becoming low scrub southward. In both cases, the species is Avicennia marina var. australis. In Western Australia, A. marina reaches Bunbury (33° 19' S). In the northern hemisphere, scrubby Avicennia germinans in Florida occurs as far north as St. Augustine and Cedar Point.

Records exist of A. germinans and Rhizophora mangle in Bermuda, likely supplied by the Gulf Stream. In southern Japan, Kandelia obovata occurs to about 31° N.

Mangrove forests

Mangrove forests, also called mangrove swamps or mangals, occur in tropical and subtropical tidal areas, including estuaries and marine shorelines. The intertidal existence to which these trees are adapted limits the number of species able to thrive. High tide brings salt water; solar evaporation after the tide recedes increases soil salinity, while the returning tide flushes soils back to seawater salinity. At low tide, organisms face increased temperature and reduced moisture before being cooled and flooded again.

Thus, plants must tolerate broad ranges of salinity, temperature, and moisture, so only a select few tree species make up the mangrove community. About 110 species are considered mangroves, though a given swamp typically features only a small number—Caribbean forests often have only three or four tree species. By comparison, tropical rainforests contain thousands of tree species, but mangrove ecosystems provide habitat for a great variety of other species, including up to 174 species of marine megafauna.

Mangrove plants require physiological adaptations to low oxygen, high salinity, and tidal flooding. Each species has its own solutions, which may explain distinct zonation on some shorelines. Small environmental variations within a mangal can lead to differing coping methods, so species mix is partly determined by tolerances to tidal flooding and salinity, and may be influenced by factors such as crabs preying on seedlings.

Mangrove microbiome

Plant microbiomes are crucial to mangrove health and productivity. Knowledge of these microbiomes has been applied to produce inocula for crop protection, stimulating growth by releasing phytohormones and enhancing uptake of phosphorus and nitrogen. However, most plant microbiome studies have focused on model plants and crops such as Arabidopsis thaliana, rice, barley, wheat, maize, and soybean, with less information on tree species. Plant microbiomes are shaped by plant-related factors (genotype, organ, species, health status) and environmental factors (land use, climate, nutrient availability).

Plant species and genotypes play significant roles in shaping rhizosphere and plant microbiomes, as tree genotypes and species are associated with specific microbial communities. Different plant organs also host specific microbial communities depending on plant-associated and environmental conditions. Mangrove roots harbor microbial taxa that contribute to ecological functions. Like typical terrestrial plants, mangroves depend on mutually beneficial interactions with microbes.

Microbes in developed roots help transform nutrients into usable forms and provide phytohormones that suppress phytopathogens or help mangroves withstand heat and salinity. In return, root-associated microbes receive carbon metabolites from the plant via root exudates, establishing close mutualistic associations.

Did You Know?

• The etymology of the English term "mangrove" is speculative and disputed, possibly deriving from Portuguese mangue, Spanish mangle, or Cariban and Arawakan languages such as Taíno.
• Mangrove environments in the Eastern Hemisphere harbor six times as many tree and shrub species as those in the New World.
• The first mangroves were marine taxa adapted to coastal, brackish environments, documented as early as the Pennsylvanian period.