How Do Mangroves Survive in Salt Water?

If we look at the east coast of Sumatra, there is a stretch of forest that borders directly with the sea. Land and sea border areas like this, generally cannot be inhabited by plants. Then how can this one plant live in an area that is flooded with seawater with a high salt content?

These plants are mangroves. Mangroves are plants that live in intertidal areas in tropical and subtropical regions. Intertidal areas are areas that are affected by the tides and ebbs of seawater, generally the salt content in the soil is high. This makes other plants unable to live well. Moreover, the soil cavity or substrate in the border area is filled with water instead of oxygen, so the oxygen content is low. However, in areas with these limitations, mangroves actually stand up vigorously. Mangroves make several adaptations and adjustments so that they can grow and develop properly. Some store the excess salt in their organs, some modify the shape of the roots so that they have air ducts to store more oxygen, some have roots that pop out to get additional oxygen. Each type develops its own adaptations.

The high salt content is one of the factors that makes not just any plant can live in the land and sea border areas. Mangroves have several ways to overcome this high salt content. Some species such as Rhizophora and Bruguiera carry out ultrafiltration in their roots. Through this mechanism, salt is filtered first in the cell membrane of the root cortex, before water enters their tissues. This method reduces almost 90% of salt. Some other species, such as Avicennia and Acanthus, secrete through multicellular glands found in leaves. Lumnitzera and Excoecaria have very large vacuoles in the leaves. These leaves are then dropped after they are thick and juicy. Meanwhile, the excess salt on the stems is stored in the bark of the stem.

If agricultural land usually requires loosening with the aim of increasing the oxygen content in the soil, then the substrate in the intertidal area does not have high oxygen. This is because the space between the substrates, which should be filled with oxygen, is instead filled with seawater. To overcome this, many types of mangroves make root modifications. A number of species have breathing roots and other modifications according to their type and position in the tidal zone. The roots of mangroves also have thick aerenchymic tissue that reaches 70% of the root volume as an air duct. Species such as Avicennia also have lentiles on the roots as an additional entrance for oxygen. As for saving water, mangroves also have their own way. Some species, such as Sonneratia, have hidden stomata in an effort to reduce water evaporation. Generally, mangrove leaves have a thick waxy layer that reduces water loss from the leaf surface.

This ability to survive is what allows mangroves to occupy areas that cannot be occupied by other plants. This is also proof that mangroves are extraordinary plants. It is even more extraordinary that Indonesia is one of the countries with the largest mangrove area in the world. There are many benefits that coastal communities get from the existence of mangroves. Not only as a harsh dam of sea waves, but also as a place for the development of several species of fish and storing carbon reserves that affect the climate on Earth. Research related to the physiology of mangroves has not been widely done in Indonesia. Various mangrove adaptation mechanisms still need further research, for example related to aeration mechanisms and the physiological role of lenticells.

This plant is able to adapt to various conditions and natural limitations. However, this plant certainly does not have a way to adapt to human pressure. So the next time we see a mangrove forest, realize that the ecosystem is not without value. The mangrove land is not empty land, but an ecosystem that provides many benefits.

Resources and further reading

For the mechanism of mangroves to cope with salt, save water, and their leaf structure:

Naskar, S. & Palit, P.K. (2015). Anatomical and physiological adaptations of mangroves. Wetlands Ecology and Management, 23, 357–370.

For mangrove root forms, aerenchym, lenticell, and how mangroves breathe in soil without oxygen:

Srikanth, S., Lum, S.K.Y. & Chen, Z. (2016). Mangrove root: adaptations and ecological importance. Trees, 30, 451–465.

For the anatomy of leaves and salt glands in different types of tropical mangroves:

Vinoth, R., Kumaravel, S. & Ranganathan, R. (2019). Anatomical and physiological adaptation of mangrove wetlands in east coast of Tamil Nadu. World Scientific News, 129, 161–179.

For an overview of mangrove ecosystems, their distribution in the world, and the threats they face:

Friess, D.A. (2016). Mangrove forests. Current Biology, 26(16), R746–R748