Tropical seagrasses manipulate sediment biogeochemistry to thrive in low nutrient environments – Biologisk Institut - Københavns Universitet

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04. december 2017

Tropical seagrasses manipulate sediment biogeochemistry to thrive in low nutrient environments

Marine biology

Seagrasses are marine flowering plants that thrive in shallow coastal environments, where they provide multiple important ecosystem services such as enhancing marine biodiversity, increasing water quality, providing shelter and food to numerous juvenile marine fish and crustaceans, and by mitigating climate change due to their extraordinarily high carbon burial capacity. However, seagrass meadows are currently disappearing globally at alarming rates mainly as a result of human activity, including coastal development, anchoring of boats and harbour dredging.

In tropical low-nutrient environments, seagrasses form densely-vegetated, multi-species meadows despite of strong phosphorus and iron binding in the sandy, carbonate-rich sediments surrounding their roots, and the mechanisms enabling the success of tropical seagrasses have remained unclear. In a new study published in the leading environmental science journal Environmental Science & Technology, a team of Danish, Austrian and Australian researchers used a novel combination of high-resolution, two-dimensional chemical imaging techniques to demonstrated that tropical seagrasses can manipulate the chemical microenvironment and biogeochemical processes in their rhizosphere leading to a release of soluble phosphate and iron from the sediment matrix.

“Seagrasses are anchored in the seabed, despite high concentrations of phytotoxic hydrogen sulfide in the sediment surrounding them. The plants are able to prevent hydrogen sulfide from entering the tissue by sealing off mature parts of their root structure. They also release oxygen from the root-tips and root/shoot junctions, which chemically re-oxidize hydrogen sulfide to e.g. non-toxic sulfuric acid or stimulates sulfide-oxidizing bacteria in the surrounding sediment. Organic compounds are also released into the root zone which can affect pH and microbial activity in sediment,” explains lead author Kasper E. Brodersen, who is a postdoc in the Marine Biology Section at University of Copenhagen’s Department of Biology.

Photo: Multi-species seagrass meadow around Green Island, Queensland, Australia. Credit: Maria Mosshammer

A new method reveals the chemical context
For the first time, and with the help of new imaging techniques, the research group was able to show the chemical dynamics of oxygen and pH, as well as phosphorus and other chemicals in tropical seagrass root zones. Measurements demonstrated that seagrass root zones exhibited a wide variety of chemical microenvironments affected by the release of oxygen and organic compounds.

“Nutrient concentrations are generally much higher in the sediment as compared to the water, but can be largely inaccessible to plants due to strong binding to the sediment matrix. Our study showed that tropical seagrasses can solubilize such sediment-bound nutrients by releasing oxygen and dissolved organic carbon from their root-tips and root/shoot junctions. This led to local acidification releasing phosphate from the calcium carbonate sediment matrix, as well as stimulation of sulphide-producing sulphate-reducing bacteria, which also results in further phosphate release and solubilisation of iron oxides into reduced iron, which can be taken up by the seagrass. This is important knowledge when seeking to understand why certain plants thrive in nutrient-poor areas. Our novel method thus provides us with insight into how seagrasses manipulate the chemical microenvironment and influence the availability of nutrients around their roots,” continues Kasper E. Brodersen. 

The plants exploit nutrients in various ways
The research group thus found two different mechanisms by which the availability of essential iron and phosphate nutrients could be increased.

1) The release of oxygen from roots in certain microenvironments stimulates the oxidation of hydrogen sulfide, which causes an acidification of the sediment environment. As a result, phosphorous is mobilized from the calcium carbonate sediment and becomes accessible to seagrass.

2) In other microenvironments around seagrass roots, the plants release organic compounds that stimulate sulfate respiring bacteria in the sediment. This increases the formation of hydrogen sulfide, which reacts with iron hydroxides and calcium carbonate in the sediment. Finally, both phosphate and reduced iron are released, which can then be absorbed by plants. By actively manipulating chemical conditions in their root zones, tropical seagrasses ensure access to essential nutrients, which explains their wide distribution in nutrient-poor tropical coastal waters.   

The results of this project and new combination of methods serves as a foundation for similar studies that look into how marine plants interact with sediments and microbes. Seagrasses and other marine plants are important components of coastal ecosystems and are threatened by environmental changes including eutrophication, coastal development and climate change. A better understanding of how marine plants interact with microorganisms and chemical microenvironments is essential for understanding how stress factors impact plants and their ability to adapt.