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Rhizome, Root/Sediment Interactions, Aerenchyma and Internal Pressure Changes in Seagrasses

  • Kasper Elgetti Brodersen
  • Michael Kühl
  • Daniel A. Nielsen
  • Ole Pedersen
  • Anthony W. D. Larkum
Chapter

Abstract

Life in seawater presents several challenges for seagrasses owing to low O2 and CO2 solubility and slow gas diffusion rates. Seagrasses have evolved numerous adaptations to these environmental conditions including porous tissue providing low-resistance internal gas channels (aerenchyma) and carbon concentration mechanisms involving the enzyme carbonic anhydrase. Moreover, seagrasses grow in reduced, anoxic sediments, and aerobic metabolism in roots and rhizomes therefore has to be sustained via rapid O2 transport through the aerenchyma. Tissue aeration is driven by internal concentration gradients between leaves and belowground tissues, where the leaves are the source of O2 and the rhizomes and roots function as O2 sinks. Inadequate internal aeration e.g., due to low O2 availability in the surrounding water during night time, can lead to sulphide intrusion into roots and rhizomes, which has been linked to enhanced seagrass mortality. Under favourable conditions, however, seagrasses leak O2 and dissolved organic carbon into the rhizosphere, where it maintains oxic microzones protecting the plant against reduced phytotoxic compounds and generates dynamic chemical microgradients that modulate the rhizosphere microenvironment. Local radial O2 loss from belowground tissues of seagrasses leads to sulphide oxidation in the rhizosphere, which generates protons and results in local acidification. Such low-pH microniches can lead to dissolution of carbonates and protolytic phosphorus solubilisation in carbonate-rich sediments. The seagrass rhizosphere is also characterised by numerous high-pH microniches indicative of local stimulation of proton consuming microbial processes such as sulphate reduction via root/rhizome exudates and/or release of alkaline substances. High sediment pH shifts the sulphide speciation away from H2S towards non-tissue-penetrating HS ions, which can alleviate the belowground tissue exposure to phytotoxic H2S. High sulphide production can also lead to iron and phosphorus mobilization through sulphide-induced reduction of insoluble Fe(III)oxyhydroxides to dissolved Fe(II) with concomitant phosphorus release to the porewater. Adequate internal tissue aeration is thus of vital importance for seagrasses as it ensures aerobic metabolism in distal parts of the roots and provides protection against intrusion of phytotoxins from the surrounding sediment.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Kasper Elgetti Brodersen
    • 1
  • Michael Kühl
    • 1
    • 2
  • Daniel A. Nielsen
    • 1
  • Ole Pedersen
    • 3
    • 4
  • Anthony W. D. Larkum
    • 1
  1. 1.Climate Change ClusterUniversity of Technology SydneyUltimo, SydneyAustralia
  2. 2.Kasper Elgetti Brodersen, Marine Biological Section, Department of BiologyUniversity of CopenhagenHelsingørDenmark
  3. 3.Freshwater Biological Laboratory, Department of BiologyUniversity of CopenhagenCopenhagenDenmark
  4. 4.School of Plant BiologyThe University of Western AustraliaCrawleyAustralia

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