Oxygen dynamics in the rhizosphere of Cymodocea rotundata

Abstract

The spatial distribution of oxygen and the dynamics of the oxic mlcrozone around roots of Cymodocea rotundata were studied using oxygen microelectrodes under constant light conditions and during light-dark transitions. Under daylight steady state conditions, oxygen was present at concentrations up to 75 % of air saturation at the root surface, and the oxic microzone around the roots was 80 pm thick. Steady state oxygen concentrations were reached within 1.5 h after light-dark shifts. Under darkness, free oxygen, at about 20% of air saturation, was still present on the root surface at steady state, but the thickness of the oxic microzone shrank to 50 pm. The oxygen present in the rhizosphere during darkness was supplied from the water column to roots via, primarily, gas-phase diffusion in leaves and rhizomes. The oxic microzone around roots comprised about 0.5% of the total volume of the seagrass rhlzosphere, and the root-mediated oxygen supply was estimated to be insignificant for the whole sediment oxygen budget contributing about 1 % of total oxygen consumption only. However, the continuous oxygen supply may ensure a persistent oxic environment for belowground tissues of C. rotundata and, hence, protect the plant from reduced phytotoxins.

The role of organic acid exudates in liberating phosphorus from seagrass‐vegetated carbonate sediments

“…We used the large decrease in sulfides found in the root zone to approximate the O 2 release from roots based on stoichiometric ratios, individual root tips m 22 and sulfide oxidation area 21 d 21 . This calculation yielded a daily flux of O 2 from the roots of 0.96 mmol Pedersen et al 1998;Frederiksen and Glud 2006) and the tight coupling between sulfate reduction and sulfide oxidation, these processes would result in the net production of acidity (Burdige and Zimmerman 2002;Hu and Burdige 2007). The resulting acid produced by sulfide oxidation and sulfate reduction is likely influenced by the release of OA, because OA can provide the organic matter necessary for sulfate reduction.…”

“…Recently, Burdige et al (2008) modeled carbonate dissolution rates in seagrass sediments based on oxygen release from seagrass roots and showed that rates of dissolution were positively correlated with seagrass density. Seagrasses are known release O 2 from root tips (Pedersen et al 1998;Frederiksen and Glud 2006) and the root tips are expected to be the dominant region of OA exudation, which may cause pockets of high rates of dissolution and reduced P i adsorption to occur around root tips. It is likely that most exudate activity occurs in this microenvironment, because the processes that consume O 2 and OA in the sediments are rapid.…”

Impact of sea grass density on carbonate dissolution in Bahamian sediments

“…In temperate clastic sediments Zostera marina (eel grass) is known to transport O 2 through its roots and rhizomes to the surrounding sediments, markedly affecting redox conditions in the rhizosphere (Sand-Jensen et al 1982;Smith et al 1984;Jaynes and Carpenter 1986;Pedersen et al 1998). Sea grasses such as Thalassia testudinum are characterized by a significant amount of below-ground biomass and may transport photosynthetically derived O 2 into tropical carbonate sediments, thereby providing the missing O 2 needed to balance the carbonate dissolution budget without enhancing the removal of carbonate dissolution end products.…”

Toxic effects of increased sediment nutrient and organic matter loading on the seagrass Zostera noltii

“…At low sulfide levels, seagrass is usually able to cope with sulfide by oxidizing it in the root zone (Holmer and Bondgaard, 2001;Marba et al, 2006;Pedersen et al, 1998).…”