Seagrass-Mediated Phosphorus and Iron Solubilization in Tropical Sediments

Abstract

Tropical seagrasses are nutrient-limited owing to the strong phosphorus fixation capacity of carbonate-rich sediments, yet they form densely vegetated, multispecies meadows in oligotrophic tropical waters. Using a novel combination of high-resolution, two-dimensional chemical imaging of O2, pH, iron, sulfide, calcium, and phosphorus, we found that tropical seagrasses are able to mobilize the essential nutrients iron and phosphorus in their rhizosphere via multiple biogeochemical pathways. We show that tropical seagrasses mobilize phosphorus and iron within their rhizosphere via plant-induced local acidification, leading to dissolution of carbonates and release of phosphate, and via local stimulation of microbial sulfide production, causing reduction of insoluble Fe(III) oxyhydroxides to dissolved Fe(II) with concomitant phosphate release into the rhizosphere porewater. These nutrient mobilization mechanisms have a direct link to seagrass-derived radial O2 loss and secretion of dissolved organic carbon from the below-ground tissue into the rhizosphere. Our demonstration of seagrass-derived rhizospheric phosphorus and iron mobilization explains why seagrasses are widely distributed in oligotrophic tropical waters.

Seagrass rhizosphere microenvironment alters plant‐associated microbial community composition

“…1). This is in accordance with a recent study (Brodersen et al, 2017) that demonstrated Fe 31 reduction, and thus Fe 21 Fig. 1.…”

“…C and D. Microsensor measurements in a presterilized environment, that is, sterilized artificial sediment matrix and below-ground tissue surface. mobilisation, in the seagrass rhizosphere predominantly during night-time (Brodersen et al, 2017). Moreover, at the seagrass-driven rhizospheric oxic/anoxic interface, we observed a rapid decrease in rhizosphere pH by 2 pH units ( Fig.…”

“…Nielsen and colleagues (), for example, showed higher rates of N 2 ‐fixation and sulphate reduction around and on below‐ground tissues of seagrasses as compared to rates in the sediment, and this study also showed that root/rhizome‐associated SRB fixed more dinitrogen than needed for their own growth. In addition, the occurrence of SRB within the rhizosphere may lead to increased phosphorus solubilisation owing to reduction of insoluble Fe 3+ oxyhydroxides that can promote the release of previously adsorbed phosphate to the pore‐water (Pagès et al ., ; Brodersen et al ., ).…”

“…Some of the limitations of using an artificial sediment matrix mainly relate to it being a simplification of complex natural sediments, where other elements and especially solid-phase species (such as Fe 31 oxyhydroxides) also play important roles for reactions affecting distributions of O 2 , sulphide and pH in the seagrass rhizosphere (Brodersen et al, 2017). Insoluble Fe 31 oxyhydroxides are, for example, likely to accumulate in the seagrass rhizosphere during day-time as a result of chemical and biological oxidation of dissolved Fe 21 and precipitated FeS (Brodersen et al, 2017). At night time, in contrast, precipitated Fe 31 oxyhydroxides are reduced via sulphide, which regenerates dissolved Fe 21 leading to consumption of sulphide through precipitation of FeS (Brodersen et al, 2017).…”

Strong leaf surface basification and CO2limitation of seagrass induced by epiphytic biofilm microenvironments

“…The internal tissue aeration of seagrasses is affected by numerous O 2 sources and sinks (Borum, Sand-Jensen, Binzer, Pedersen, & Greve, 2006) that determine how much O 2 is transported along concentration gradients in the aerenchymal tissue down to the below-ground tissues (Pedersen, Binzer, & Borum, 2004;Borum et al, 2005; Brodersen, Kuhl, Nielsen, Pedersen & Larkum, 2018a), where it supports aerobic metabolism (Colmer, 2003), reoxidates intruded sulphide (Holmer & Hasler-Sheetal, 2014), and excess O 2 is released into the rhizosphere (Pedersen, Borum, Duarte, & Fortes, 1998). The O 2 release sustains oxic microzones protecting seagrasses from phytotoxic sulphide intrusion (Brodersen, Nielsen, Ralph, & Kühl, 2015;Koren, Brodersen, Jakobsen, & Kühl, 2015;Brodersen, Koren, Lichtenberg, & Kühl, 2016) and enables vital nutrient mobilization processes (Brodersen,Koren, Moßhammer, Ralph, Kühl, & Santner, 2017, Brodersen et al, 2018b. Adequate internal plant aeration is thus vital for the health and fitness of seagrasses.…”