Sugars dominate the seagrass rhizosphere

Abstract

Seagrasses are among the most efficient sinks of carbon dioxide on Earth. While carbon sequestration in terrestrial plants is linked to the microorganisms living in their soils, the interactions of seagrasses with their rhizospheres are poorly understood. Here, we show that the seagrass, Posidonia oceanica excretes sugars, mainly sucrose, into its rhizosphere. These sugars accumulate to µM concentrations—nearly 80 times higher than previously observed in marine environments. This finding is unexpected as sugars are readily consumed by microorganisms. Our experiments indicated that under low oxygen conditions, phenolic compounds from P. oceanica inhibited microbial consumption of sucrose. Analyses of the rhizosphere community revealed that many microbes had the genes for degrading sucrose but these were only expressed by a few taxa that also expressed genes for degrading phenolics. Given that we observed high sucrose concentrations underneath three other species of marine plants, we predict that the presence of plant-produced phenolics under low oxygen conditions allows the accumulation of labile molecules across aquatic rhizospheres.

The Diversity and Functional Capacity of Microbes Associated with Coastal Macrophytes

“…Indeed, we found an abundance of genes for dissolved organic matter assimilation and transport in all metagenomes, suggesting that heterotrophy may be common in macrophyte-associated microbial communities ( 43 ). Improved characterization of the components of dissolved organic matter and the genomes of hosts will allow us to better assess the complementarity in resource supply by hosts and resource use by microbes, with critical implications for the carbon cycle ( 58 ).…”

Changes in the Rhizosphere Prokaryotic Community Structure of Halodule wrightii Monospecific Stands Associated to Submarine Groundwater Discharges in a Karstic Costal Area

“…Glycine, serine and threonine metabolism, described as processes helping to maintain the redox balance and energy levels in plants, arginine and proline metabolism (highly beneficial for plants exposed to stress conditions), and metabolism of terpenoids and polyketids, also involved in plant metabolism, were the amino acid metabolism related profiles [ 46 , 70 ]. Moreover, starch and sucrose metabolism, recently acknowledged as the major sugar excreted by Posidonia oceanica , was also more expressed for these sites [ 6 ]. Among other, sites S5 and S6 were differentially abundant for genetic information processes, vital for proliferation and growth of microorganisms, such as, Translation, Replication and Repair, Folding, sorting and degradation and replication and repair, and Energy metabolism, which included Carbon fixation pathways in prokaryotes and methane metabolism, usually occurring in nutrient-poor environments [ 71 ].…”

“…As the only marine angiosperms, seagrasses root system oxygenates the surrounding thin zone of sediment, known as rhizosphere [ 3 ]. In this zone, oxygen and dissolved organic carbon, such as sucrose, recently found as the major sugar excreted by seagrasses, leak from roots creating a dynamic environment for a diverse group of microorganisms critical for plant nutrient acquisition, host defense to pathogens and biogeochemical cycling [ 3 , 4 , 5 , 6 , 7 ]. Dominant bacterial members in belowground compartments usually belong to the classes Alpha- , Gamma- , Delta- , Epsilonproteobacteria and Bacteroidetes [ 3 ], and the three most important functional groups for seagrass ecology are considered aerobic heterotrophs, sulfate-reducing and nitrogen fixing bacteria [ 5 ].…”

Not all pathways are the same – unique adaptations to submerged environments emerge from comparative seagrass genomics

“…Other forms of complex seagrass-bacteria mutualistic relationships have recently been discovered (Tarquinio et al ., 2019), including the deposition of sugars unavailable to bacteria due to phenolic compounds (Sogin et al ., 2021) or methane production by archaea living in seagrass meadows (Schorn et al ., 2022). Future studies will focus on the interaction between the loss of ethylene genes in seagrasses and the role of host microbiomes in providing critical processes for survival (Zilber-Rosenberg and Rosenberg, 2008).…”