No abstract

Root functional diversity of submerged vegetation exerts a major effect on nitrogen (N) cycling in lake sediments. This fact, however, is neglected in current N-balance models because the links between the engineering role of plants and in situ microbial N cycling are poorly understood. We hypothesized that macrophyte species with high root oxygen loss (ROL) capacity promote the highest denitrification because of a higher abundance of ammonia oxidizers and tighter coupling between nitrifiers and denitrifier communities. We sampled five small ultraoligotrophic shallow lakes with abundant macrophyte cover including sediments dominated either by Isoetes spp. (high ROL), mixed communities of natopotamids (low ROL), and unvegetated sandy sediments. At each site, we quantified denitrification (DNT) rates and proxies for the abundance of denitrifiers (nirS and nirK genes), and both ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) and the diversity of nirS-harboring bacteria. Vegetated sediments showed significantly higher abundances of N-cycling genes than bare sediments. Plant communities dominated by Isoetes generated sediments with higher redox and NO 2 3 concentrations and significantly higher DNT rates than natopotamidsdominated landscapes. Accordingly, increasing DNT rates were observed along the gradient from low ROL plants-bare sediments-high ROL plants. Significantly higher abundance of the archaeal amoA gene was recorded in sediments colonized by high ROL plants unveiling a key biogeochemical role for AOA in coupling macrophyte landscape and ecosystem denitrification.The global nitrogen cycle has been strongly modified by massive industrial fixation of nitrogen gas (N 2 ) for human use and by fossil-fuel combustions (Gruber and Galloway 2008). Current concentrations of bio-available nitrogen (N) forms are higher than ever in the human era (Fowler et al. 2013). Terrestrial and aquatic denitrification (DNT) and anammox processes are of special interest because they represent the only permanent removal pathway whereby bioavailable N is returned to inert N 2 gas (Rockstr€ om et al. 2009). The presence and specific composition of rooted plants is a major factor influencing DNT rates of soils and sediments (RisgaardPetersen and Jensen 1997). Plants can alter physicochemical factors known to control denitrification rates such as pH, oxygen, carbon sources and nitrate concentrations (Griffiths et al. 1997;Gacia et al. 2009). These modifications, in turn, influence the activity, diversity and abundance of rhizosphere nitrifiers and denitrifiers populations although few studies have reported quantitative evidences of these plantmicrobial interactions in lake ecosystems (Kofoed et al. 2012). Commonly, nitrification (NT) produces NO 2 3 under oxic conditions after ammonification while DNT requires suboxic conditions and is highly dependent on both NO 2 3 transport from aerobic to anaerobic zones and changes of the in situ redox conditions (Seitzinger et al. 2006).Submersed aquatic plants (SAV)...

Summary• Underwater photosynthesis by aquatic plants is often limited by low availability of CO 2 , and photorespiration can be high. Some aquatic plants utilize crassulacean acid metabolism (CAM) photosynthesis. The benefits of CAM for increased underwater photosynthesis and suppression of photorespiration were evaluated for Isoetes australis, a submerged plant that inhabits shallow temporary rock pools.• Leaves high or low in malate were evaluated for underwater net photosynthesis and apparent photorespiration at a range of CO 2 and O 2 concentrations.• CAM activity was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and also by changes in the titratable acidity (lmol H + equivalents) of leaves. Leaves high in malate showed not only higher underwater net photosynthesis at low external CO 2 concentrations but also lower apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O 2 concentrations, including values below air equilibrium. At a high O 2 concentration of 2.2-fold the atmospheric equilibrium concentration, net photosynthesis was reduced substantially and, although it remained positive in leaves containing high malate concentrations, it became negative in those low in malate.• CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O 2 and CO 2 concentration ranges in floodwaters, via increased CO 2 fixation and suppression of photorespiration.

A unique type of vernal pool are those formed on granite outcrops, as the substrate prevents percolation so that water accumulates in depressions when precipitation exceeds evaporation. The O2 dynamics of small, shallow vernal pools with dense populations of Isoetes australis were studied in situ, and the potential importance of the achlorophyllous leaf bases to underwater net photosynthesis (PN) and radial O2 loss to sediments is highlighted. O2 microelectrodes were used in situ to monitor pO2 in leaves, shallow sediments, and water in four vernal pools. The role of the achlorophyllous leaf bases in gas exchange was evaluated in laboratory studies of underwater PN, loss of tissue water, radial O2 loss, and light microscopy. Tissue and sediment pO2 showed large diurnal amplitudes and internal O2 was more similar to sediment pO2 than water pO2. In early afternoon, sediment pO2 was often higher than tissue pO2 and although sediment O2 declined substantially during the night, it did not become anoxic. The achlorophyllous leaf bases were 34% of the surface area of the shoots, and enhanced by 2.5-fold rates of underwater PN by the green portions, presumably by increasing the surface area for CO2 entry. In addition, these leaf bases would contribute to loss of O2 to the surrounding sediments. Numerous species of isoetids, seagrasses, and rosette-forming wetland plants have a large proportion of the leaf buried in sediments and this study indicates that the white achlorophyllous leaf bases may act as an important area of entry for CO2, or exit for O2, with the surrounding sediment.