The role of dissolved organic and inorganic nitrogen for growth of macrophytes in coastal waters of the Baltic Sea

Highlights

• Uptake of dissolved organic nitrogen by rooted macrophytes

• Dissolved organic nitrogen is preferred over nitrate but not over ammonium.

• Roots and shoots are equally responsible for nutrient supply.

• Charophytes are able to transport dissolved inorganic nitrogen.

Abstract

Macrophytes and phytoplankton compete for inorganic nitrogen during growth, even in eutrophied coastal waters containing relatively high nitrogen concentrations. In this study we investigated, whether dissolved organic nitrogen (DON) serves as an additional nitrogen source for rooted submerged macrophytes in several key species in the nutrient-rich inner coastal waters of the Darss-Zingster-Bodden chain, located in the southern Baltic Sea. The uptake and translocation of ¹⁵N-labeled dissolved inorganic nitrogen (DIN, ammonium and nitrate) and DON (amino-acid mixture) were measured for three common species: Chara aspera, Chara tomentosa, and Stuckenia pectinata. A two-compartment-device was used to discriminate between the roles of roots and shoots in N uptake. The results showed that DON and DIN were taken up by all species, but ammonium (mean 0.116%¹⁵N mg DW^− 1 h^− 1) was preferred over amino acids (mean 0.024%¹⁵N mg DW^− 1 h^− 1) which were preferred over nitrate (mean 0.007%l¹⁵N mg DW^− 1 h^− 1). To our knowledge, this is the first study to demonstrate the uptake of DON in charophytes and the submerged angiosperm S. pectinata. Both nitrate and ammonium, as DIN, were translocated in the basipetal and acropetal directions in Characeae, which was unexpected given the lack of vascular bundles in these species. By contrast nutrient transport was below the detection limit in the vascular macrophyte S. pectinata. The translocation of DON was not observed in any species or in any direction. Our findings suggest that rooted plants have an advantage over phytoplankton based on their ability to assimilate and transport nutrients not only from the water column but also from the sediments, whereas phytoplankton can only use nutrients of the water column.

Introduction

The nutritional pollution through human activity and agriculture in catchment areas of coastal zones and estuaries have led to the increasing eutrophication of coastal waters and thus to a shift in plant communities, from macrophytes to phytoplankton dominated systems (Gocke et al., 2003, Kovtun et al., 2009, Munkes, 2005, Schumann et al., 2006). Macrophytes can therefore be used as an indicator species in assessments of the good environmental status of a water body. However, to do so requires a detailed understanding of the mechanisms underlying nutrient uptake and the growth of these key species. Abiotic factors such as light limitation and sedimentation were shown to indirectly influence growth (Angelstein et al., 2009, Kovtun-Kante et al., 2014, Schaible and Schubert, 2008). Another important aspect is the competition for nutrients between macrophytes and phytoplankton. A number of studies have examined the role of phosphorus as a limiting factor (Angelstein and Schubert, 2008, Reid et al., 2000, Rip et al., 2007). Although nitrogen limitation is less well explored, it has been documented in freshwater and marine environments (Bianchi and Engelhaupt, 2000, Elser et al., 2007, Guildford and Hecky, 2000).

In examining the mechanisms of nitrogen limitation, both the sources (sediment vs. water column) of the different nitrogen species and the ability of primary producers to assimilate them must be considered. The two forms of nitrogen, dissolved organic nitrogen (DON) and inorganic nitrogen (DIN), differ in their availability (Stepanauskas et al., 2000). DON accounts for anywhere between 20 and 90% of the total nitrogen pool (Petrone et al., 2009, Seitzinger and Sanders, 1997). However, its concentration was previously thought to be small and was usually not included in studies of the nitrogen uptake by phototroph organism. In addition, 20–30 years ago DON was considered to be largely refractory and was thus ignored as a nutrient source. This erroneous conclusion was based on the complex composition of DON, which includes the poorly decomposable humic and fulvic fractions. However, once DON was identified as a nutrient source the conversion of DON into biomass by phytoplankton and microorganisms was demonstrated on short time by several groups (Andersson et al., 2006, Berg et al., 1997, Berman and Chava, 1999, Bronk et al., 1994, Fiedler et al., 2015). The uptake of DON by macrophytes is of particular interest only since then Tyler et al. (2005) showed that the nitrogen requirement of non-rooted red and green algae can be satisfied to a significant extent by DON. Subsequently, the uptake of DON was also demonstrated in seagrasses (La Nafie et al., 2014, Van Engeland et al., 2011, Vonk et al., 2008) and seaweeds (Phillips and Hurd, 2004), but whether it also occurs in other rooted macrophytes is unknown. Unlike phytoplankton, which derives their nutrients only from the water column, rooted submerged macrophytes are also able to use nutrients from the sediments. Thus, studies of the uptake of nutrients by macrophytes must consider both the roots and shoots. Nutrient uptake by the roots of submerged aquatic plants and the mechanism of nutrient transport has been often discussed, but are still subjects of debate in the literature (Agami and Waisel, 1986a, Takayanagi et al., 2012, Wilson et al., 1988).

In comparative terms, the roots of submerged vascular macrophytes often comprise ≤ 10% of the total algal biomass (Brenkert and Amundsen, 1982). This low biomass of roots compared to shoots suggests that main function is to anchor plants in the sediments, with nutrient acquisition playing only a minor role (Sutcliffe, 1959). Many studies have provided support for this hypothesis, by showing that the nitrogen requirement of macrophytes can be fulfilled solely by uptake via the shoots (Madsen and Cedergreen, 2002). By contrast others have shown that both shoots and roots substantially contribute to the nutrient supply (Carignan and Kalff, 1980, Nichols and Keeney, 1976), albeit in different, species-specific proportions. Due to the lack of transpiration in submerged plants, nutrient transport must rely on alternative mechanisms (Raven, 2003) as e.g. cytoplasmatic streaming. Most submerged macrophytes have vascular bundles, which in rooted macrophytes allow the transport of phosphorus e.g. in both directions, downwards (basipetal) and upwards (acropetal) (Angelstein and Schubert, 2008, Littlefield and Forsberg, 1965). In plants such as Characeae, which lack vascular bundles, nitrogen uptake mechanism via cell wall and intracellular translocation remains to be explained.

Previous studies on uptake of nutrients have either disregarded or at least tried to remove the biofilm before the experiments. To our knowledge, there is no possibility to obtain the macrophytes axenic. In this study, under natural conditions occurring biofilm (belong to the bacteria, diatoms and attached algae) gathered at least quantitatively. Which proportions the bacterial biofilm is involved in the uptake of nitrogen components, was not an aim of this experiment. Macrophytes and its bacterial biofilm were considered together.

We hypothesized that: (1) DON provides an alternative to DIN as a nitrogen source that allows the successful growth of macrophytes, as suggested in other studies (Mozdzer et al., 2010). (2) The uptake of either nitrogen source is achieved via roots and also shoots in same ratios. Thus, the aims of this study were (1) to demonstrate the uptake of DON vs. DIN (nitrate and ammonium) by rooted submerged macrophytes. (2) to determine whether both, shoots and roots, are responsible for nutrient uptake and (3) whether transport occurs from roots to shoots and vice versa. We used ¹⁵N-labeled ammonium, nitrate, and an amino-acid mixture (as DON) and examined the uptake and translocation of these nitrogen sources in three common macrophytes found in inner coastal, heavily eutrophic waters. In addition, the microbial biofilm in nutrient uptake was considered.