The shift from macrophytic to algal particulate organic matter favours dissimilatory nitrate reduction to ammonium over denitrification in a eutrophic lake

Abstract

1. In eutrophic lakes, the shift from a macrophyte-dominated state to an algaedominated state changes the particulate organic matter (POM) sources, which in turn alters organic matter quality that is released and that sinks to the benthos.However, the influences of this shift on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are unclear in eutrophic lakes.2. Here, we elucidated how various POM sources influenced these nitrate reduction pathways in a eutrophic lake. The lake sediments from August were incubated under algae, macrophyte, and soil treatments in aquarium tanks. Potential denitrification and DNRA rates and related functional gene abundances were measured periodically. Meanwhile, the n-alkanes composition of POM and fluorescence excitation-emission matrices of dissolved organic matter were measured before and during incubations, respectively, to aid in the analysis of the potential mechanisms by which organic matter quality affect nitrate reduction.3. The results indicated that algal detritus was more labile and decomposed faster than other POM, coupled with higher DNRA rates and nrfA gene abundances.Macrophyte treatments resulted in nitrate accumulation and the increase of gene abundances related to denitrification, but no increase in denitrification rates was observed. In soil treatments, nitrate reduction processes were not significantly influenced by the addition of POM. Overall, compared with other sources, algae-derived POM can produce high-quality organic carbon, low dissolved oxygen and nitrate concentrations, which is more conducive to DNRA than denitrification. 4. A loss of macrophytes and increase in algal biomass will induce changes in autochthonous POM quality and potentially drive nitrogen recycling in eutrophic lakes.

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes

“…Lakes, where active chemical reactions occur, incur massive deposits of organic matter from aquatic and surrounding terrestrial organisms, greatly influencing the composition of DOM in the water. − Mineralization of DOM derived from phytoplankton facilitates the rerelease of phosphorus (P), supporting the growth of algal blooms. , In addition, labile algal detritus is more beneficial to dissimilatory nitrate reduction to ammonium (DNRA), while macrophyte detritus promotes nitrate accumulation and an elevated abundance of denitrification genes . In addition to endogenous DOM, terrestrial DOM directly changes the composition of DOM in lakes, which then changes the nutrient status of these water bodies. ,− This may be because microbes show different hydrolytic abilities for DOM of varying molecular weights and sources, which thus directly affect the distribution of bioavailable nutrients. , …”

“…15,16 In addition, labile algal detritus is more beneficial to dissimilatory nitrate reduction to ammonium (DNRA), while macrophyte detritus promotes nitrate accumulation and an elevated abundance of denitrification genes. 17 In addition to endogenous DOM, terrestrial DOM directly changes the composition of DOM in lakes, which then changes the nutrient status of these water bodies. 7,18−20 This may be because microbes show different hydrolytic abilities for DOM of varying molecular weights and sources, which thus directly affect the distribution of bioavailable nutrients.…”

High Inter‐ and Intra‐Lake Variation in Sediment Phosphorus Pools in Shallow Lakes

“…The sources of sedimented organic matter influence sediment P composition and lability (Twilley et al., 1986). For example, if phytoplankton detritus dominates organic matter sedimentation, then sediment organic P will be associated with more labile materials (Jiang et al., 2022). Inter‐lake variation in sediment P composition, due to differences in exogeneous inputs and in situ processing, may also cause the dominant mechanisms of internal P loading to vary among different waterbodies.…”