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Sediment plays a key role in organic matter ( OM) and internal nutrient cycling in lakes. The role of sediment as a source of OM and its potential bottom-up effects on the pelagic food web have rarely been studied. Particularly, the influence of the biochemical composition of sediment OM on pelagic compartments remains largely unknown., During a 5-month experiment, we studied the influence of two different sediments added at the bottom of large replicated mesocosms on the biomass of seston and zooplankton, and their elemental and lipid compositions. The influence of sediment treatments on sedimentation rates, elemental and biochemical compositions and potential biodegradability of recently sedimented OM ( c. 1 week) was also examined., The two added sediments ( S1 and S2) presented contrasting elemental and biochemical compositions and potential biodegradabilities. According to their contents of organic carbon, nitrogen, proteins, sugars and polyunsaturated fatty acids, S2 appeared to be much more biodegradable than S1. Therefore, the S2 sediment was expected to release more nutrients and OM to the water column than S1, leading to changes in communities, stoichiometry and lipid compositions of pelagic compartments., Probably due to its very poor content of labile compounds, the presence of S1 at the bottom of the mesocosms did not induce changes in the biomass of seston and zooplankton. Only few changes in the stoichiometry of these compartments were observed. In contrast, S2 sediment released more phosphorus and dissolved OM into the water column than S1. As a result, the S2 treatment induced an increase in seston biomass and therefore in zooplankton biomass via herbivory., None of the sediment treatments affected the lipid composition of seston and zooplankton. Moreover, neither S1 nor S2 induced changes in the sedimentation rates, elemental and lipid compositions, and potential biodegradability of recent sediments. Our mesocosm experiment suggests that differences in the quality of lake sediments lead to moderate changes in the pelagic communities in the absence of planktivorous or omnivorous fish., Our results could partly explain the efficiency of biomanipulation for improving water quality of eutrophic lakes despite potential nutrient release from sediment. [ABSTRACT FROM AUTHOR]

Highlights: [•] New approach for characterization of soil organic matter (SOM) proposed. [•] SOM analyzed using microwave assisted acid and base hydrolysis. [•] Key structural information about hydrophilic constituents of SOM obtained. [•] Microwave assisted acid hydrolysis revealed unaltered lignin constituents. [Copyright &y& Elsevier]

Sedimentation is a key process in lake functioning, and plays an important role in nutrient and carbon cycles at both regional and global scales. Several biological processes have been shown as quantitatively affecting sedimentation, but very few works have tried to relate the structure of aquatic communities and the quality of sinking organic matter. We tested in a mesocosm study how food-web structure affects quantitatively and qualitatively sedimentation in eutrophic systems. We carried out a long-term experiment (14 months) in large replicated enclosures either dominated by planktivorous fish or fishless. Food-web structure modified the specific composition of zooplankton communities and phytoplankton biomass, as expected by the trophic cascade theory. Planktivorous fish had a strong positive effect on gross sedimentation rate, but the fraction of suspended particulate material that sank only slightly differed between treatments. The density of transparent exopolymer particles (TEP) was a good descriptor of sedimentation rate, highlighting the key role of these polysaccharidic particles in sinking processes. In fish enclosures, sediment elemental ratios were positively related to seston elemental ratios, suggesting the dominance of algal sedimentation. In fishless mesocosms, N/P ratios of sinking material and of zooplankton showed a strong negative relationship, indicative of a major contribution of zooplankton-egested material to sinking particles. Analyses of free lipids in sediments confirmed the distinct origins of sinking material. Despite the absence of clear elemental composition distinctions between the two types of sediment, food-web structure strongly modified sediment biochemical composition. [ABSTRACT FROM AUTHOR]

The effects of food web structure on the quantity and biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms were examined. Food web structure was manipulated by addition of zooplanktivorous fish. Biochemical characterisations were carried out using lipid biomarkers (sterols, fatty acids, chlorophyll-derived compounds and long-chain alkanediols). Fish addition decreased zooplankton biomass and increased seston biomass and deposited sediment through a trophic cascade. Fish presence strongly influenced the biochemical characteristics of seston and sediment. In contrast, food web structure had a minor impact on the lipid biomarker composition of zooplankton. Although the relative abundance of sterols in the different compartments did not differ strongly between treatments, sterol profiles in seston and sediment depended on food web structure. The predominance of Δ-sterols in seston and sediment in the fish treatment indicated a major contribution of Chlorophyceae. In contrast, the distribution of sterols in seston and sediment in the fishless treatment, dominated by cholesterol, indicated a major zooplanktonic input. The distribution of fatty acids and the relative abundance of chlorophyll-derived compounds and long-chain alkanediols agreed with the predominant contribution of phytoplankton or zooplankton to seston and sediment in the two treatments. The relative abundance of bacterial biomarkers suggested that the contribution of bacteria was rather low. The high relative abundance of polyunsaturated fatty acids (PUFAs) and the absence of stanols in sediments suggested low microbial reworking of organic matter in the recently accumulated sediments. The trophic cascade, generated by the addition of fish, increased the relative abundance of PUFAs in deposited organic matter, thus enhancing sediment quality and potential degradability. [ABSTRACT FROM AUTHOR]

Abstract: A comparison was made between the composition of the recalcitrant organic matter (ROM) isolated from a sandy forest soil, as revealed with microwave assisted extractions and/or hydrolysis, and using common pyrolysis techniques. Successive microwave irradiation treatments were performed in H2O, 0.1 and 1M HCl and 0.1 and 1M KOH. At each step the insoluble residue was examined via Curie point pyrolysis (CuPy) and Curie point thermally assisted hydrolysis and methylation (CuTHM). Sequential irradiation treatment resulted in ca. 35% degradation of the ROM. Compounds released on microwave irradiation in H2O and in HCl were dominated by glucose, suggesting the occurrence of carbohydrate-containing molecular associations in the soil organic matter (SOM) which were not disrupted during acid hydrolysis and extraction as applied for the isolation of the ROM. The product distribution from the microwave irradiation in KOH showed an important contribution to the ROM from the higher plant polyesters cutin and suberin, and to a lesser extent from lignin. Different lignin-derived compounds were specifically released upon microwave acid or base hydrolysis. This suggested that two types of lignin monomers, ether- or ester-linked, occurred in the ROM. The changes in the composition of the CuPy pyrolysates of the residues from the different microwave hydrolyses are consistent with the near complete removal of carbohydrates by microwave HCl hydrolysis. The changes in the composition of the CuTHM pyrolysates of the residues from the different microwave acid and base hydrolyses are in agreement with a major release of cutin- and suberin-derived compounds upon microwave KOH hydrolysis. The CuPy and CuTHM pyrolysates of the final residue consist predominantly of lignin-derived compounds. The study emphasizes the potential of microwave assisted hydrolysis to give a better estimate of the actual contribution of cutin to ROM than pyrolysis. However, the technique appears to be unable to completely release the lignin-based constituent of the ROM. Microwave irradiation appears to provide great potential as a tool for extraction and chemical characterisation of complex OM and could be an attractive additional technique to pyrolysis. [Copyright &y& Elsevier]

Abstract: The composition of humic acids (HAs) isolated from an agricultural soil and a lignite deposit was examined via H2O2 and RuO4 oxidation. The oxidation digests were separated into lipophilic and hydrophilic components. Information with regard to the source, degree of humification and preservation of easily degradable constituents of the HAs was obtained and results were compared with those obtained earlier for base hydrolysates of solvent-extracted fractions. H2O2 oxidation of both HAs afforded lipophilic fractions containing high molecular weight compounds. The composition of the base hydrolysates of the lipophilic fractions strongly differed with the origin of the HA. The lipophilic components of the soil HA derived mainly from the higher plant polyesters cutin and suberin. The lipophilic components of the lignite HA predominantly comprised long chain alkanoic acids and alkanols. The patterns for the hydrophilic components released upon H2O2 oxidation were found to be identical irrespective of the origin of the HA. The hydrophilic fractions comprised aliphatic (poly)carboxylic acids related to carbohydrate moieties and benzene polycarboxylic acids. The relative abundance of benzene polycarboxylic acids increased with the degree of humification. For both HAs, RuO4 oxidation resulted in a lipophilic fraction containing low molecular weight products identical to those found in the base hydrolysate of the lipophilic fraction released upon H2O2 oxidation. The hydrophilic components released upon RuO4 oxidation were independent of the HA origin and consisted mainly of monosaccharides and disubstituted aromatic compounds. In agreement with the greater aromaticity of lignite HA, the aromatic compound/carbohydrate ratio was higher for lignite HA than soil HA. The results show that the fused aromatic structures had a small size and that carbohydrates could escape degradation during the humification process. [Copyright &y& Elsevier]

Abstract: Base- and acid-hydrolysable fractions of humic acids (HAs) isolated from a forest soil, an agricultural soil and a lignite deposit were analysed, and comparisons were made between the base hydrolysable lipid (bound lipid), carbohydrate and amino acid signatures of the different humic acids. Bound lipids differ depending on the humic acid origin. Their composition were rather similar for the two soil humic acids, with three main lipid classes identified: (i) aliphatic components, (ii) aromatic components and (iii) sterols and triterpenols. The aliphatic subfraction was the most abundant and consisted predominantly of cutin- and suberin-derived moieties some of which could be clearly related to the vegetation. A minor bacterial input was indicated by the presence of short chain α- and β-hydroxyalkanoic acids. Aromatic subfraction contributed to a low amount to the total base hydrolysates and consisted mainly of lignin-derived methoxyphenols. Present in trace amounts, sterols and triterpenols are mainly of higher plant origin. The base hydrolysate from lignite humic acid markedly differs. Bound lipids released from lignite HA comprised almost exclusively aliphatic components, largely dominated by long chain alkanoic acids. Lignin-derived moieties, hardly detected, consisted solely of vanillic and 4-hydroxybenzoic acids indicating a much higher degree of lignin alteration in lignite humic acid. Sterols and triterpenols were absent. Although the composition of monosaccharides released upon acid hydrolysis was rather uniform irrespective of the humic acid origin, the distribution changed with the degree of humification of the HAs. Ratios of (Galactose+Mannose) to (Xylose+Arabinose) increased from soil to lignite humic acids. The high values of the ratios indicate that carbohydrates are primarily of microbial origin. In all humic acids neutral and acidic protein amino acids dominated. Non protein amino acids were only minor components consisting mainly of hydroxy proline and ornithine. The amino acid distributions of both soil HAs were similar. The amino acid distribution of lignite HA resembled that of soil HAs except for the following differences: (1) the absence of hydroxy proline and the greater abundance of ornithine suggesting a higher microbial contribution to the amino acids as the degree of humification increases, (2) the higher contribution of polar amino acids suggesting a preferential preservation of these amino acids possibly by interaction with the humic acid surface through hydrogen bonds. [Copyright &y& Elsevier]

The purified cell walls of mother cells (CWM) were isolated from three strains of trilaminar sheath (TLS)- and algaenan-containing freshwater microalgae Chlorella emersonii, Tetraedron minimum and Scenedesmus communis. The chemical structures of CWM and algaenans were investigated by means of tetramethylammonium hydroxide (TMAH) hydrolysis and tetramethylammonium hydroxide thermochemolysis. The compounds released were characterised by 1H and 13C-NMR, gel permeation chromatography and desorption chemical ionisation mass spectrometry. The results show that the outer cell walls of the microalgae are constituted, at least in part, of linear (poly)esters containing extremely long chain alcohol and acid moieties (up to C80) and that algaenans are mainly composed of extremely long chain (di)carboxylic acids up to C120. The present results which are in direct contrast to the previous three-dimensional architecture proposed for algaenans, led us to re-interpret the algaenan structure. [Copyright &y& Elsevier]

New strains of the hydrocarbon rich alga Botryococcus braunii Kützing were isolated from water samples collected in three tropical freshwater lakes. These strains synthesize lycopadiene, a tetraterpenoid metabolite, as their sole hydrocarbon. The morphological and ultrastructural characteristics of these algae are similar to those reported for previously described strains which produce either alkadienes or botryococcenes. The pyriform shaped cells are embedded in a colonial matrix formed by layers of closely appressed external walls; this dense matrix is impregnated by the hydrocarbon and some other lipids. We believe the new strains synthesizing lycopadience form a third chemical race in B. branuii, besides the alkadiene and botryococcene races, rather than a different species. Like the other two types of hydrocarbons, lycopadiene was produced primarily during the exponential and linear growth phases. The major fatty acid in the three races was oleic acid. This fatty acid was predominant in the alkadiene race; palmitic and octacosenoic acid also were present in appreciate amounts in the three races. Cholest-5-en-3β-ol, 24-methylcholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol occurred in the three races; three unidentified sterols also were detected in the lycopadiene race. Moreover, the presence of very long chain alkenyl-phenols in the lipids of algae of the alkadiene race was not observed in the botryococcene and lycopadiene races. Of the polysaccharides released in the medium, galactose appeared as a primary component: it predominated in the botryococcene race. The other major constituents were fucose for the lycopadiene race. Although morphologically similar, some important chemical differences exist among algae classified as B. braunii. [ABSTRACT FROM AUTHOR]

In the context of global change, eroded soil carbon fate and its impact on aquatic ecosystems CO 2 emissions are subject to intense debates. In particular, soil carbon mineralization could be enhanced by its interaction with autochthonous carbon, a process called priming effect, but experimental evidences of this process are scarce. We measured in a microcosm experiment simulating oligo-mesotrophic and eutrophic aquatic conditions how quickly soil organic matter (SOM) sampled in diverse ecosystems was mineralized as compared to mineralization within soil horizons. For both nutrient loads, 13C-glucose was added to half of the microcosms to simulate exudation of labile organic matter (LOM) by phytoplankton. Effects of LOM on soil mineralization were estimated using the difference in δ 13C between the SOM and the glucose. After 45 days of incubation, the mean SOM mineralization was 63% greater in the aquatic context, the most important CO 2 fluxes arising during the first days of incubation. Nutrients had no significant effect on SOM mineralization and glucose addition increased by 12% the mean SOM mineralization, evidencing the occurrence of a priming effect. [ABSTRACT FROM AUTHOR]

Declines in availability of plant resources to pollinators are a major cause of pollinator loss. The management of plant communities to enhance floral resources is often proposed as a way to sustain pollinator populations. Nectar, the main energetic resource for pollinators, plays a central role in behaviour and composition of pollinator communities. Abiotic and biotic factors are known to influence nectar traits at both the species and community levels, but the impact of plant community composition itself has never been investigated. Below-ground interactions in plant communities can induce changes in plant development through (i) plant-derived litter amendment of the soil and (ii) competition for soil resources between plants. We tested how plant below-ground interactions affect above-ground nectar traits involved in plant attractiveness to pollinators. A short-term pot experiment was carried out with three temperate grassland species Mimulus guttatus, Lamium amplexicaule, and Medicago sativa, showing distinct litter stoichiometry and competitive abilities for soil resources. Litter amendment (none, mono and tri-specific litter) and plant interaction treatments (monocultures, two- and three-species mixtures) were crossed in a factorial design. Litter amendment to the soil led to an increase in total nectar sugar content in L. amplexicaule plants but not in the two other species. We also found that the presence of M. guttatus, a competitive species, reduced the total nectar sugar content in L. amplexicaule through a concomitant decrease in nectar volume per flower and in floral display size, but not in other species. Species-specific responses of nectar traits to variation in soil nitrogen availability were thus observed, suggesting consequences for plant species and community attractiveness to pollinators. However, we did not find evidence that the legume M. sativa affected nectar traits of any neighbouring plants. Synthesis. Our results demonstrate that litter inputs and competition between plants for soil resources can alter nectar traits linked to plant attractiveness to pollinators. This supports the idea that below-ground plant-plant interactions for soil resources can influence above-ground plant-plant interactions for pollination services. This offers promising perspectives in studying the role of below-ground-above-ground interactions on higher trophic levels. [ABSTRACT FROM AUTHOR]