Benthic primary production decreases internal phosphorus loading from lake sediments under light supplement.

In aquatic ecosystems, light penetrating the sediment surface in shallow lakes may regulate the internal phosphorus (P) release through benthic primary production, which subsequently affects oxidation, pH levels, and alkaline phosphatase activity in the upper sediment. To study the effects of light exposure on the P dynamics at the sediment-water interface under eutrophic conditions, a two-month mesocosm experiment was conducted in twelve cement tanks (1000 L each). The tanks were equipped with Light-Emitting Diode (LED) lights, and surface sediments collected from eutrophic Lake Nanhu (China) were exposed to four different light intensities (0, 50, 100, 200 µmol m-2 s-1). The results revealed that: 1) Both the total phosphorus concentration and the phosphorus release flux from the sediment were lower in the light treatments (mean value, 0.59-0.71 mg L-1 and 0.00-0.01 mg m-2 d-1, respectively) than in the control treatment (0.77 mg L-1 and 0.01 mg m-2 d-1, respectively), indicating that light supplement could decrease the internal P release. 2) Benthic primary production promoted by light directly absorbed soluble reactive phosphorus and decreased the internal P release. The resulting improved production could also increase dissolved oxygen concentrations at the sediment-water interface, thus indirectly inhibiting internal P release. 3) The relative contributions of direct absorption and indirect inhibition on the internal P release ranged between 23% to 69% and 31% to 77% depending on the light intensity.

Mechanisms of high ammonium loading promoted phosphorus release from shallow lake sediments: A five-year large-scale experiment.

The unprecedented global increase in the anthropogenic-derived nitrogen (N) input may have profound effects on phosphorus (P) dynamics and may potentially lead to enhanced eutrophication as demonstrated in short-term mesocosm experiments. However, the role of N-influenced P release is less well studied in large-scale ecosystems. To gain more insight into ecosystem effects, we conducted a five-year large-scale experiment in ten ponds (700-1000 m2 each) with two types of sediments and five targeted total N concentrations (TN) by adding NH4Cl fertilizer (0.5, 1, 5, 10, and 25 mg N L-1). The results showed that: (ⅰ) The sediment P release increased significantly when TN exceeded 10-25 mg N L-1. (ⅱ) The most pronounced sediment P release increase occurred in summer and from sediments rich in organic matter (OMSed). (ⅲ) TN, algal biomass, fish biomass, non-algal turbidity, sediment pH, and OMSed were the dominant factors explaining the sediment P release, as suggested by piecewise structural equation modeling. We propose several mechanisms that may have stimulated P release, i.e. high ammonium input causes a stoichiometric N:P imbalance and induce alkaline phosphatase production and dissolved P uptake by phytoplankton, leading to enhanced inorganic P diffusion gradient between sediment and water; higher pelagic fish production induced by the higher phytoplankton production may have led increased sediment P resuspension through disturbance; low oxygen level in the upper sediment caused by nitrification and organic decomposition of the settled phytoplankton and, finally, long-term N application-induced sediment acidification as a net effect of ammonium hydrolysis, nitrification, denitrification; The mechanisms revealed by this study shed new light on the complex processes underlying the N-stimulated sediment P release, with implications also for the strategies used for restoring eutrophicated lakes.

Integrated Analyses of miRNome and Transcriptome Reveal the Critical Role of miRNAs Toward Heat Stress Response in Isochrysis galbana.

Isochrysis galbana is widely used in aquaculture as a bait microalgal species. High temperature (HT) can severely impair the development of I. galbana, exerting adverse effects on its yield. MicroRNAs (miRNAs) play an essential role in modulating stress-responsive genes. However, the role of miRNAs in response to HT in microalgae remains largely unexplored. In the present study, we identified several conserved and novel miRNAs in I. galbana through miRNome sequencing. Among these identified miRNAs, 22 miRNAs were differentially expressed in response to heat stress, and their target genes were predicted accordingly. Moreover, a comprehensive and integrated analysis of miRNome and transcriptome was performed. We found that six potential reversely correlated differentially expressed miRNA (DEM) and differentially expressed gene (DEG) pairs were associated with heat stress response (HSR) in I. galbana. The expressions of DEMs and DEGs were further verified using real-time quantitative PCR (RT-qPCR). Integrated analyses showed that miRNAs played fundamental roles in the regulatory network of HSR in I. galbana mainly by regulating some heat-responsive genes, including heat shock proteins (HSPs), reactive oxygen species (ROS) signaling-related genes, and specific key genes in the ubiquitination pathway. Our current study identified the first set of heat-responsive miRNAs from I. galbana and helped elucidate the miRNA-mediated HSR and resistance mechanisms in I. galbana. This new knowledge could provide ways to enhance its heat stress tolerance.

Responses of coastal sediment phosphorus release to elevated urea loading.

Increased urea is one of the common nitrogen forms polluting coastal waters and affecting nutrient dynamics. To investigate the effects of urea on sediment phosphorus (P) release, we carried out a 2-month mesocosm experiment with six targeted loadings of urea (0-0.6 mg N L-1 d-1). Results showed that: i) urea was rapidly transformed into ammonium and then nitrate (NO3-). ii) When nitrogen occurred as urea or ammonium, minor P release was observed. iii) After urea were mostly converted to NO3-, P release became clearer. iv) NO3- had a dual effect by promoting P release through decreasing sediment pH and increasing alkaline phosphatase activity or by inhibiting P release through improving sediment oxidation. v) The overall effects of urea on P release depended on the ultimate NO3- concentrations, being prominent when NO3- ≥ 11 mg N L-1. Our findings are of relevance when determining nitrogen reduction targets needed for combating eutrophication.

Decreasing toxicity of un-ionized ammonia on the gastropod Bellamya aeruginosa when moving from laboratory to field scale.

Along with a steady increasing use of artificial nitrogen fertilizer, concerns have been raised about the effects that high nitrogen loading may have on ecosystems. Due to the toxicity of unionized ammonia (NH3), tolerance criteria have been proposed for ambient water management in many countries; however, these are mainly based on acute or chronic tests carried out under lab conditions run with purified water. Aiming at understanding the responses of organisms to natural exposure to high ammonia concentrations, a Viviparidae gastropod, Bellamya aeruginosa, was tested at three experimental scales: standard 96-h lab test, one-month cage test in 6 experimental ponds with continuous nitrogen inputs, and intensive investigation of the B. aeruginosa from these ponds in spring and winter. The results were: 1) 96-h LC50 in the standard lab test was 0.56 mg NH3-N/L and 343.3 mg TAN/L (total ammonia expressed as N, standardized at pH 7 and 20 â„ƒ). 2) In the one-month cage test, the survival rate was 97% when NH3-N was 0.61 mg/L (i.e., a higher concentration than the lab 96-h LC50) and the body size of the gastropods actually increased with increasing NH3-N concentrations. 3) In the winter-spring investigation, little effect of ammonia on the standing crops of gastropods was found, and the body size of the gastropods tended to increase with increasing ammonia concentrations (NH3-N concentration range of 0.05 ~ 2.06 mg/L). Thus, B. aeruginosa showed higher tolerance to ammonia exposure (NH3-N concentration < 0.81 mg/L) in the field than under laboratory conditions. Our study points to the necessity of considering the relevant scale when determining criteria for ammonia toxicity in water management.

RNA-seq Insights Into the Impact of Alteromonas macleodii on Isochrysis galbana.

Phycospheric bacteria may be the key biological factors affecting the growth of algae. However, the studies about interaction between Isochrysis galbana and its phycospheric bacteria are limited. Here, we show that a marine heterotrophic bacterium, Alteromonas macleodii, enhanced the growth of I. galbana, and inhibited non-photochemical quenching (NPQ) and superoxide dismutase (SOD) activities of this microalgae. Further, we explored this phenomenon via examining how the entire transcriptomes of I. galbana changed when it was co-cultured with A. macleodii. Notable increase was observed in transcripts related to photosynthesis, carbon fixation, oxidative phosphorylation, ribosomal proteins, biosynthetic enzymes, and transport processes of I. galbana in the presence of A. macleodii, suggesting the introduction of the bacterium might have introduced increased production and transport of carbon compounds and other types of biomolecules. Besides, the transcriptome changed largely corresponded to reduced stress conditions for I. galbana, as inferred from the depletion of transcripts encoding DNA repair enzymes, superoxide dismutase (SOD) and other stress-response proteins. Taken together, the presence of A. macleodii mainly enhanced photosynthesis and biosynthesis of I. galbana and protected it from stress, especially oxidative stress. Transfer of fixed organic carbon, but perhaps other types of biomolecules, between the autotroph and the heterotroph might happen in I. galbana-A. macleodii co-culture. The present work provides novel insights into the transcriptional consequences of I. galbana of mutualism with its heterotrophic bacterial partner, and mutually beneficial associations existing in I. galbana-A. macleodii might be explored to improve productivity and sustainability of aquaculture algal rearing systems.

Effects of nitrate on phosphorus release from lake sediments.

Phosphorus (P) release from sediment is a key process affecting the effectiveness of eutrophication mitigation. We hypothesized that high nitrate (NO3-) input may have dual effect on sediment P release: reduce the sediment P release by improving the oxidation of sediment or promote P release by stimulating the growth of phytoplankton and increase the decomposition rates and oxygen consumption at the sediment water interface. To test the effect of different NO3- concentrations, we conducted a three-month experiment in 15 cement tanks (1 m3), with five targeted concentrations of NO3-: control, 2 mg L-1, 5 mg L-1, 10 mg L-1, and 15 mg L-1. The results showed that: i) when NO3- was maintained at high levels: NO3-≥5-7 mg L-1 (range of median values), there was no effect of NO3- on net P release from the sediment, likely because the positive effects of NO3- (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO3- addition was terminated NO3- dropped sharply to a low level (NO3-≤0.4 mg L-1), followed by a minor P release in the low N treatments but a significant P release in the high N treatments, which likely reflect that the inhibition effect of NO3- on P release decreased, while the promotion effects at high NO3- concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO3- loading on stimulating P release from the sediment, being clear at high NO3- exceeding 5-7 mg L-1.

High ammonium loading can increase alkaline phosphatase activity and promote sediment phosphorus release: A two-month mesocosm experiment.

In aquatic ecosystems, ammonium is one of the dominant substances in the effluent discharge from wastewater treatment plants and its impact has been widely explored as it is thought, in its toxic form (NH3), to cause stress on organisms. Little is, however, known about its potential effect on the release of phosphorus (P) from the sediment. In a two-month mesocosm (150 L) experiment, we tested if high loading of ammonium promotes sediment P release and investigated the dominant underlying mechanisms. A gradient of five target ammonium loading levels was used by adding NH4Cl fertilizer: no addition/control (N0), 3 (N1), 5 (N2), 10 (N3), and 21 (N4) mg NH4Cl L-1 (NH4Cl expressed as nitrogen). We found that: 1) significant sediment P release for N3 and N4 but minor release or retention for N0, N1, and N2 were detected both by the total phosphorus concentration (TP) in the overlying water and in situ measurements of diffusive gradients in thin-films (DGT) at the sediment-water interface; 2) overall, TP correlated significantly and positively with total nitrogen (TN) concentrations in the water. Further correlation and path analyses suggested that stimulated alkaline phosphatase activity (APA) was likely the dominant mechanisms behind the ammonium-induced sediment P release and decreased dissolved oxygen (DO) levels (an approximate reduction from 9.2 to 6.6 mg O2 L-1) was likely a contributing factor, particularly in the beginning of the experiment.

Higher Tolerance of Canopy-Forming Potamogeton crispus Than Rosette-Forming Vallisneria natans to High Nitrogen Concentration as Evidenced From Experiments in 10 Ponds With Contrasting Nitrogen Levels.

Due to excess nutrient loading, loss of submersed macrophytes is a worldwide phenomenon in shallow lakes. Phosphorus is known to contribute significantly to macrophyte recession, but the role of nitrogen has received increasing attention. Our understanding of how high nitrogen concentrations affect the growth of submersed macrophytes, particularly under natural conditions, is still limited. In this study, we conducted experiments with canopy-forming Potamogeton crispus in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100 mg L-1) and compared the results with those of our earlier published experiments with rosette-forming Vallisneria natans performed 1 year before. Canopy-forming P. crispus was more tolerant than rosette-forming V. natans to exposure to high NH4 concentrations. This is probably because canopy-forming species reach the water surface where there is sufficient light for production of carbohydrates, thereby allowing the plants to partly overcome high NH4 stress. Both the canopy-forming P. crispus and the rosette-forming V. natans showed clear declining trends with increasing chlorophyll a in the water. Accordingly, shading by phytoplankton might be of key importance for the decline in submersed macrophytes in this experiment. Both experiments revealed free amino acids (FAA) to be a useful indicator of physiological stress by high ammonium but is not a reliable indicator of macrophyte growth.