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.

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 Different Concentrations of Ammonia Nitrogen on the Growth and Enzyme Activity of Four Common Algae Strains].

Treating swine wastewater with a high ammonia nitrogen content with microalgae cultures has proved difficult. In this paper, the strains Chlamydomonas 715, Botryococcus braunii 357, Porphyridium cruentum 806, and Scenedesmus obliquus 417 were tested. Ammonia nitrogen concentrations of 50 mg·L-1, 500 mg·L-1, and 2000 mg·L-1 applied to the media according to the concentrations of biogas slurry. This allowed the effect of different concentrations of ammonia nitrogen on the growth and cell enzyme activity of microalgae to be tested. The results showed that the growth of Chlamydomonas 715 and Scenedesmus obliquus 417 was inhibited at different concentrations of ammonia nitrogen, and the biomass and biomass productivities were lower than for the normal media. However, the biomass and biomass productivity of Porphyridium cruentum 806 in 50 mg·L-1 ammonia nitrogen were 1.78 g·L-1 and 0.16 g·(L·d)-1, respectively, which were higher than the values obtained using KOCK medium. Furthermore, the biomass and biomass productivity of Botryococcus braunii 357 in 500 mg·L-1 ammonia nitrogen were 1.95 g·L-1 and 0.18 g·(L·d)-1, respectively, which were higher than the values obtained using BG11 medium. The SOD, POD, and CAT of all algae species showed a decreasing tendency in response to an increase in the concentration of ammonia nitrogen, as did MDA. These results provide a theoretical basis for the treatment of swine wastewater with high ammonia nitrogen content using microalgae cultures.

Bacterial Community Diversity and Screening of Growth-Affecting Bacteria From Isochrysis galbana Following Antibiotic Treatment.

Algal cultures are generally co-cultures of algae and bacteria, especially when considering outdoor cultivation. However, the effects of associated bacteria on algal growth remain largely unexplored, particularly in the context of Isochrysis galbana. In the present study, we investigated the effects of antibiotic on the growth of I. galbana and its associated bacterial community. We found advantageous responses of I. galbana to antibiotic exposure, evidenced by the increased growth, and the maximal photochemical efficiency of PSII (Fv/Fm). Since antibiotics can cause major disturbances within bacterial community, we further conducted 16S rDNA amplicon sequencing to determine the changes of bacterial community diversity following antibiotic treatment. We found that antibiotic treatment considerably and negatively affected the abundance and diversity of bacterial community, and 17 significantly decreased bacterial species in the antibiotic-treated medium, including Pseudomonas stutzeri, were identified. Further co-culture experiments revealed that P. stutzeri inhibited the growth of I. galbana, and the inhibitory activity was retained in the cell-free bacterial filtrate. These results indicated that the negative effect of bacteria was not exclusively transmitted through contact with I. galbana but could be also mediated via secretory compounds. Taken together, our findings not only fully characterized the bacterial community associated with I. galbana and how the bacterial community changed in response to antibiotic perturbations, but also provided a valuable information about the interactions between I. galbana and its associated bacteria, which might help improve the yield, and quality of I. galbana during its cultivation processes.

Site-Directed Mutagenesis from Arg195 to His of a Microalgal Putatively Chloroplastidial Glycerol-3-Phosphate Acyltransferase Causes an Increase in Phospholipid Levels in Yeast.

To analyze the contribution of glycerol-3-phosphate acyltransferase (GPAT) to the first acylation of glycerol-3-phosphate (G-3-P), the present study focused on a functional analysis of the GPAT gene from Lobosphaera incisa (designated as LiGPAT). A full-length cDNA of LiGPAT consisting of a 1,305-bp ORF, a 1,652-bp 5'-UTR, and a 354-bp 3'-UTR, was cloned. The ORF encoded a 434-amino acid peptide, of which 63 residues at the N-terminus defined a chloroplast transit peptide. Multiple sequence alignment and phylogeny analysis of GPAT homologs provided the convincible bioinformatics evidence that LiGPAT was localized to chloroplasts. Considering the conservation of His among the G-3-P binding sites from chloroplastidial GPATs and the substitution of His by Arg at position 195 in the LiGPAT mature protein (designated mLiGPAT), we established the heterologous expression of either mLiGPAT or its mutant (Arg195His) (sdmLiGPAT) in the GPAT-deficient yeast mutant gat1Δ. Lipid profile analyses of these transgenic yeasts not only validated the acylation function of LiGPAT but also indicated that the site-directed mutagenesis from Arg(195) to His led to an increase in the phospholipid level in yeast. Semi-quantitative analysis of mLiGPAT and sdmLiGPAT, together with the structural superimposition of their G-3-P binding sites, indicated that the increased enzymatic activity was caused by the enlarged accessible surface of the phosphate group binding pocket when Arg(195) was mutated to His. Thus, the potential of genetic manipulation of GPAT to increase the glycerolipid level in L. incisa and other microalgae would be of great interest.

Fabrication of (Nd(0.01)La(x)Y(0.99-x))2O3 nanoparticles and transparent ceramics by combustion synthesis.

Y2O3 acts as the matrix material when doped with different content of La2O3 for reducing sintering temperature and refining grains. The (Nd(0.01)La(x)Y(0.99-x))2O3 nanoparticles and transparent ceramics are fabricated by a combustion synthesis. The powder feature is characterized by TEM. The microstructure, mechanical properties and transmittance of the samples are examined by SEM, HV-1000 hardness tester and fluorescence analyzer respectively. The results show that the (Nd(0.01)La(x)Y(0.99-x))2O3 nanoparticles are homogeneous in size and nearly spherical with average diameter in the range of 40-60 nm. There are no other phases except the Y2O3 cubic phase in the (Nd(0.01)La(x)Y(0.99-x))2O3 nanoparticles. The grains of the samples significantly reduce with increasing La2O3 content. The hardness and fracture toughness increase rapidly first and then gradually tend to plateau with increasing La2O3 content. The transmittance of sample also increases gradually with increasing La2O3, the largest transmittance exceeds 77% when the La2O3 content is x = 0.12.