Understanding the effects of salinity on phytoplankton communities across different periods is essential for elucidating ecological processes in saline lake ecosystems. In this study, we investigated phytoplankton community structure in Yuncheng Salt Lake through four sampling surveys corresponding to distinct hydrological stages (spring recharge, summer brine concentration, autumn crystallization, and winter freeze-up). By integrating microscopic identification and enumeration, physicochemical parameter measurement, and multivariate statistical analyses, we assessed phytoplankton community composition, diversity, and environmental drivers. A total of 80 phytoplankton genera belonging to 6 phyla were identified. The community exhibited significant spatiotemporal heterogeneity, with a clear differentiation pattern along salinity differences. Phytoplankton richness, evenness, and total abundance gradually decreased with salinity increase. Redundancy analysis and canonical correspondence analysis further confirmed salinity as the dominant driver shaping community structure. Linear mixed-effects models revealed that salinity exerted significant and consistent fixed effects on all community attributes, while temporal variations, through periodic fluctuations in environmental factors such as water temperature and monsoonal conditions, modulated the salinity‑driven community responses. This study provides new insights into the spatiotemporal dynamics of phytoplankton communities in salt lakes and offers a scientific basis for the ecological management and conservation of inland saline waters.

Robust microalgal activity is critical for the microalgal-bacterial symbiosis system (MBSS) to enable wastewater resource recovery, but microalgal performance can be affected by high concentrations of sludge. Ca2+ may regulate microalgal performance. Nevertheless, the mechanism of Ca2+-mediated regulation, particularly under high concentrations of sludge, remains unclear. This study integrated physiological and genomic analyses to investigate microalgal responses to sludge (100-800 mg/L) and Ca2+ supplementation (10-50 mM). Results showed that high-concentration sludge (400-800 mg/L) reduced microalgal growth, pigment synthesis, and photosynthetic efficiency by 65.6%-86.6%, 20.1%-39.2%, and 1.6%-7.0%, respectively, while Ca2+ restored these parameters by up to 39.9%, 39.7%, and 8.5%. At the genetic level, Ca2+ activated microalgal Ca2+ signaling pathways (43.9%-226.4% increase in CaM, CDPK, and CBL). It upregulated antioxidant enzyme genes (76.1%-373.0% increase in SOD, CAT, and POD) to mitigate cell damage and photosynthetic genes (e.g. 95.0%-260.9% increase in psbA and rbcL) to restore chloroplast function. Concurrently, Ca2+ promoted bacterial central carbon metabolism genes (e.g., 1.6%-26.2% increase in CS, IDH and OGDH) to increase CO2 release for microalgal carbon fixation and recruited siderophore-producing bacteria (e.g., 120.6%-154.3% increase in Sphingopyxis) to improve iron bioavailability for microalgal photosynthesis. Therefore, a positive feedback loop was formed through the supplementation of Ca2+. Microalgal photosynthesis supplied organic carbon/O2 for bacteria, while bacterial metabolism provided CO2 and iron for microalgae. Collectively, Ca2+ optimized microalgal activity via cross-kingdom coordination of carbon-iron metabolism, offering a mechanistic basis for optimizing MBSS applications in wastewater treatment and biological resource recovery by using Ca2+ as an effective regulator.

Microalga Coelastrella has garnered increasing attention in recent years due to its high content of natural carotenoids. However, this microalga still lacks systematic optimization of its cultivation conditions and the corresponding elucidation of underlying mechanisms. Meanwhile, its application scope remains to be expanded. This study aimed to systematically evaluate the effects of high light intensity, salinity stress, and nitrogen deprivation in the induction stage on the carotenoid accumulation of Coelastrella sp. TX using a two-stage cultivation strategy. Combined high light intensity (140 mu mol m_ 2 s_ 1) and nitrogen deprivation resulted in the highest carotenoid production. The microalgal biomass increased significantly by 74.23 % and the carotenoid yield increased by 67.80 % under these conditions compared with the non-stressed culture. These highly accumulated carotenoid components included lutein, beta-carotene, neoxanthin, and alpha-carotene. Transcriptomic analysis revealed that the combined stress significantly upregulated the expression of numerous key genes (PSY, PDS, ZDS, CRTISO, LCYB, LCYE, and CYHB) in the carotenoid synthesis pathway, promoting the accumulation of these carotenoids. Additionally, carotenoid accumulation, together with the upregulation of photoprotection genes PsbS and PsbO in Photosystem II, enhanced the photoprotective capacity of the microalgae under high light intensity and nitrogen deprivation, maintaining its high photosynthetic activity. Further investigation confirmed that Coelastrella sp. TX at 40 mg/L optimized both the cell density and nutritional quality of Paramecium, a common ciliate used as aquaculture feed, indicating its potential application in the aquaculture industry.

Nanoplastics have more severe effects on cells than microplastics, but knowledge of their effects on phytoplankton, particularly freshwater microalgae, is limited. Here, we investigated the effects and adaptive mechanisms of Chlamydomonas reinhardtii exposed to polystyrene nanoplastics (0, 10, 25, 50, and 100 mg/L) over 14 days. Algal cell growth, photosynthesis, oxidative stress, microstructure, transcriptomics and metabolomics were analyzed. The results showed that algal cells recovered after adapting to the nanoplastics, which exhibited a "transient stress-induced proliferation" and "heteroaggregation". Under long-term exposure (14 days), high concentration nanoplastics (100 mg/L) significantly promoted algal growth, with biomass increasing by 41% compared to the control group (p < 0.05). Additionally, antioxidant enzyme activities, such as superoxide dismutase and peroxidase, were enhanced, and the contents of reactive oxygen species and malondialdehyde also increased. Transcriptomic analysis identified 956 differentially expressed genes in the 100 mg/L group, with 761 genes downregulated. GO enrichment analysis revealed significant impairment in pathways related to protein synthesis and metabolism. Metabolomic analysis showed that the nanoplastics treatment group had 492 upregulated and 573 downregulated metabolites, primarily involving amino acid and nucleotide metabolism pathways. Amino acids played a crucial role among the studied metabolites. In summary, C. reinhardtii adapts to nanoplastics stress through coordinated heteroaggregation, enhanced protein synthesis, metabolic adjustment, and vesicular transport. The heteroaggregation between nanoplastics and microalgae offers promising insights for bioremediation strategies against nanoplastic pollution.

Phytoplankton communities are of vital importance to the functioning of freshwater ecosystems, but the role of the metabolic capacity of the community in regulating community dynamics under natural conditions has yet to be sufficiently considered. This study investigated 26 lakes situated along the eastern section of the Hu Line, combining field surveys with metagenome-assembled analyses to ascertain the factors responsible for the divergence in Chlorophyta and Bacillariophyta communities. The results demonstrated that the diversity of Chlorophyta was markedly higher than that of Bacillariophyta whereas the abundance was significantly lower. These discrepancies in community attributes were predominantly attributable to variations in the response of the two algal groups to nutrients. The abundance and diversity of diatom metabolic genes were significantly higher than those of green algae. The greater diversity and extent of metabolic genes in Bacillariophyta confer enhanced metabolic capacity and, consequently, greater adaptive capacity. Such differences in metabolic gene composition may be attributed to the disparate evolutionary pathways that these organisms have followed.

Salt lakes account for nearly half of the world's inland water area and play an irreplaceable role as "carbon conversion and stabilization factories," making substantial contributions to the global carbon cycle. Central to this function is the transformation of dissolved organic carbon (DOC) and its accumulation into recalcitrant dissolved organic carbon (RDOC), which together underpin internal carbon processing in these systems. However, the pathways through which DOC is converted to RDOC in salt lakes, and how these pathways are shaped by salinity and microbial communities, remain poorly resolved. Here, using Yuncheng Salt Lake as a within-lake system, we combined field-based in situ characterization with long-term incubation experiments to examine how contrasting salinity regimes were associated with microbial and dissolved organic matter (DOM) variation. Higher salinity was associated with reduced bacterial and dissolved organic matter diversity, stronger deterministic bacterial assembly, and a restructured bacteria-DOM association network. Under the standardized nutrient-replete incubation conditions used here, samples from the higher-salinity regime exhibited higher biodegradable DOC, lower RDOC preservation, and greater overall DOC loss over the 100-day experimental timescale. Salinity-related differences in microbial community composition, metabolomic profiles, and dissolved organic matter characteristics were closely associated with variation in RDOC dynamics, suggesting that these carbon-processing differences were accompanied by coordinated microbial and metabolic reorganization. Together, these results provide process-relevant, condition-specific evidence that contrasting salinity regimes within Yuncheng Salt Lake were associated with differences in microbe-DOM coupling and in DOC/RDOC outcomes.

Aged nanoplastics (ANPs) have emerged as a significant risk factor for microalgal wastewater treatment. However, Ca2+ appearing to have a moderating effect on this potential threat. This study investigated the response of Chlorococcum sphacosum to ANPs exposure and the protective role of Ca2+ through integrated physiological and multi-omics analyses. Exposure to 1 mg/LANPs reduced the removal of NH4+-N, TP, and COD by an average of 17.2%. Key physiological parameters were also suppressed, with biomass, chlorophyll a content, and photosynthetic efficiency (Fv/Fm) decreasing on average by 44.2%. Transcriptome analysis revealed that genes related to photosynthesis and energy metabolism were downregulated on average by 73.4%, highlighting the molecular basis for disrupted energy homeostasis and redox balance. Metabolomic analysis confirmed that key antioxidant metabolites, including glutathione, l-glutamic acid, linoleic acid, putrescine, and azelaic acid, decreased on average by 62.6%. These disturbances progressively caused structural deterioration, characterized by thylakoid disassembly, plasmolysis, and cell wall degradation, which impaired cellular activity and physiological function. Notably, 10 mM Ca2+ supplementation effectively counteracted these adverse effects. Ca2+ supplementation improved nutrient removal, restored physiological performance, increased antioxidant levels, mitigated transcriptional repression of photosynthetic and energy metabolism genes, and reversed the suppression of antioxidant biosynthesis. Overall, this study systematically elucidates the Ca2+-mediated resilience network in microalgae under ANPs exposure. These findings provide mechanistic insights and practical strategies for sustaining microalgae-based wastewater treatment in the presence of nanoplastic contamination.

The microalgal-bacterial symbiotic system (MBSS) represents a promising sustainable technology for wastewater treatment, and its performance is tightly linked to operational parameters. Nevertheless, the systematic understanding of how the ammonium-to-nitrate ratio drives MBSS performance remains largely insufficient. Here MBSS was operated under a constant total nitrogen concentration with varying ammonium-to-nitrate ratios (4: 0, 3: 1, 2: 2, 1: 3, 0: 4). By integrating physicochemical analysis, amplicon sequencing, nitrogen cycling functional prediction, and multivariate statistics, it was found that the ammonium-to-nitrate ratio significantly regulated MBSS performance and bacterial community dynamics. Specifically, high-ammonium conditions (4: 0 ratio) promoted microalgal growth, total phosphorus (TP) removal, and chemical oxygen demand (COD) removal, whereas high-nitrate conditions (0: 4 ratio) enhanced total inorganic nitrogen (TIN) removal. Functional prediction revealed distinct nitrogen metabolic pathways under different conditions. Under high-ammonium conditions, bacteria preferentially employed nitrogen-removal pathways (e.g., amoA → hao → narB → nirS → norB → nosZ) that operated in concert with microalgae. Conversely, under high-nitrate conditions, bacteria competed with microalgae for nitrogen, employing pathways such as napA/nasA → nirK → norB. Co-occurrence network analysis indicated that high-ammonium conditions promoted a tightly connected, stable community. Furthermore, five keystone taxa (Pseudoxanthomonas, Cloacibacterium, Dyella, Paraburkholderia, and Pandoraea) were identified, collectively explaining 74.2% of system performance variation. Comprehensive assessment via microalgal-bacterial multifunctionality index, developed by combining key data on pollutant removal efficiency and bacterial community characteristics, confirmed that MBSS achieved optimal overall performance when ammonium served as the sole nitrogen source. This work clarified the regulatory mechanism of the ammonium-to-nitrate ratio in MBSS, providing a theoretical basis for optimizing nitrogen-containing wastewater treatment.

The technology of microalgae-bacteria consortia (MBC) for wastewater treatment is currently facing a variety of challenges. One of the main issues is the construction of structurally and functionally stable symbiont. Ca2+ may be involved in this process, but the underlying mechanism is not well understood. Here the response of MBC to the regulation of Ca2+ was systematically explored from the perspectives of extracellular polymeric substances (EPS) and bacterial communities. The results showed that the exogenous addition of Ca2+ (10-50 mM) not only promoted the production of extracellular polysaccharides and proteins of MBC, but also increased the proportion of some functional groups and components of EPS, such as CO and α-helix. The change of EPS characteristics was conducive to provide more sites for bining Ca2+, which in turn favored the formation of compact MBC via overcoming electrostatic repulsive effect. Besides, the supplementation of Ca2+ favored the recruitment of more EPS-producing bacteria (such as Rhodobacter, Pedobacter, Rhizorhapis, and Sphingopyxis) and indole acetic acid producing bacteria (such as Hydrogenophaga and Agromyces). The enrichment of these functional bacteria not only promoted the adhesion between bacteria and microalgae, but also promoted the growth of symbiotic microalgae, which contributed to the formation of stable large-sized MBC. The change in structure and function of MBC was ultimately reflected in the improved performance in treating municipal wastewater. The findings of this study provided insights into the mechanism underlying the enhanced performance of MBC for wastewater treatment under the influence of Ca2+.

Freshwater brown algae have been acknowledged as the least diverse of all groups of freshwater algae. Morphological and molecular investigation were conducted on a freshwater brown alga collected in this study. Morphological characterization and phylogenetic tree based on the rbcL gene confirmed its attribution to Heribaudiella fluviatilis. The complete organelle genomes of H. fluviatilis were determined and assembled from high-throughput sequencing. A circular tetrad structure of 128,880 bp was identified for the chloroplast, and the mitochondrial genome was assembled as a circle with 43,657 bp in length. This study provides the first and only record of organelle genomic information of the freshwater Phaeophyceae, which are important for understanding the endosymbiosis theories. Concatenated (Bayesian inference, maximum likelihood under MFP and GHOST models) and coalescent methods were used to construct the phylogenetic relationship among secondary plastid-bearing lineages based on organelle genomes. Obvious conflicts were observed in grouping relationships of chloroplast and mitochondrial genomes, even among different evolutionary models and tree-constructing methods. The transcriptome of H. fluviatilis was sequenced and used to construct the phylogenetic relationship among secondary plastid-bearing lineages. The phylogenetic relationships in nuclear and mitochondrial genomes were consistent in grouping CHSR taxa (cryptophytes, haptophytes, stramenopiles and rhizarians) in an independent branch. The phylogenetic relationship constructed from nuclear transcriptomes was highly reliable by owning robust supporting values on each node and was consistent with the serial endosymbiosis theory on evolution of red secondary plastid-derived lineages.

In recent years, there has been a marked increase in the frequency and duration of cyanobacterial blooms, which has attracted widespread attention from all sectors of society. However, the processes and pathways leading to the formation of cyanobacterial blooms are largely stochastic, and a targeted quantitative model incorporating multiple drivers is required. A series of fieldwork and analytical procedures were conducted on 11 lakes in Beijing-Tianjin-Hebei and neighboring regions. The objective of this research was to investigate the effects of location, climate, habitat, metabolism and biology on the abundance of cyanobacteria. The findings indicated that location factors exerted an indirect influence on cyanobacterial density, with a calculated effect size of -0.001. This observation is consistent with the documented prevalence of cyanobacterial bloom outbreaks, suggesting that cyanobacterial abundance is not directly associated with location. Furthermore, the water environment (0.35 indirect and -0.37 direct effects) and climatic conditions (-0.2 indirect and 0.16 direct effects) had significant direct and indirect effects on cyanobacterial cell densities. It was also determined that high nutrients and high temperatures remained the basis for promoting cyanobacterial blooms. It is noteworthy that the microbial community exerted a substantial inhibitory effect on the proliferation of the cyanobacterial community (1.13 direct inhibitory effect). It is hypothesized that the negative effect is derived from two primary sources: the competition for ecological niches by organisms exhibiting similarity, and the direct inhibitory effect of certain bacteria on the growth of cyanobacteria, a negative correlation between acidobacteriota, bacteroide and a variety of cyanobacteria was observed in our study. The effect of specific bacteria is found to be significantly negatively correlated with the prevalence of a wide range of cyanobacteria, which is consistent with the results of many extant studies. These extant studies indicate that the formation of cyanobacterial communities is primarily influenced by microorganisms in the environment. The results of this study will provide new and complementary evidence on the mechanisms of cyanobacterial bloom occurrence and development and the design of management strategies.

Background:The genus Urostipulosphera was established in 2019. Its morphological characteristics are similar to Uroglena and Uroglenopsis, but it differs from both. This genus connects individual cells into communities through a dichotomous branching structure, which distinguishes it from Uroglena and Uroglenopsis. Although there are nine species in this genus, molecular data are available for only four species. Molecular data and species distribution records for Urostipulosphaera are severely lacking. Species of this genus have been recorded in a few locations in Europe and North America. New information:We identified three samples that were discovered in Shanxi Province, China, based on morphological characteristics and molecular phylogenetic analysis. Results of the polygenic phylogenetic tree, based on SSU, rbcL and ITS sequences, showed that the three samples were clustered with Urostipulosphaeraarticulata U5-5 from Czechia, with a high support rate of 100/1.00. Morphological observations further supported this result. Therefore, the three samples were identified as Urostipulosphaeraarticulata. This is the first report in China and enriches the geographic diversity of this species. Moreover, we found the long flagellum with mastigonemes under the scanning electron microscope complementing the description of the morphological characteristics of this species. The ITS2 secondary structure of the specimens differed from that of Urostipulosphaeraarticulata U5-5, which exhibited a loop and five-arm structure. A Bayesian relaxed clock analysis indicated that the genus Urostipulosphaera originated in the Early Cretaceous, approximately 109.85 million years ago (Ma) and diverged into two clades around 79.81 Ma.

Phytoplankton are vital components of freshwater ecosystems, serving as primary producers and ecological indicators of freshwater health. While substantial research has explored the dynamics and potential drivers of phytoplankton communities in freshwater lakes, the influence of altitude-a crucial geographic factor-remains unelucidated. In this study, we investigated 26 lakes located along the eastern section of the Hu Line (Heihe-Tengchong Line) from China, focusing on how altitudinal gradients shape phytoplankton communities. Our findings reveal that cyanobacteria increasingly dominate community abundance with rising altitude. However, this dominance results in a reduction of community diversity, as cyanobacteria outcompete other taxa, thereby narrowing the ecological niches available. Further analysis indicates that the effects of altitude are mediated primarily by dominant cyanobacteria, which are equipped with gas vesicles that provide an adaptive advantage under low atmospheric pressure conditions associated with higher altitudes. This physiological trait allows cyanobacteria to maintain buoyancy and occupy favorable niches in the water column, enhancing their proliferation at the expense of overall community diversity. The study underscores the critical role of altitude in modulating phytoplankton community structure through its direct influence on cyanobacteria. These findings contribute new insights into the ecological processes that regulate freshwater ecosystems across altitudinal gradients, and highlight the need for targeted management strategies in regions where cyanobacterial dominance (cyanobacterial blooms) may compromise water quality and biodiversity.

Microbial communities are integral to the biogeochemical cycles of urban rivers. Nevertheless, there has been limited research on the seasonal variations of periphyton microbial communities and the mechanisms underlying their assembly. In this study, we employed high-throughput sequencing to investigate the diversity dynamics and assembly mechanisms of periphyton microbial communities (including prokaryotic and eukaryotic) in the urban section of the Fenhe River. Our findings indicated that the prokaryotic communities were predominantly composed of Proteobacteria, while the eukaryotic communities were dominated by Rotifera, with significant seasonal variations in their proportions. The diversity of both prokaryotes and eukaryotes exhibited a downward trend from spring to autumn. Co-occurrence network analysis revealed that the microbial community displayed closer network connections and greater complexity during the spring season, suggesting more intense interactions among periphyton microbes to adapt to seasonal environmental shifts. Stochastic processes significantly influenced the assembly of prokaryotic and eukaryotic communities during the spring and summer seasons, while deterministic processes were more dominant in autumn. Both abundant and rare taxa were predominantly shaped by stochastic processes. The variations in the structure and assembly of periphyton microbial communities across different seasons were primarily affected by temperature and nitrate nitrogen. Overall, our study elucidated the impacts of seasonal changes on the diversity and assembly of periphyton microbial communities, thereby deepening the understanding of urban river ecosystems from the microbial ecology perspective.

Given their varied physiological and ecological attributes, phytoplankton frequently exhibit disparate responses to changes in environmental conditions. Therefore, when studying modifications in the structure of a phytoplankton community, it is necessary to consider such differences and investigate the underlying mechanisms. Through a field sampling study of 26 lakes, we found that the differing responses of phytoplankton to changes in environmental factors were partly related to their genome size. The Cyanobacteria and Chrysophyta, which possess smaller genomes, exhibited a relatively low degree of sensitivity to alterations in nutrient concentrations. However, they demonstrated a stronger dependence on the metabolic processes of environmental microorganisms compared to other phytoplankton with larger genomes. Furthermore, the Cyanobacteria and Chrysophyta were more closely associated with environmental nutrients and water chemistry and were more actively involved in interspecific interactions among phytoplankton. By randomly deleting sample points, we verified the generality of the above conclusions. However, more refined grouping analyses indicate that diatoms exhibited different ecological characteristics due to their physiological and morphology. The present study offers new insights into the mechanisms underlying the ecological adaptations of phytoplankton and may provide new guidance for the management of Cyanobacterial blooms. Further studies could take a more refined approach to investigate the effect of phytoplankton “effective genome size” on their adaptive capacity.

Most biotic and abiotic interactions among aquatic species in freshwater ecosystems remain unknown, which frequently results in the assumption that dominant species play the most crucial ecological role. This tendency is particularly evident in studies of phytoplankton communities that has predominantly focused on common and abundant members such as cyanobacteria, which overlooks the ecological contributions of rare (non-dominant and uncommon) species. This study investigated freshwater ecosystems east of the Hu Huanyong Line, examining the ecological roles of common and rare algal genera. Results indicated that common algal genera significantly influence community dynamics and occupy central positions in ecological networks. By contrast, rare algal genera, situated at network peripheries, were crucial for ecosystem stability and maintaining ecosystem stability through distinct “presence or absence patterns”. Furthermore, the diversity and abundance of rare algae were negatively correlated with multiple nutrient factors (−0.347, −0.327), contrasting the positive relationship observed for common algae (0.378, 0.292), which indicates their vulnerability to eutrophication. Overall, this study emphasizes the importance of conserving rare algal species in order to maintain ecosystem stability and biodiversity in the face of global environmental change.

The genus Epipyxis, belonging to the family Dinobryaceae, has been documented to have only sporadic occurrences in freshwater habitats. However, the species diversity of this genus remains largely unexplored due to the scarcity of available molecular sequences. This limitation has significantly hindered a comprehensive understanding of both the species diversity and evolutionary relationships of the genus Epipyxis. In this study, a new species Epipyxis fenheensis sp. nov. was described from Shanxi Province, China, based on detailed morphological observations and phylogenetic analyses. This species was characterized by a tube-like lorica, a spindle protoplast, two heterokont flagella, and oval or elliptic scales. In addition, phylogenetic analysis based on multi-genes (SSU, LSU, and rbcL) indicated that strain SX231009 was closely related to E. pulchra. Given its distinct morphological characteristics and independent phylogenetic position, we propose the designation of this strain as a new species, E. fenheensis sp. nov. The results of this study significantly expand the known diversity of the genus Epipyxis and provide valuable insights into the regional biodiversity and evolutionary history of freshwater chrysophytes.

Undaria pinnatifida, a nutrient-rich seaweed, holds potential for the alcoholic beverage industry. This study optimized the ultrasonic processing of Undaria blend liquor (UBL) and the fermentation of Undaria fermented wine (UFW) while identifying volatile components and assessing antioxidant properties. After optimization, UBL had a polysaccharide content (PC) of 0.66 g/L and an alcohol content (AC) of 39.2 % vol, while UFW showed a PC of 9.81 g/L and an AC of 8.3 % vol. HS-SPME-GC × GC-TOF-MS analysis identified 34 characteristic volatile compounds, with esters as the predominant class. UBL was featured by notably high levels of ester compounds, while UFW contained fatty acids leading to distinct flavor profiles. Antioxidant assays revealed that both beverages demonstrated free radical scavenging activity in a dose-dependent manner. These findings highlight the potential of Undaria as a novel resource for developing functional and flavorful alcoholic beverages, contributing to innovation in the food and liquor industries.

Although the effect of aeration intensity on the performance of microalgae-based wastewater treatment systems has been widely studied, the impact of aeration mode has received less attention. This study explored how different aeration modes influence nitrogen metabolism in microalgae-based wastewater systems using metagenomic analysis. Both continuous aeration (CA) and intermittent aeration (IA) supported rapid bacterial growth and effective pollutant removal. Compared to CA, IA and no-aeration modes significantly enhanced bacterial nitrification and denitrification. Key nitrogen-metabolizing genera such as Paracoccus, Acidovorax, and Rhizobium played major roles in nitrogen cycling. Their abundances were closely associated with NH4+-N, NO3--N, NO2--N, total phosphorus, chemical oxygen demand, dissolved oxygen, bacterial number, and total biomass. Overall, environmental changes induced by different aeration modes significantly shaped indigenous bacterial communities involved in nitrogen metabolism, thereby influencing the abundances of nitrogen metabolism-related genes and, ultimately, nitrogen removal performance.

Complex relationships exist between diatoms, bacterial communities and dissolved organic matter in aquatic ecosystems. Clarifying their relationships is essential to reveal biogeochemical cycles. In this study, the diatom Cyclostephanos invisitatus was cultured in standard silicon (SS), no silicon (NS), and excessive silicon (ES). Integrated 16S rDNA high-throughput sequencing and excitation-emission matrix parallel factor analysis (EEM-PARAFAC) were used to investigate diatom growth, bacterial community and dissolved organic matter (DOM) characteristics. It was found that the abundance of diatoms decreased in no silicon and excessive silicon conditions. Further, the composition and function of the associated bacterial communities were affected. The Proteobacteria and Cyanobacteria were more highly represented in the attached bacteria, whereas the Proteobacteria and Bacteroidetes were more highly represented in the free-living bacteria. Alpha diversity showed different trends with incubation time in all treatments. Moreover, genes (K01995 to K01998) involved in environmental information processing were more abundant in diatoms during the stationary phase in attached bacterial community. Stochastic processes play a key role in the formation of bacterial communities, both for attached and free-living bacteria. In addition, the differences in fluorescence components (protein and humic-like) and spectral indices (fluorescence index, biological index, freshness index, and humification index) revealed intricate relationship between microalgae and bacteria. Overall, this work confirmed the importance of silicon concentration in regulating diatom-associated bacterial communities and DOM characteristics.