Physico-chemical characterisation of irrigation basin waters and inventory study of their algal communities.

To cope with the water shortage in Sous Massa region of Morocco, agricultural producers in the region have resorted to different types of water supply basins, known as "irrigation basins" but the phenomenon of eutrophication has hindered the continuity of agricultural productivity by altering the quality of the water used for irrigation on the one hand, and causing economic damage to agricultural producers due to the clogging of the water pumping network on the other. We began by characterising the physico-chemical quality of the water to determine the causes of its high nutrient content, then we determined the taxonomy of the algal species in the irrigation basins to which we had access. A qualitative study of the water in the irrigation basins in order to better explain the inventory obtained from the taxonomic identification of the algal biomass collected, which proved the existence of new species, not previously identified, characterising the freshwaters of the Moroccan region, is under the scope of this work. The species studied belong mainly to the following groups: green algae (11 genera of Chlorophyta and 7 genera of Charophyta), blue algae (7 genera of Cyanobacteria), brown algae (7 genera of Diatoms), and one genus of Euglenophyta.

Metagenomics datasets of water and sediments from eutrophication-impacted artificial lakes in South Africa.

We present metagenomes of 16 samples of water and sediment from two lakes, collected from eutrophic and non-eutrophic areas, including pooled samples enriched with phosphate and nitrate. Additionally, we assembled 167 bacterial metagenome-assembled genomes (MAGs). These MAGs were de-replicated into 83 unique genomes representing different species found in the lakes. All the MAGs exhibited >70% completeness and <10% contamination, with 79 MAGs being classified as 'nearly complete' (completeness >90%), while 54 falling within 80-90% range and 34 between 75-80% complete. The most abundant MAGs identified across all samples were Proteobacteria (n = 80), Firmicutes_A (n = 35), Firmicutes (n = 13), and Bacteriodota (n = 22). Other groups included Desulfobacteria_I (n = 2), Verrucomicrobiota (n = 4), Campylobacterota (n = 4) and Actinobacteriota (n = 6). Importantly, phylogenomic analysis identified that approximately 50.3% of the MAGs could not be classified to known species, suggesting the presence of potentially new and unknown bacteria in these lakes, warranting further in-depth investigation. This study provides valuable new dataset on the diverse and often unique microbial communities living in polluted lakes, useful in developing effective strategies to manage pollution.

Dissolved trace elements and nutrients in the North Sea-a current baseline.

Primary production is an important driver of marine carbon storage. Besides the major nutrient elements nitrogen, phosphorus, and silicon, primary production also depends on the availability of nutrient-type metals (e.g., Cu, Fe, Mo) and the absence of toxicologically relevant metals (e.g., Ni, Pb). Especially in coastal oceans, carbon storage and export to the open ocean is highly variable and influenced by anthropogenic eutrophication and pollution. To model future changes in coastal carbon storage processes, a solid baseline of nutrient and metal concentrations is crucial. The North Sea is an important shelf sea, influenced by riverine, atmospheric, Baltic Sea, and North Atlantic inputs. We measured the concentrations of dissolved nutrients (NH4+, NO3-, PO43-, and SiO44-) and 26 metals in 337 water samples from various depths within the entire North Sea and Skagerrak. A principal component analysis enabled us to categorize the analytes into three groups according to their predominant behavior: tracers for seawater (e.g., Mo, U, V), recycling (e.g., NO3-, PO43-, SiO44-), and riverine or anthropogenic input (e.g., Ni, Cu, Gd). The results further indicate an increasing P-limitation and increasing anthropogenic gadolinium input into the German Bight.

Unveiling the role of novel carbohydrate-binding modules in laminarin interaction of multimodular proteins from marine Bacteroidota during phytoplankton blooms.

Laminarin, a ß(1,3)-glucan, serves as a storage polysaccharide in marine microalgae such as diatoms. Its abundance, water solubility and simple structure make it an appealing substrate for marine bacteria. Consequently, many marine bacteria have evolved strategies to scavenge and decompose laminarin, employing carbohydrate-binding modules (CBMs) as crucial components. In this study, we characterized two previously unassigned domains as laminarin-binding CBMs in multimodular proteins from the marine bacterium Christiangramia forsetii KT0803T, thereby introducing the new laminarin-binding CBM families CBM102 and CBM103. We identified four CBM102s in a surface glycan-binding protein (SGBP) and a single CBM103 linked to a glycoside hydrolase module from family 16 (GH16_3). Our analysis revealed that both modular proteins have an elongated shape, with GH16_3 exhibiting greater flexibility than SGBP. This flexibility may aid in the recognition and/or degradation of laminarin, while the constraints in SGBP could facilitate the docking of laminarin onto the bacterial surface. Exploration of bacterial metagenome-assembled genomes (MAGs) from phytoplankton blooms in the North Sea showed that both laminarin-binding CBM families are widespread among marine Bacteroidota. The high protein abundance of CBM102- and CBM103-containing proteins during phytoplankton blooms further emphasizes their significance in marine laminarin utilization.

Particulate and water-mobilizable phosphorus from a watershed with a plain river network contributes equal amounts of algal available phosphorus to its downstream lake.

This research was designed to estimate the contributions of phosphorus (P) in different factions from an upstream plain river network to algal growth in a downstream shallow eutrophic lake, Taihu Lake, in China. During three flow regimes, the P fractions in multiple phases (particulate, colloidal and dissolved phases) and their algal availabilities were assessed via bioassays with Dolichospermum flos-aquae as the test organism. The P partitioning patterns among multiple phases were strongly affected by the concentration of total suspended solids (TSS) that changed with the river flow regime, with stronger disturbance of sediments at lower water levels (low flow) and weaker disturbance of sediments at higher water levels (high flow) in the plain river network. The median TSS concentration across the river network decreased from 157.4 mg/L during low flow to 31.8 mg/L during high flow, and the median particulate P concentration decreased from 0.132 mg/L to 0.093 mg/L. The particulate P contributed equally to the amount of algal available P (AAP) as did the water-mobilizable P (colloidal plus dissolved phase) in the rivers flowing into Taihu Lake. The annual average concentrations of particulate algal available P (P-AAP), colloidal algal available P (C-AAP) and dissolved algal available P (D-AAP) were estimated to be 0.032 mg/L, 0.012 mg/L and 0.019 mg/L, respectively, during 2012-2018, accounting for 50.8 %, 19.0 % and 30.2 %, respectively, of the total AAP. At the watershed scale, controlling P drainage from downstream urbanized areas should be emphasized. Additionally, controlling sediment resuspension or reducing the TSS concentration in the inflowing rivers is important for decreasing the particulate P flux to downstream lakes.

Alpha-glucans from bacterial necromass indicate an intra-population loop within the marine carbon cycle.

Phytoplankton blooms provoke bacterioplankton blooms, from which bacterial biomass (necromass) is released via increased zooplankton grazing and viral lysis. While bacterial consumption of algal biomass during blooms is well-studied, little is known about the concurrent recycling of these substantial amounts of bacterial necromass. We demonstrate that bacterial biomass, such as bacterial alpha-glucan storage polysaccharides, generated from the consumption of algal organic matter, is reused and thus itself a major bacterial carbon source in vitro and during a diatom-dominated bloom. We highlight conserved enzymes and binding proteins of dominant bloom-responder clades that are presumably involved in the recycling of bacterial alpha-glucan by members of the bacterial community. We furthermore demonstrate that the corresponding protein machineries can be specifically induced by extracted alpha-glucan-rich bacterial polysaccharide extracts. This recycling of bacterial necromass likely constitutes a large-scale intra-population energy conservation mechanism that keeps substantial amounts of carbon in a dedicated part of the microbial loop.

Reduced precipitation can induce ecosystem regime shifts in lakes by increasing internal nutrient recycling.

Eutrophication is a main threat to continental aquatic ecosystems. Prevention and amelioration actions have been taken under the assumption of a stable climate, which needs reconsideration. Here, we show that reduced precipitation can bring a lake ecosystem to a more productive regime even with a decline in nutrient external load. By analyzing time series of several decades in the largest lake of the Iberian Peninsula, we found autocorrelated changes in the variance of state variables (i.e., chlorophyll and oxygen) indicative of a transient situation towards a new ecosystem regime. Indeed, exceptional planktonic diatom blooms have occurred during the last few years, and the sediment record shows a shift in phytoplankton composition and an increase in nutrient retention. Reduced precipitation almost doubled the water residence time in the lake, enhancing the relevance of internal processes. This study demonstrates that ecological quality targets for aquatic ecosystems must be tailored to the changing climatic conditions for appropriate stewardship.

Succession of particle-attached and free-living bacterial communities in response to microalgal dynamics induced by the biological cyanocide paucibactin A.

Microalgae, including cyanobacteria and eukaryotic algae, are hotspots of primary production and play a critical role in global carbon cycling. However, these species often form blooms that poses a threat to aquatic ecosystems. Although the use of bacteria-derived cyanocides is regarded as an environmentally friendly method for controlling cyanobacterial blooms, only a few studies have examined their potential impact on ecosystems. This study is the first to explore the response of particle-attached (PA) and free-living (FL) bacteria to the dynamics of microalgal communities induced by the biological cyanocide paucibactin A. The microalgal community dynamics were divided into two distinct phases [phase I (days 0-2) and phase II (days 3-7)]. In phase I, paucibactin A caused a sudden decrease in the cyanobacterial concentration. Phase II was characterized by increased growth of eukaryotic microalgae (Scenedesmus, Pediastrum, Selenastrum, and Coelastrum). The stability of the bacterial community and the contribution of stochastic processes to community assembly were more pronounced in phase II than in phase I. The microalgal dynamics triggered by paucibactin A coincided with the succession of the PA and FL bacterial communities. The lysis of cyanobacteria in phase I favored the growth of microbial organic matter degraders in both the PA (e.g., Aeromonas and Rheinheimera) and FL (e.g., Vogesella) bacterial communities. In phase II, Lacibacter, Phycisphaeraceae, and Hydrogenophaga in the PA bacterial community and Lacibacter, Peredibacter, and Prosthecobacter in the FL bacterial community showed increased relative abundances. Overall, the FL bacterial community exhibited greater sensitivity to the two sequential processes compared with the PA bacterial community. These results highlight the need for studies evaluating the impact of biological cyanocides on aquatic ecosystems when used to control natural cyanobacterial blooms.

Deciphering the environmental adaptation and functional trait of core and noncore bacterial communities in impacted coral reef seawater.

Microorganisms play pivotal roles in different biogeochemical cycles within coral reef waters. Nevertheless, our comprehension of the microbially mediated processes following environmental perturbation is still limited. To gain a deeper insight into the environmental adaptation and nutrient cycling, particularly within core and noncore bacterial communities, it is crucial to understand reef ecosystem functioning. In this study, we delved into the microbial community structure and function of seawater in a coral reef under different degrees of anthropogenic disturbance. To achieve this, we harnessed the power of 16S rRNA gene high-throughput sequencing and metagenomics techniques. The results showed that a continuous temporal succession but little spatial heterogeneity in the bacterial communities of core and noncore taxa and functional profiles involved in nitrogen (N) and phosphorus (P) cycling. Eutrophication state (i.e., nutrient concentration and turbidity) and temperature played pivotal roles in shaping both the microbial community composition and functional traits of coral reef seawater. Within this context, the core subcommunity exhibited a remarkably broader habitat niche breadth, stronger phylogenetic signal and lower environmental sensitivity when compared to the noncore taxa. Null model analysis further revealed that the core subcommunity was governed primarily by stochastic processes, while deterministic processes played a more significant role in shaping the noncore subcommunity. Furthermore, our observations indicated that changes in function related to N cycling were correlated to the variations in noncore taxa, while core taxa played a more substantial role in critical processes such as P cycling. Collectively, these findings facilitated our knowledge about environmental adaptability of core and noncore bacterial taxa and shed light on their respective roles in maintaining diverse nutrient cycling within coral reef ecosystems.

Indirect Impact of Eutrophication on Occurrence, Air-Water Exchange, and Vertical Sinking Fluxes of Antibiotics in a Subtropical River.

A significant challenge that warrants attention is the influence of eutrophication on the biogeochemical cycle of emerging contaminants (ECs) in aquatic environments. Antibiotics pollution in the eutrophic Pearl River in South China was examined to offer new insights into the effects of eutrophication on the occurrence, air-water exchange fluxes (Fair-water), and vertical sinking fluxes (Fsinking) of antibiotics. Antibiotics transferred to the atmosphere primarily through aerosolization controlled by phytoplankton biomass and significant spatiotemporal variations were observed in the Fair-water of individual antibiotics throughout all sites and seasons. The Fsinking of ∑AB14 (defined as a summary of 14 antibiotics) was 750.46 ± 283.19, 242.71 ± 122.87, and 346.74 ± 249.52 ng of m-2 d-1 in spring, summer, and winter seasons. Eutrophication indirectly led to an elevated pH, which reduced seasonal Fair-water of antibiotics, sediment aromaticity, and phytoplankton hydrophobicity, thereby decreasing antibiotic accumulation in sediments and phytoplankton. Negative correlations were further found between Fsinking and the water column daily loss of antibiotics with phytoplankton biomass. The novelty of this study is to provide new complementary knowledge for the regulation mechanisms of antibiotics by phytoplankton biological pump, offering novel perspectives and approaches to understanding the coupling between eutrophication and migration and fate of antibiotics in a subtropical eutrophic river.

Temperature and nutrients drive distinct successions between diatoms and dinoflagellates over the past 40 years: Implications for climate warming and eutrophication.

Diatoms and dinoflagellates are two typical functional groups of phytoplankton, playing important roles in ecosystem processes and biogeochemical cycles. Changes in diatoms and dinoflagellates are thought to be one of the possible mechanisms for the increase in harmful algal blooms (HABs), due to changing hydrological conditions associated with climate change and human activities. However, little is known about their ability to adapt to changing ocean environments, thus making it difficult to know whether and how they are adapting. By analyzing a 44-year monitoring dataset in the central Bohai Sea during 1978-2021, we found that the abundance ratio of diatoms to dinoflagellates showed a decreasing trend seasonally and ecologically, indicating that the phytoplankton community underwent distinct successional processes from diatom dominance to diatom-dinoflagellate co-dominance. These processes exhibited varying responses to temperature, nutrient concentrations and ratios, and their interactions, of which temperature primarily drove the seasonal succession whereas nutrients were responsible for the ecological succession. Specifically, diatoms showed a preference for lower temperatures and higher DIP concentrations, and were able to tolerate lower DIN at lower temperatures. In contrast, dinoflagellates tended to prevail at conditions of warming and high N/P ratios. These different traits of diatoms and dinoflagellates reflected the fact that warming as a result of rising temperature and eutrophication as a consequence of nutrient input would favor dinoflagellates over diatoms. Moreover, the increasing dominance of dinoflagellates indicated that dinoflagellate blooms were likely to become more frequent and intense in the central Bohai Sea.

The origins of odor (ß-cyclocitral) under different water nutrient conditions: Algae or submerged plants?

Among the problems caused by water eutrophication, the issue of odor compounds has attracted notable attention. ß-Cyclocitral, a widely distributed and versatile odor compound, is commonly derived from both algae and aquatic plants. Planting aquatic plants is a common method of water purification. However, there is limited study on their impact on ß-cyclocitral levels in water. Here, we conducted a study on the ß-cyclocitral levels in water and the submerged plant leaves under three nutrient levels and six plant density treatments. Our findings revealed the following: (1) Chlorophyll-a (Chla), ß-cyclocitral in the water (Wcyc), ß-cyclocitral in Potamogeton lucens leaves (Pcyc) and the biomass of the submerged plants increase with rising nutrient concentration, which increased about 83 %, 95 %, 450 %, 320 % from eutrophic treatment to oligotrophic treatment, respectively. (2) In water, ß-cyclocitral is influenced not only by algae but also by submerged plants, with primary influencing factors varying across different nutrient levels and plant densities. The main source of ß-cyclocitral in water becomes from plants to algae as the water eutrophication and plant density decrease. (3) As submerged plants have the capability to emit ß-cyclocitral, the release of ß-cyclocitral increases with the density of submerged plants. Hence, when considering planting submerged plants for water purification purposes, it is crucial to carefully manage submerged plant density to mitigate the risk of odor pollution emanating from aquatic plants. This study offers fresh insights into selecting optimal water density for submerged plants and their role in mitigating the release of ß-cyclocitral.

Bloom dynamics under the effects of periodic driving forces.

Phytoplankton bloom received considerable attention for many decades. Different approaches have been used to explain the bloom phenomena. In this paper, we study a Nutrient-Phytoplankton-Zooplankton (NPZ) model consisting of a periodic driving force in the growth rate of phytoplankton due to solar radiation and analyse the dynamics of the corresponding autonomous and non-autonomous systems in different parametric regions. Then we introduce a novel aspect to extend the model by incorporating another periodic driving force into the growth term of the phytoplankton due to sea surface temperature (SST), a key point of innovation. Temperature dependency of the maximum growth rate (µmax) of the phytoplankton is modelled by the well-known Q10 formulation: [Formula: see text] , where µ0 is maximum growth at 0oC. Stability conditions for all three equilibrium points are expressed in terms of the new parameter ρ2, which appears due to the incorporation of periodic driving forces. System dynamics is explored through a detailed bifurcation analysis, both mathematically and numerically, with respect to the light and temperature dependent phytoplankton growth response. Bloom phenomenon is explained by the saddle point bloom mechanism even when the co-existing equilibrium point does not exist for some values of ρ2. Solar radiation and SST are modelled using sinusoidal functions constructed from satellite data. Our results of the proposed model describe the initiation of the phytoplankton bloom better than an existing model for the region 25-35° W, 40-45° N of the North Atlantic Ocean. An improvement of 14 days (approximately) is observed in the bloom initiation time. The rate of change method (ROC) is applied to predict the bloom initiation.

Insights into cyanobacterial blooms through the lens of omics.

Cyanobacteria are oxygen-producing photosynthetic bacteria that convert carbon dioxide into biomass upon exposure to sunlight. However, favorable conditions cause harmful cyanobacterial blooms (HCBs), which are the dense accumulation of biomass at the water surface or subsurface, posing threats to freshwater ecosystems and human health. Understanding the mechanisms underlying cyanobacterial bloom formation is crucial for effective management. In this regard, recent advancements in omics technologies have provided valuable insights into HCBs, which have raised expectations to develop more effective control methods in the near future. This literature review aims to present the genomic architecture, adaptive mechanisms, microbial interactions, and ecological impacts of HCBs through the lens of omics. Genomic analysis indicates that the genome plasticity of cyanobacteria has enabled their resilience and effective adaptation to environmental changes. Transcriptomic investigations have revealed that cyanobacteria use various strategies for adapting to environmental stress. Additionally, metagenomic and metatranscriptomic analyses have emphasized the significant role of the microbial community in regulating HCBs. Finally, we offer perspectives on potential opportunities for further research in this field.

Assessment of in-situ monitoring and tracking the vertical migration of cyanobacterial blooms using LISST-HAB.

Cyanobacterial harmful algal blooms (cyanoHABs) are becoming increasingly common in aquatic ecosystems worldwide. However, their heterogeneous distributions make it difficult to accurately estimate the total algae biomass and forecast the occurrence of surface cyanoHABs by using traditional monitoring methods. Although various optical instruments and remote sensing methods have been employed to monitor the dynamics of cyanoHABs at the water surface (i.e., bloom area, chlorophyll a), there is no effective in-situ methodology to monitor the dynamic change of cell density and integrated biovolume of algae throughout the water column. In this study, we propose a quantitative protocol for simultaneously measurements of multiple indicators (i.e., biovolume concentration, size distribution, cell density, and column-integrated biovolume) of cyanoHABs in water bodies by using the laser in-situ scattering and transmissometry (LISST) instrument. The accuracy of measurements of the biovolume and colony size of algae was evaluated and exceeded 95% when the water bloom was dominated by cyanobacteria. Furthermore, the cell density of cyanobacteria was well estimated based on total biovolume and mean cell volume measured by the instrument. Therefore, this methodology has the potential to be used for broader applications, not only to monitor the spatial and temporal distribution of algal biovolume concentration but also monitor the vertical distribution of cell density, biomass and their relationship with size distribution patterns. This provides new technical means for the monitoring and analysis of algae migration and early warning of the formation of cyanoHABs in lakes and reservoirs.

Functional trait-based phytoplankton biomass and assemblage analyses in the pre-growing season for comprehensive algal bloom risk assessment.

Algal bloom (AB) risk assessment is critical for maintaining ecosystem health and human sustainability. Previous AB risk assessments have focused on the potential occurrence of ABs and related factors in the growing season, whereas their hazards, especially in the pre-growing season, have attracted less attention. Here, we performed a comprehensive AB risk assessment, including water trophic levels, phytoplankton biomass, functional trait-based assemblages, and related environmental factors, in the pre-growing season in Dongting Lake, China. Although mesotrophic water and low phytoplankton biomass suggested low AB potential, toxic taxa, which constituted 13.28% of the phytoplankton biomass, indicated non-negligible AB hazards. NH4+ and water temperature were key factors affecting phytoplankton motility and toxicity. Our study establishes a new paradigm for quantitative AB risk assessment, including both potential AB occurrence and hazards. We emphasize the importance of phytoplankton functional traits for early AB warning and NH4+ reduction for AB control in the pre-growing season.

Responses to phytoplankton community succession and expression of key functional genes in plateau lakes under 17ß-estradiol interference.

Steroid estrogens (SEs) have garnered global attention because of their potential hazards to human health and aquatic organisms at low concentrations (ng/L). The ecosystems of plateau freshwater lakes are fragile, the water lag time is long, and pollutants easily accumulate, making them more vulnerable to the impact of SEs. However, the knowledge of the impact of SEs on the growth and decomposition of phytoplankton communities in plateau lakes and the eutrophication process is limited. This study investigated the effects and mechanisms of SEs exposure on dominant algal communities and the expression of typical algal functional genes in Erhai Lake using indoor simulations and molecular biological methods. The results showed that phytoplankton were sensitive to 17ß-estradiol (E2ß) pollution, with a concentration of 50, and 100 ng/L E2ß exposure promoting the growth of cyanophyta and chlorophyta in the short term; this poses an ecological risk of inducing algal blooms. E2ß of 1000 ng/L exposure led to cross-effects of estrogenic effects and toxicity, with most phytoplankton being inhibited. However, small filamentous cyanobacteria and diatoms exhibited greater tolerance; Melosira sp. even exhibited "low inhibition, high promotion" behavior. Exposure to E2ß reduced the Shannon-Wiener diversity index (H'), Pielou index (J), and the number of dominant algal species (S) in phytoplankton communities, leading to instability in community succession. E2ß of 50 ng/L enhanced the expression levels of relevant functional genes, such as ftsH, psaB, atpB, and prx, related to Microcystis aeruginosa. E2ß of 50 ng/L and 5 mg/L can promote the transcription of Microcystis toxins (MC) related genes (mcyA), leading to more MC production by algal cells.

Spatial and Temporal Resolution of Cyanobacterial Bloom Chemistry Reveals an Open-Ocean Trichodesmium thiebautii as a Talented Producer of Specialized Metabolites.

While the ecological role that Trichodesmium sp. play in nitrogen fixation has been widely studied, little information is available on potential specialized metabolites that are associated with blooms and standing stock Trichodesmium colonies. While a collection of biological material from a T. thiebautii bloom event from North Padre Island, Texas, in 2014 indicated that this species was a prolific producer of chlorinated specialized metabolites, additional spatial and temporal resolution was needed. We have completed these metabolite comparison studies, detailed in the current report, utilizing LC-MS/MS-based molecular networking to visualize and annotate the specialized metabolite composition of these Trichodesmium blooms and colonies in the Gulf of Mexico (GoM) and other waters. Our results showed that T. thiebautii blooms and colonies found in the GoM have a remarkably consistent specialized metabolome. Additionally, we isolated and characterized one new macrocyclic compound from T. thiebautii, trichothilone A (1), which was also detected in three independent cultures of T. erythraeum. Genome mining identified genes predicted to synthesize certain functional groups in the T. thiebautii metabolites. These results provoke intriguing questions of how these specialized metabolites affect Trichodesmium ecophysiology, symbioses with marine invertebrates, and niche development in the global oligotrophic ocean.

Spatio-temporal distribution of sea surface chlorophyll-a in coral reefs of the South China Sea over the past decade based on Landsat-8 Operational Land Images.

The concentration of chlorophyll-a (Chl-a) in seawater reflects phytoplankton growth and water eutrophication, which are usually assessed for evaluation of primary productivity and carbon source/sink of coral reefs. However, the precise delineation of Chl-a concentration in coral reefs remains a challenge when ocean satellites with low spatial resolution are utilized. In this study, a remote sensing inversion model for Chl-a was developed in fringing reefs (R2 = 0.76, RMSE =0.41 µg/L, MRE = 14 %) and atolls (R2 = 0.79, RMSE =0.02 µg/L, MRE = 8 %), utilizing reflectance data from the sensitive band of the Landsat-8 Operational Land Imagers (OLI) with a spatial resolution of 30 m. The aforementioned model was utilized to invert high-resolution distribution maps of Chl-a concentration in six major coral reef regions of the South China Sea from 2013 to 2022 and subsequently used to analyze the variations in Chl-a concentration and its influencing factors. The results indicate a Chl-a concentration gradient among coral reefs Daya Bay, Weizhou Island, Luhuitou, Xuwen, Huangyan Island, and Xisha Island in that order. The Chl-a concentration in coral reefs exhibited an overall increasing trend, with significant seasonal fluctuations, characterized by higher concentrations during winter and spring and lower concentrations during summer and autumn. The concentration of Chl-a in coral reefs was positively correlated with the average wind speed.

Phycospheric bacteria limits the effect of nitrogen and phosphorus imbalance on diatom bloom.

Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.