[Research advances in communication interactions among the symbionts of "bacteria-zooxanthellae-coral"]. / çŠç‘šå ±ç”Ÿä½“ä¸­"细菌-虫黄藻-宿主"ä¸‰è§’å ³ç³»çš„é€šè®¯äº¤æµ.

"Bacteria-zooxanthellae-coral" is a pair of typical triangular relationships in the marine ecosystem. There are complex flows of material, information, and energy in this system. The balance and stability of the symbionts is an important guarantee for maintaining the health of coral reef ecosystems. Many studies have been conducted on the interaction of coral symbionts in the past 20 years, which help clarify the material metabolism and nutrient exchange between "bacteria-zooxanthellae-coral" and their interaction with the environment. Due to the complexity of this symbiotic system, the mechanisms of some phenomena are still not well understood, especially for the communication among the symbionts. The interaction mediated by signal molecules is the internal driving force for the homeostatic maintenance and efficient operation of coral symbionts. In this review, we tried to summarize the latest research progress by focusing on the chemical signaling molecules in coral symbiotic system, including the communications between the bacteria and bacteria, bacteria and corals, bacteria and zooxanthellae, and zooxanthellae and corals. The main signals molecules include quorum sensing (QS) molecules, dimethylsulfoniopropionate (DMSP), glycans signals, lipid signals, and the noncoding RNAs. We focused on the functional mode and ecological significance of signal molecules in symbionts, and selectively exemplified microbial cooperation and competition mediated by QS signals, the interaction between bacteria and corals under the regulation of DMSP, and the response process of corals and zooxanthellae to noncoding RNAs under environmental stresses. We proposed the future research focus and possible directions, including the expansion of research dimensions, the application of new technologies and new methods, and the construction of ecological models. This work would help improve the understanding of interactions between "bacteria-zooxanthellae-coral". The exploration about the ways based on communication language would provide new ideas for the restoration and protection of coral reef ecosystems.

The Microbial Mechanisms of a Novel Photosensitive Material (Treated Rape Pollen) in Anti-Biofilm Process under Marine Environment.

Marine biofouling is a worldwide problem in coastal areas and affects the maritime industry primarily by attachment of fouling organisms to solid immersed surfaces. Biofilm formation by microbes is the main cause of biofouling. Currently, application of antibacterial materials is an important strategy for preventing bacterial colonization and biofilm formation. A natural three-dimensional carbon skeleton material, TRP (treated rape pollen), attracted our attention owing to its visible-light-driven photocatalytic disinfection property. Based on this, we hypothesized that TRP, which is eco-friendly, would show antifouling performance and could be used for marine antifouling. We then assessed its physiochemical characteristics, oxidant potential, and antifouling ability. The results showed that TRP had excellent photosensitivity and oxidant ability, as well as strong anti-bacterial colonization capability under light-driven conditions. Confocal laser scanning microscopy showed that TRP could disperse pre-established biofilms on stainless steel surfaces in natural seawater. The biodiversity and taxonomic composition of biofilms were significantly altered by TRP (p < 0.05). Moreover, metagenomics analysis showed that functional classes involved in the antioxidant system, environmental stress, glucose−lipid metabolism, and membrane-associated functions were changed after TRP exposure. Co-occurrence model analysis further revealed that TRP markedly increased the complexity of the biofilm microbial network under light irradiation. Taken together, these results demonstrate that TRP with light irradiation can inhibit bacterial colonization and prevent initial biofilm formation. Thus, TRP is a potential nature-based green material for marine antifouling.

Identification of a RelA/SpoT Homolog and Its Possible Role in the Accumulation of Astaxanthin in Haematococcus pluvialis.

A RelA/SpoT homolog, HpRSH, was identified in Haematococcus pluvialis. HpRSH was found to catalyze Mg2+-dependent guanosine tetraphosphate (ppGpp) synthesis and Mn2+-dependent ppGpp hydrolysis, respectively. The transcription of HpRSH was significantly upregulated by environmental stresses, such as darkness, high light, nitrogen limitation, and salinity stress. The intracellular ppGpp level was also increased when exposed to these stresses. In addition, the classical initiator of stringent response, serine hydroxamate (SHX), was found to upregulate the transcription of HpRSH and increase the level of ppGpp. Moreover, stringent response induced by SHX or environmental stresses was proven to induce the accumulation of astaxanthin. These results indicated that stringent response regulatory system involved in the regulation of astaxanthin biosynthesis in H. pluvialis. Furthermore, stringent response was unable to induce astaxanthin accumulation under dark condition. This result implied that stringent response may regulate astaxanthin biosynthesis in a light-dependent manner.

Bisphenol A biodegradation by Sphingonomas sp. YK5 is regulated by acyl-homoserine lactone signaling molecules.

Microbial degradation is an effective approach for the removal of Bisphenol A (BPA). During the biodegradation process, quorum sensing (QS) is a phenomenon that enables bacteria to coordinate collective behaviors based on cell density-dependent chemical signals. However, whether the degradation of BPA can be facilitated by this QS system (such as acyl-homoserine lactone, AHL) is unclear. To answer this question, the bifunctional Sphingonomas sp. strain YK5 that had BPA-degrading and AHL-producing properties was used. Biochemical analysis revealed that this bacterial strain mainly produced C8-HSL signals. Gene knockout experiments indicated that the AHL-system (LuxI1/LuxI2) was required for efficient BPA degradation. RT-PCR analyses revealed that the AHL system positively regulated the relative expression of genes (bisdA, CYP450, hapA, ligAB, and proB) involved in BPA degradation. Given that AHL signaling may be a common trait among BPA-degrading microorganisms and AHL system can regulate the degradation activity, manipulation of this system may be a valuable strategy to control BPA biodegradation.

The structure and assembly mechanisms of plastisphere microbial community in natural marine environment.

The microbial colonization profiles on microplastics (MPs) in marine environments have recently sparked global interest. However, many studies have characterized plastisphere microbiomes without considering the ecological processes that underly microbiome assembly. Here, we carried out a three-timepoint exposure experiment at 1-, 4-, and 8-week and investigated the colonization dynamics for polyethylene, polypropylene, polystyrene, polyvinyl chloride, and acrylonitrile-butadiene-styrene MP pellets in natural coastal water. Using high-throughput sequencing of 16S rRNA, we found diversity and evenness were higher (p < 0.05) in the plastisphere communities than those in seawater, and microorganisms colonizing were co-influenced by environmental factors, polymer types, and exposure duration. Functional potential and co-occurrence network analysis revealed that MP exposure enriched the xenobiotic biodegradation potential and reduced the complexity of the MP microbial network. Simultaneously, null-model analyses indicated that stochastic processes contributed a bigger role than deterministic processes in shaping plastisphere microbial community structure with dispersal limitations contributing to a greater extent to microbial succession trajectories. These results implied the plastic surface had a more important role as a raft onto which microbes attach rather than selectively recruiting plastic-specific microbial colonizers. Our work strengthened the understanding of the ecological mechanisms by which microbial community patterns are controlled during colonization by plastic-associated microbes.

Oyster Biodeposition Alleviates Sediment Nutrient Overload: A Case Study at Shenzhen Bay, China.

Oysters are ecological engineers, and previous studies have examined their role as competent facilitators of ecological restoration. However, the decisive role of oysters in the aquatic environment is still debatable because oyster biodeposition (OBD) may also increase the nutrients enriched in sediments. In order to better interpret this problem, we sampled sediment cores from representative oyster culture areas and uncultured areas in Shenzhen Bay. The results have shown that the TOC (total organic carbon) and TN (total nitrogen) decreased significantly (p < 0.05) at the surface sediment layer (0-20-cm deep) and the sediment layer (20-40-cm deep) of the oyster site compared with the reference site. The decreased TOC and TN were also observed at 60- to 100-cm sediment depth in the oyster site. This indicated that the OBD significantly impacted the concentration of TOC and TN in the sediment. To confirm the alleviative role of OBD, we conducted stable isotope (δ13C and δ15N) analyses, which further demonstrated the presence of heavier and less lighter forms of organic carbon and nitrogen sediment. The surface sediment layer (0-20 cm) at the oyster site showed 8% more δ13C‰ compared with the control site (p < 0.05), reflecting the reduction in the TOC. In order to reveal the potential microbial mechanisms involved in OBD, we performed a functional analysis using the Geochip5 advanced microarray technology. Regarding carbon metabolism, we observed that genes (encoding pullulanase, glucoamylase, exoglucanase, cellobiase, and xylanase) involved in the degradation of relatively labile C-based molecules (e.g., starch, cellulose, and hemicellulose) were highly represented in an experimental area (p < 0.05). In addition, microbes in the experimental area exhibited a greater capacity for degrading recalcitrant C (e.g., lignin), which involves glyoxal oxidase (glx), manganese peroxidase (mnp), and phenol oxidase. Among the genes controlling nitrogen metabolism, the genes involved in denitrification, assimilation, ammonification, and nitrification were differentially expressed compared with the control area. These results indicated that microbial metabolic roles might have enhanced the C/N-flux speed and reduced the overall nutrient status. We concluded that OBD alleviates sediment nutrient overload under oyster farming from a microbial ecological perspective in a rapidly urbanized coastal area.

Transcriptome analysis expands the potential roles of quorum sensing in biodegradation and physiological responses to microcystin.

Bacterial degradation is one of the most efficient ways to remove microcystins (MCs), the most frequently detected toxin in cyanobacterial blooms. Using Novosphingobium sp. ERW19 as a representative strain, our laboratory previously demonstrated that quorum sensing (QS), the cell density-dependent gene regulation system, positively regulates biodegradation of MCs via transcriptional activation of mlr-pathway-associated genes. Increasing evidence indicates that QS is involved in a wide spectrum of regulatory circuits, but it remains unclear which physiological processes in MC degradation besides the expression of MC-degrading genes are also subject to QS-dependent regulation. This study used transcriptome analysis to identify QS-regulated genes during degradation of MCs. A large percentage (up to 32.6%) of the genome of the MC-degrading bacterial strain Novosphingobium sp. ERW19 was significantly differentially expressed in the corresponding QS mutants. Pathway enrichment analysis of QS-regulated genes revealed that QS mainly influenced metabolism-associated pathways, particularly those related to amino acid metabolism, carbohydrate metabolism, and biodegradation and metabolism of xenobiotics. In-depth functional interpretation of QS-regulated genes indicated a variety of pathways were potentially associated with bacterial degradation or physiological responses to MCs, including genes involved in cell motility, cytochrome P450-dependent metabolism of xenobiotics, glutathione S-transferase (GST), envelope stress response, and ribosomes. Furthermore, QS may be involved in regulating the initial and final steps of the catabolic pathway of phenylacetic acid, an intermediate product of MC degradation. Collectively, this global survey of QS-regulated genes in a MC-degrading bacterial strain facilitates a deeper understanding of QS-controlled processes that may be important for bacterial degradation of MCs or may contribute to the physiological responses of bacteria to MCs.

The communities and functional profiles of virioplankton along a salinity gradient in a subtropical estuary.

Viruses are the major drivers shaping microorganismal communities, and impact marine biogeochemical cycling. They are affected by various environmental parameters, such as salinity. Although the spatiotemporal distribution and dynamics of virioplankton have been extensively studied in saline environments, few detailed studies of community structure and function of viruses along salinity gradients have been conducted. Here, we used the 16S and 18S rRNA gene amplicon and metagenomic sequencing from a subtropical estuary (Pearl River Estuary, PRE; located in Shenzhen, Guangdong Province, China) to explore how viral community composition and function vary along a salinity gradient. Results showed that the detected viruses were mainly bacteriophages. The double-stranded DNA viruses were the most abundant (especially Siphoviridae, Myoviridae, Mimiviridae, Phycodnaviridae, and Podoviridae), followed by a small number of single-stranded DNA (Circoviridae) and RNA (Retroviridae) viruses. Viral biodiversity significantly declined and community structure varied greatly along the salinity gradient. The salinity, ammonium and dissolved oxygen were dominated factors influencing the community composition of viruses. Association network analysis showed that viruses had a negative effect on multiple host taxa (prokaryotic and eukaryotic species). Metagenomic data revealed that the main viral functional potential was involved in organic matter metabolism by carbohydrate-active enzymes (CAZymes). Deeper comparative functional analyses showed that viruses in the low-salinity environment had more carbohydrate-binding module and glycosidase hydrolases activities than those under high-salinity conditions. However, an opposite pattern was observed for carbohydrate esterases. These results suggest that virus-encoded CAZyme genes may alter the bacterial metabolism in estuaries. Overall, our results demonstrate that there is a spatial heterogeneity in the composition and function of virioplankton along a salinity gradient. This study enhances our understanding of viral distribution and their contribution to regulating carbon degradation throughout environments with varying salinities in subtropical estuaries.

Stringent Response Regulates Stress Resistance in Cyanobacterium Microcystis aeruginosa.

Cyanobacterial blooms are serious environmental issues in global freshwater ecosystems. Nitrogen limitation is one of the most important strategies to control cyanobacterial blooms. However, recent researches showed that N limitation does not effectively control the bloom; oppositely, N limitation induces N-fixing cyanobacterial blooms. The mechanism underlying this ecological event is elusive. In this study, we found that N limitation enhances stress tolerance of Microcystis aeruginosa by triggering stringent response (SR), one of the most important bacterial adaptive responses to environmental stresses. Initiation of SR exerted protective effects on the cells against salt and oxidative stresses by promoting colony formation, maintaining membrane integrity, increasing photosynthetic performance, reducing ROS production, upregulating stress-related genes, etc. These protections possibly help M. aeruginosa maintain their population number during seasonal N limitation. As SR has been proven to be involved in nitrogen fixing under N limitation conditions, the potential role of SR in driving the shift and succession of cyanobacterial blooms was discussed. Our findings provide cellular evidence and possible mechanisms that reducing N input is ineffective for bloom control.

Temporal heterogeneity of microbial communities and metabolic activities during a natural algal bloom.

Elucidating the interactions between algae and associated microbial communities is critical for understanding the mechanisms that mediate the dynamic of harmful algal blooms (HABs) in marine environment. However, the microbial functional profiles and their biogeochemical potential in HABs process remains elusive, especially during a complete natural HAB cycle. Here, we used pyrosequencing and functional gene array (GeoChip) to investigate microbial community dynamics and metabolic potential during a natural dinoflagellate (Noctiluca scintillans) bloom. The results shown that bacterioplankton exhibited significant temporal heterogeneity over the course of the bloom stages. Microbial succession was co-driven by environmental parameters and biotic interactions. The functional analysis revealed significant variations in microbial metabolism during matter cycling. At bloom onset-stage, metabolic potential associated with iron oxidation and transport was elevated. Carbon fixation and degradation, denitrification, phosphorus acquisition, and sulfur transfer/oxidation were significantly enhanced at the plateau stage. During the decline and terminal stages, oxidative stress, lysis of compounds, and toxin degradation & protease synthesis increased. This work reveal phycosphere microorganisms can enhanced organic C decomposition capacity, altered N assimilation rate and S/P turnover efficiency, and balancing of the Fe budget during HAB process. The ecological linkage analysis has further shown that microbial composition and functional potential were significantly linked to algal blooms occurrence. It suggest that structural variability and functional plasticity of microbial communities influence HAB trajectory.

Two hierarchical LuxR-LuxI type quorum sensing systems in Novosphingobium activate microcystin degradation through transcriptional regulation of the mlr pathway.

Microcystins (MCs) are the most common cyanotoxins produced by harmful cyanobacterial blooms and pose an increasing global threat to human health and ecosystems. Microbial degradation represents an efficient and sustainable approach for the removal of MCs. Although the enzymatic pathway for biodegradation of MCs has been characterized, the regulatory mechanisms underlying the degradation processes remain unclear. Quorum sensing (QS) is a cell-density-dependent regulatory mechanism that enables bacteria to orchestrate collective behaviors. The acyl-homoserine lactone (AHL)-mediated QS system regulates the biodegradation of many organic pollutants. However, it is not known whether this QS system is involved in the degradation of MCs. This study aimed to fill this knowledge gap. In this study, the proportion of culturable AHL-producers increased significantly after enrichment of MCs, and AHL-based QS systems were present in all genome-sequenced MC-degrading strains, supporting the hypothesis that QS participates in the degradation of MCs. Two bifunctional Novosphingobium strains (with MC-degrading and AHL-producing abilities) were isolated using a novel primer pair targeting mlrA, the marker gene of mlr degradation pathway. Biochemical and genetic analysis revealed that the MC-degrading bacterium Novosphingobium sp. ERW19 encodes two hierarchical regulatory QS systems designated novR1/novI1 and novR2/novI2. Gene knockout and complementation experiments indicated that both systems were required for efficient degradation of MCs. Transcriptomic analyses revealed that the QS systems positively regulate degradation of MCs through transcriptional activation of MC-degrading genes, especially mlrA. Given that QS may be a common trait within mlr pathway-dependent MC-degrading bacterial strains and the degradation activity is directly regulated by QS, manipulation of the QS systems may be a promising strategy to control biodegradation of MCs.

[Research progress and ecological roles of phytoplankton cysts]. / æµ®æ¸¸æ¤ç‰©å­¢å›Šçš„ç ”ç©¶è¿›å±•ä¸Žç”Ÿæ€æ„ä¹‰.

Phytoplankton have a complex life-cycle. Resting cysts are formed like terrestrial plants. Cysts play important roles in resisting environmental pressure, surviving adverse environment, maintaining physiological homeostasis, and maintaining species reproduction. In some ecological events, cysts were served as "seed bank" for periodical algal blooming. However, compared with the mature cells, the ecophysiological research on cysts is relatively limited. To deepen our understanding on life-cycle and ecological roles of cysts, we reviewed the researches on cysts in the past two decades. Firstly, we outlined the types (resting cysts, zoocysts, mature cysts) and factors that affected the formation of cysts of typical dinoflagellates and diatoms. Secondly, we summarized the formation mechanism, physiological characteristics and germination factors of cysts. Finally, we discussed the significance of cysts in algal evolution and in ocean carbon cycle. This review, summarizing the adaptability and plasticity of cysts, would shed light on ecological mechanism of algae survival strategy.

Opportunistic bacteria use quorum sensing to disturb coral symbiotic communities and mediate the occurrence of coral bleaching.

Coral associated microorganisms, especially some opportunistic pathogens can utilize quorum-sensing (QS) signals to affect population structure and host health. However, direct evidence about the link between coral bleaching and dysbiotic microbiomes under QS regulation was lacking. Here, using 11 opportunistic bacteria and their QS products (AHLs, acyl-homoserine-lactones), we exposed Pocillopora damicornis to three different treatments: test groups (A and B: mixture of AHLs-producing bacteria and cocktail of AHLs signals respectively); control groups (C and D: group A and B with furanone added respectively); and a blank control (group E: only seawater) for 21 days. The results showed that remarkable bleaching phenomenon was observed in groups A and B. The operational taxonomic units-sequencing analysis shown that the bacterial network interactions and communities composition were significantly changed, becoming especially enhanced in the relative abundances of Vibrio, Edwardsiella, Enterobacter, Pseudomonas, and Aeromonas. Interestingly, the control groups (C and D) were found to have a limited influence upon host microbial composition and reduced bleaching susceptibility of P. damicornis. These results indicate bleaching's initiation and progression may be caused by opportunistic bacteria of resident microbes in a process under regulation by AHLs. These findings add a new dimension to our understanding of the complexity of bleaching mechanisms from a chemoecological perspective.

Functional profiles of phycospheric microorganisms during a marine dinoflagellate bloom.

Harmful algal blooms (HABs) are an ecological concern but relatively few studies have investigated the functional potential of bacterioplankton over a complete algal bloom cycle, which is critical for determining their contribution to the fate of algal blooms. To address this point, we carried out a time-series metagenomic analysis of the functional features of microbial communities at three different Gymnodinium catenatum bloom stages (pre-, peak-, and post-bloom). Different microbial composition were observed during the blooming stages. The environmental parameters and correlation networks co-contribute to microbial variability, and the former explained 38.4% of total variations of the bacterioplankton community composition. Functionally, a range of pathways involved in carbon, nitrogen, phosphorus and sulfur cycling were significantly different during the various HAB stages. Genes associated with carbohydrate-active enzymes, denitrification, and iron oxidation were enriched at the pre-bloom stage; genes involved in reductive citrate cycle for carbon fixation, carbon degradation, nitrification and phosphate transport were enhanced at the peak stage; and relative gene abundance related to sulfur oxidation, vitamin synthesis, and iron transport and storage was increased at the post-bloom stage. The ecological linkage analysis has shown that microbial functional potential especially the C/P/Fe metabolism were significantly linked to the fate of the algal blooms. Taken together, our results demonstrated that microorganisms displayed successional patterns not only at the community level, but also in the metabolic potential on HAB's progression. This work contributes to a growing understanding of microbial structural elasticity and functional plasticity and shed light on the potential mechanisms of microbial-mediated HAB trajectory.

Temporal Variability of Virioplankton during a Gymnodinium catenatum Algal Bloom.

Viruses are key biogeochemical engines in the regulation of the dynamics of phytoplankton. However, there has been little research on viral communities in relation to algal blooms. Using the virMine tool, we analyzed viral information from metagenomic data of field dinoflagellate (Gymnodinium catenatum) blooms at different stages. Species identification indicated that phages were the main species. Unifrac analysis showed clear temporal patterns in virioplankton dynamics. The viral community was dominated by Siphoviridae, Podoviridae, and Myoviridae throughout the whole bloom cycle. However, some changes were observed at different phases of the bloom; the relatively abundant Siphoviridae and Myoviridae dominated at pre-bloom and peak bloom stages, while at the post-bloom stage, the members of Phycodnaviridae and Microviridae were more abundant. Temperature and nutrients were the main contributors to the dynamic structure of the viral community. Some obvious correlations were found between dominant viral species and host biomass. Functional analysis indicated some functional genes had dramatic response in algal-associated viral assemblages, especially the CAZyme encoding genes. This work expands the existing knowledge of algal-associated viruses by characterizing viral composition and function across a complete algal bloom cycle. Our data provide supporting evidence that viruses participate in dinoflagellate bloom dynamics under natural conditions.

Complete genome sequence of Acinetobacter baumanni J1, a quorum sensing-producing algicidal bacterium, isolated from Eastern Pacific Ocean.

The Acinetobacter baumanni J1 isolated from surface water of the Eastern Pacific Ocean, demonstrated significant algicidal activity on the algae Alexandrium tamarense. Interestingly, this strain showed the ability to produce an acyl-homoserine lactone (AHL) quorum sensing molecule. To better understand its AHL producing mechanism and its ecological functions, the genome of A. baumanni strain J1 was completely sequenced. The genome contained a circular chromosome of 3,948,465 bp with an average GC content of 39.9 mol%. A total of 3707 protein coding genes, 41 tRNA genes and 16 rRNA genes were obtained. In silico genome annotation identified a LuxI putative gene located on contig 4. Subsequent thin-layer chromatography analysis indicated that C8-AHL could be produced by A. baumanni J1, which confirmed the authenticity of the LuxI gene. Taken together, this work describes an algicidal bacterium that is capable of producing an AHL molecule, which may represent a valuable tool for developing microbial methods to control harmful algae.

Transcriptomic Profile and Sexual Reproduction-Relevant Genes of Alexandrium minutum in Response to Nutritional Deficiency.

Alexandrium minutum is a typical marine toxic dinoflagellate responsible for producing paralytic shellfish poisoning (PSP) toxins. Until now, we know little about the genomic information of A. minutum, so a transcriptome study was conducted to clarify the physiological adaptations related to nutritional deficiency. Here, we performed RNA-Seq analysis to assess the gene expression patterns of A. minutum under N and P deficient conditions for 0 (control), 6, and 72 h. Main differences between the control and experimental groups were observed in hydrolase activity and fatty acid, lipid, protein, and P metabolism. Activities of photosystem I (PSI) and PSII were significantly down-regulated, and the endocytosis pathway (clathrin-dependent endocytosis) was significantly enriched under N and P stress compared with the control, indicating that A. minutum shifts its trophy pattern under N and P stress. We also identified several unigenes related to the process of sexual reproduction, including sex determination, sperm-egg recognition, sex differentiation, mating, and fertilization. Approximately 50% of the successfully annotated unigenes were differentially expressed between the short-term stimulated sample (6 h) and control (R). However, the expression level of most unigenes returned to normal levels after 72 h, indicating that N and P stress plays a limited role in the induction of sexual reproduction. Furthermore, the quantitative real-time PCR (qRT-PCR) results of the five representative sex-related unigenes were consistent with sequencing data, which confirmed the authenticity of transcriptomic analysis. Also, qRT-PCR analysis showed that the long and short form transcripts of the saxitoxin biosynthesis gene (sxtA) were down-regulated under the nutrient deficient condition compared with the control, indicating that N and P stress regulates sxtA expression. Overall, transcriptome analysis of A. minutum revealed that N and P deficiency induced responses associated with stress response, photosynthetic efficiency, toxin biosynthesis, and sexual reproduction. Our data indicate that algae change their trophic modes (to facultative mixotrophy) and related physiological reactions under stress conditions; this possibly represents an ecological adaption strategy in the algal life cycle.

Phycosphere Microbial Succession Patterns and Assembly Mechanisms in a Marine Dinoflagellate Bloom.

Given the ecological significance of microorganisms in algal blooming events, it is critical to understand the mechanisms regarding their distribution under different conditions. We tested the hypothesis that microbial community succession is strongly associated with algal bloom stages, and that the assembly mechanisms are cocontrolled by deterministic and stochastic processes. Community structures and underlying ecological processes of microbial populations (attached and free-living bacteria) at three algal bloom stages (pre-, during, and postbloom) over a complete dinoflagellate Scrippsiella trochoidea bloom were investigated. Both attached and free-living taxa had a strong response to the bloom event, and the latter was more sensitive than the former. The contribution of environmental parameters to microbial variability was 40.2%. Interaction analysis showed that complex positive or negative correlation networks exist in phycosphere microbes. These relationships were the potential drivers of mutualist and competitive interactions that impacted bacterial succession. Null model analysis showed that the attached bacterial community primarily exhibited deterministic processes at pre- and during-bloom stages, while dispersal-related processes contributed to a greater extent at the postbloom stage. In the free-living bacterial community, homogeneous selection and dispersal limitation dominated in the initial phase, which gave way to more deterministic processes at the two later stages. Relative contribution analyses further demonstrated that the community turnover of attached bacteria was mainly driven by environmental selection, while stochastic factors had partial effects on the assembly of free-living bacteria. Taken together, these data demonstrated that a robust link exists between bacterioplankton community structure and bloom progression, and phycosphere microbial succession trajectories are cogoverned by both deterministic and random processes.IMPORTANCE Disentangling the mechanisms shaping bacterioplankton communities during a marine ecological event is a core concern for ecologists. Harmful algal bloom (HAB) is a typical ecological disaster, and its formation is significantly influenced by alga-bacterium interactions. Microbial community shifts during the HAB process are relatively well known. However, the assembly processes of microbial communities in an HAB are not fully understood, especially the relative influences of deterministic and stochastic processes. We therefore analyzed the relative contributions of deterministic and stochastic processes during an HAB event. Both free-living and attached bacterial groups had a dramatic response to the HAB, and the relative importance of determinism versus stochasticity varied between the two bacterial groups at various bloom stages. Environmental factors and biotic interactions were the main drivers impacting the microbial shift process. Our results strengthen the understanding of the ecological mechanisms controlling microbial community patterns during the HAB process.

Optimization of extraction solvents, solid phase extraction and decoupling for quantitation of free isoprenoid diphosphates in Haematococcus pluvialis by liquid chromatography with tandem mass spectrometry.

Isoprenoid diphosphates are important precursors actively participating in many downstream metabolisms; they are often in modified forms, e.g., protein-coupled or esterified form. Therefore, in vivo level of free isoprenoid diphosphates is quite low, ˜0.07 nmol/g fresh weight in plants. In order to directly measure the isoprenoid diphosphate pool during stress-induced accumulation of astaxanthin in Haematococcus pluvialis, the present study optimized several pretreatment procedures to enrich free isoprenoid diphosphates for high-pressure liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) detection. Specifically, different extraction solvents, e.g., water, methanol, chloroform, and mixture of water, methanol, and chloroform (1:1:1, V/V/V), and solid phase extraction (SPE) columns (OASIS@ WAX and HLB Cartridges) were compared; and gentle decoupling by NaOH or trifluoroacetic acid (TFA) was introduced to release free isoprenoid diphosphates. Results found that solvent mixture of water, methanol and chloroform (1:1:1, V/V/V) showed the highest extraction efficiency (RE) for five isoprenoid diphosphates, ranging from 76.83% to 92.43%; HLB column showed the balanced recoveries ranging from 75.29% to 87.54%; and incubation with low NaOH (˜4.7 mmol/L) at 4 °C significantly increased detectable isoprenoid diphosphates in algal cells, some of which were undetectable or in trace level before NaOH decoupling. The method was applied to H. pluvialis cells under various stresses. Low levels of isoprenoid diphosphates were determined in most of the stresses used, e.g., 0.19 ± 0.09 to 0.98 ± 0.06 mg/g fresh weight (FW) for IPP/DMAPP, 0.35 ± 0.07 mg/g FW for GGPP and undetectable for FPP and GPP; while isoprenoid diphosphates were significantly accumulated in the dark to 3.27 ± 0.05, 0.17 ± 0.09, 1.81 ± 0.16 and 0.58 ± 0.07 mg/g FW for IPP/DMAPP, GPP, FPP and GGPP, respectively. These results implied that isoprenoid diphosphates were exhausted by downstream carotenogenesis under stress. Our work emphasizes NaOH decoupling for exact quantitation of in vivo isoprenoid diphosphates.

Antibiofilm activity substances derived from coral symbiotic bacterial extract inhibit biofouling by the model strain Pseudomonas aeruginosa PAO1.

The mitigation of biofouling has received significant research attention, with particular focus on non-toxic and sustainable strategies. Here, we investigated quorum sensing inhibitor (QSI) bacteria as a means of controlling biofouling in a laboratory-scale system. Approximately, 200 strains were isolated from coral (Pocillopora damicornis) and screened for their ability to inhibit quorum sensing (QS). Approximately, 15% of the isolates exhibited QSI activity, and a typical coral symbiotic bacterium, H12-Vibrio alginolyticus, was selected in order for us to investigate quorum sensing inhibitory activity further. Confocal microscopy revealed that V. alginolyticus extract inhibited biofilm formation from Pseudomonas aeruginosa PAO1. In addition, the secondary metabolites of V. alginolyticus inhibited PAO1 virulence phenotypes by downregulating motility ability, elastase activity and rhamnolipid production. NMR and MS spectrometry suggested that the potential bioactive compound involved was rhodamine isothiocyanate. Quantitative real-time PCR indicated that the bacterial extract induced a significant downregulation of QS regulatory genes (lasB, lasI, lasR, rhlI, rhlR) and virulence-related genes (pqsA, pqsR). The possible mechanism underlying the action of rhodamine isothiocyanate analogue involves the disruption of the las and/or rhl system of PAO1. Our results highlight coral microbes as a bioresource pool for developing QS inhibitors and identifying novel antifouling agents.