Seasonal Patterns of Picocyanobacterial Community Structure in the Kuroshio Current.

The nutrient-scarce, warm, and high-salinity Kuroshio current has a profound impact on both the marine ecology of the northwestern Pacific Ocean and the global climate. This study aims to reveal the seasonal dynamics of picoplankton in the subtropical Kuroshio current. Our results showed that one of the picocyanobacteria, Synechococcus, mainly distributed in the surface water layer regardless of seasonal changes, and the cell abundance ranged from 104 to 105 cells mL-1. In contrast, the maximum concentration of the other picocyanobacteria, Prochlorococcus, was maintained at more than 105 cells mL-1 throughout the year. In the summer and the autumn, Prochlorococcus were mainly concentrated at the water layer near the bottom of the euphotic zone. They were evenly distributed in the euphotic zone in the spring and winter. The stirring effect caused by the monsoon determined their distribution in the water column. In addition, the results of 16S rRNA gene diversity analysis showed that the seasonal changes in the relative abundance of Synechococcus and Prochlorococcus in the surface water of each station accounted for 20 to 40% of the total reads. The clade II of Synechococcus and the High-light II of Prochlorococcus were the dominant strains in the waters all year round. Regarding other picoplankton, Proteobacteria and Actinobacteria occupied 45% and 10% of the total picoplankton in the four seasons. These data should be helpful for elucidating the impacts of global climate changes on marine ecology and biogeochemical cycles in the Western Boundary Currents in the future.

Suspended particulate matter-bound per- and polyfluoroalkyl substances (PFASs) in a river-coastal system: Possible correlation with transparent exopolymer particles.

The transport and ultimate fate of per- and polyfluoroalkyl substances (PFASs) are generally considered to be influenced by partitioning behavior between water, suspended particulate matters (SPM), and sediments. This study examined the distribution and partitioning of the PFASs in the water, SPM, and sediments in a densely populated urban river-coastal system. The total concentrations of eight PFASs (∑8 PFASs) in the water phase, SPM, and sediments varied from 0.59 to 7.40 ng/L, 0.54 to 9.08 ng/g, and 0.05 to 0.13 ng/g, respectively. The PFAS concentrations in the water and SPM phase decreased as the salinity increased, confirming contaminant inputs from the upstream of the river to the estuary zone. Notably, the positive correlation between SPM-bound PFASs and transparent exopolymer particles (TEPs) content, providing first evidence that TEPs may accumulate and concentrate more PFASs on the SPM. Collectively, this results offers useful information about roles of TEPs in determining environmental fate of PFASs.

Trade-Offs between Competitive Ability and Resistance to Top-Down Control in Marine Microbes.

Trade-offs between competitive ability and resistance to top-down control manifest the "kill-the-winner" hypothesis that explains how mortality caused by protists and viruses can promote bacterial diversity. However, the existence of such trade-offs has rarely been investigated in natural marine bacterial communities. To address this question, we conducted on-board dilution experiments to manipulate top-down control pressure (protists only or protists plus viruses [protists+viruses] combined) and then applied 16S rRNA gene high-throughput sequencing techniques to assess the responses of each bacterial taxon. Dilution experiments enabled us to measure the top-down-control-free growth rate as the competitive ability and top-down-control-caused mortality as the reverse of resistance to top-down control. Overall, bacterial taxa with higher top-down-control-free growth rates were accompanied by lower top-down-control-caused resistance. Furthermore, competition-resistance trade-offs were stronger and more consistent when top-down control was caused by protists+viruses combined than by protists only. When protists+viruses were diluted, the bacterial rank abundance distribution became steepened and evenness and richness were decreased. However, when protists were diluted, only richness decreased. Our results indicate the existence of competition-resistance trade-offs in marine microbes and demonstrate the positive impacts of such trade-offs on bacterial diversity. Regardless, the strength of the competition-resistance trade-offs and the impacts on bacterial diversity were contingent on whether top-down control was caused by protists+viruses combined or protists only. IMPORTANCE We addressed the "kill-the-winner" hypothesis from the perspective of its principle (the competition-resistance trade-off) in marine bacterial communities incubated in situ. Our results supported the existence of competition-resistance trade-offs and the positive effect on bacterial community diversity. The study linked theoretical expectations and complex natural systems and provided new knowledge regarding how top-down controls and competition trade-offs shaped natural bacterial communities.

Deterministic Assembly Processes Strengthen the Effects of ß-Diversity on Community Biomass of Marine Bacterioplankton.

The presence of more species in the community of a sampling site (α diversity) typically increases ecosystem functions via nonrandom processes like resource partitioning. When considering multiple communities, we hypothesize that higher compositional difference (ß diversity) increases overall functions of these communities. Further, we hypothesize that the ß diversity effect is more positive when ß diversity is increased by nonrandom assembly processes. To test these hypotheses, we collected bacterioplankton along a transect of 6 sampling sites in the southern East China Sea in 14 cruises. For any pairs of the 6 sites within a cruise, we calculated the Bray-Curtis index to represent ß diversity and summed bacterial biomass as a proxy to indicate the overall function of the two communities. We then calculated deviation of observed mean pairwise phylogenetic similarities among species in two communities from random to represent the influences of nonrandom processes. The bacterial ß diversity was found to positively affect the summed bacterial biomass; however, the effect varied among cruises. Cross-cruise comparison indicated that the ß diversity effect increased with the nonrandom processes selecting for phylogenetically dissimilar species. This study extends biodiversity-ecosystem functioning research to the scale of multiple sites and enriches the framework by considering community assembly processes. IMPORTANCE The implications of our analyses are twofold. First, we emphasize the importance of studying ß diversity. We expanded the current biodiversity-ecosystem functioning framework from single to multiple sampling sites and investigated the influences of species compositional differences among sites on the overall functioning of these sites. Since natural ecological communities never exist alone, our analyses allow us to more holistically perceive the role of biodiversity in natural ecosystems. Second, we took community assembly processes into account to attain a more mechanistic understanding of the impacts of biodiversity on ecosystem functioning.

New insights into the role of marine plastic-gels in microplastic transfer from water to the atmosphere via bubble bursting.

The pervasiveness of microplastics (MPs) in global oceans is raising concerns about their adverse impacts on ecosystems. The mechanistic understanding of MP transport is critical for evaluating its fate, flux, and ecological risks specifically. Currently, bubble bursting is believed to represent an important route for MP transfer from sea surfaces to the atmosphere. However, the detailed mechanisms of the complex physico-chemical interactions between MPs, water composition, and gel particles in the air-sea interface remain unknown. Our results suggested three steps for MP transfer between air-sea phases: (1) MPs incorporating into gel aggregates in the water column; (2) further accumulation of plastic-gel aggregate in the surface layer phase; finally (3) ejection of aggregates from the sea when bubbles of trapped air rise to the surface and burst. The water composition (e.g., high salinity, gel concentration and viscosity) can modulate plastic-gel aggregation and subsequent transport from water to the atmosphere. The possible mechanism may be closely tied to the formation of plastic-gel via cation-linking bridges, thereby enhancing plastic-gel ejection into air. Collectively, this work offers unique insights into the role of marine plastic-gels in determining MP fate and transport, especially at air-sea interfaces. The data also provide a better understanding of the corresponding mechanism that may explain the fates of missing plastics in the ocean.

Probing the outfall-related anomalous Hg levels in the Danshuei Estuarine Coastal, Taiwan.

Seasonal and spatial distributions of total mercury (THg) in the Danshuei Estuary and adjacent coastal areas near the ocean outfall of Taipei, Taiwan, have been successfully investigated from May 2003 to January 2005. We found spatio-temporal variation in THg levels in the Danshuei coastal area was the result of sources and supplies of Hg. High THg concentrations in sediments and seawater were particularly found near the effluent outfall. Average THg levels (avg.: 9-22 ng L-1) were much higher than those in surrounding coastal seawaters (avg.:1-2 ng L-1). Organic carbon contents then played vital roles in controlling water and sedimentary Hg concentrations and occurrences. Hg enrichment factor (EF) as an index of contamination status in surface sediments of the Danshuei coastal area averaged 2.0 ± 0.8 (EFs > 1), indicating an extra non-crustal source from anthropogenic loadings. It implies the Dansheui coastal environment nearby the sewer outfall is facing Hg pollution.

How Communities of Marine Stramenopiles Varied with Environmental and Biological Variables in the Subtropical Northwestern Pacific Ocean.

MArine STramenopiles (MASTs) have been recognized as parts of heterotrophic protists and contribute substantially to protist abundances in the ocean. However, little is known about their spatiotemporal variations with respect to environmental and biological factors. The objectives of this study are to use canonical correspondence analysis to investigate how MASTs communities are shaped by environmental variables, and co-occurrence networks to examine their potential interactions with prokaryotic communities. Our dataset came from the southern East China Sea (sECS) in the subtropical northwestern Pacific, and involved 14 cruises along a coastal-oceanic transect, each of which sampled surface water from 4 to 7 stations. MASTs communities were revealed by metabarcoding of 18S rDNA V4 region. Most notably, MAST-9 had a high representation in warm waters in terms of read number and diversity. Subclades of MAST-9C and -9D showed slightly different niches, with MAST-9D dominating in more coastal waters where concentrations of nitrite and Synechococcus were higher. MAST-1C was a common component of colder water during spring. Overall, canonical correspondence analysis showed that MASTs communities were significantly influenced by temperature, nitrite and Synechococcus concentrations. The co-occurrence networks showed that certain other minor prokaryotic taxa can influence MAST communities. This study provides insight into how MASTs communities varied with environmental and biological variables.

Marine microplastics in the surface waters of "pristine" Kuroshio.

Microplastics (MPs) are ubiquitous in oceans. Their transboundary transport and fate have aroused global attention. Taiwan is located close to the western boundary current-Kuroshio, is an excellent location to study of MP mobility in the global current and Pacific Garbage Patch. This study is the first investigation to understand the microplastic contamination from Taiwan to the Kuroshio. MP concentrations in the area varied from N.D. to 0.15 items m-3, with an average concentration of 0.05 ± 0.03 items m-3. The majority of MPs were polypropylene, polyethylene, polyethylene and terephthalate. We found two MP hotspots near the coastal zone. One additional hotspot was also identified in the "pristine" Kuroshio suggesting rivers and local currents may play critical roles in transporting or injecting MPs from Taiwan into the North Pacific Gyre. These findings suggest that marine environments are altered by anthropogenic disposal and provide needed data for modelling and prediction of MPs.

Importance of prey size on investigating prey availability of larval fishes.

Prey availability plays an important role in determining larval fish survival. Numerous studies have found close relationships between the density of mesozooplankton and larval fishes; however, emerging studies suggest that small-size zooplankton are more important prey for some larval fish species. One arising question is whether the size of zooplankton determines the relationship between zooplankton and larval fish community in natural environments. To address this question, we collected small-size (50-200 µm) zooplankton, mesozooplankton (> 330 µm), and larval fish using three different mesh-size (50, 330, 1000 µm, respectively) nets in the East China Sea, and examined their relationships in density. Both meso- and small-size zooplankton densities showed positive relationships with larval fish density, while the relationship is much stronger for the small-size zooplankton. Specifically, the smallest size classes (50-75 and 75-100 µm) of small-size zooplankton showed the highest positive relationships with larval fish density. Temperature, salinity, and chlorophyll-a concentration did not significantly explain larval fish density. Based on these findings, we demonstrate the importance of considering prey size when investigating prey availability for larval fishes.

Impact of upwelling on phytoplankton blooms and hypoxia along the Chinese coast in the East China Sea.

This study evaluates the rarely observed phenomenon of the simultaneous occurrences of phytoplankton blooms, hypoxia, and upwelling along the Zhejiang coast in the East China Sea. Results show that the upwelling uplifted bottom water to 5-10 m below the surface. In the upwelling region, phytoplankton blooms (Chl a = 10.9 µg L-1) occurred and hypoxia or low-oxygen appeared below the surface water. High concentrations of nitrate and phosphate were regenerated in the hypoxic regions, corresponding with mean values (± SD) of 16.9 (± 1.5) and 0.90 (± 0.14) µM, respectively. The upwelling expanded the region of hypoxic water, which nearly reached the surface, thereby increasing the threat to marine life. In addition to fluvial nutrients, the upwelling of water with high nutrient levels, especially phosphates, can enhance phytoplankton blooms. The results suggest that hypoxia can become more severe due to further decomposition of bloom-derived organic matter after blooms crash.

Nano- and microplastics trigger secretion of protein-rich extracellular polymeric substances from phytoplankton.

The substantial increase in plastic pollution in marine ecosystems raises concerns about its adverse impacts on the microbial community. Microorganisms (bacteria, phytoplankton) are important producers of exopolymeric substances (EPS), which govern the processes of marine organic aggregate formation, microbial colonization, and pollutant mobility. Until now, the effects of nano- and micro-plastics on characteristics of EPS composition have received little attention. This study investigated EPS secretion by four phytoplankton species following exposure to various concentrations of polystyrene nano- and microplastics (55 nm nanoparticles; 1 and 6 µm microparticles). The 55 nm nanoparticles induced less growth/survival (determined on a DNA basis) and produced EPS with higher protein-to-carbohydrate (P/C) ratios than the exposure to microplastic particles. The amount of DNA from the four marine phytoplankton showed a higher negative linear correlation with increasing P/C ratios, especially in response to nanoplastic exposure. These results provide evidence that marine phytoplankton are quite sensitive to smaller-sized plastics and actively modify their EPS chemical composition to cope with the stress from pollution. Furthermore, the release of protein-rich EPS was found to facilitate aggregate formation and surface modification of plastic particles, thereby affecting their fate and colonization. Overall, this work offers new insights into the potential harm of different-sized plastic particles and a better understanding of the responding mechanism of marine phytoplankton for plastic pollution. The data also provide needed information about the fate of marine plastics and biogenic aggregation and scavenging processes.

Hypoxia in autumn of the East China Sea.

Hypoxia (O2 ≤ 2 mg L-1) can severely threaten the survival of marine life and alter the biogeochemical cycles of coastal ecosystems. Its impacts are dependent on its duration. In the present study, hypoxia was observed in autumn at the end of October 2011. It may be one of the latest recorded annual hypoxic events in the East China Sea (ECS). In the hypoxic regions, a large amount of nutrients and dissolved inorganic carbon were observed to regenerate. Also, acidification (low pH) was observed. On the other hand, hypoxic dissipation may be due to the destratification caused by the upwelling of the hypoxic regions in the ECS. These results suggest that hypoxia may occur for longer periods of time than expected and, accordingly, the effects of hypoxia on the ECS ecosystems should be reconsidered and further evaluated.

Evolutionary constraints on species diversity in marine bacterioplankton communities.

Variation in microbial species diversity has typically been explained as the outcome of local ecological factors driving species coexistence, overlooking the roles of evolutionary constraints. Here, we argue that macro-evolutionary niche conservatism and unequal diversification rates among phylum-level lineages are strong determinants of diversity-environment relationships in bacterial systems. That is, apart from stochasticity, environmental effects operate most strongly on phylum composition, which in turn dictates the species diversity of bacterial communities. This concept is demonstrated using bacterioplankton in the surface seawaters of the East China Sea. Furthermore, we show that the species richness of a local bacterioplankton community can generally be estimated based on the relative abundances of phyla and their contributions of species numbers in the global seawater pool-highlighting the important influence of evolutionary constraints on local community diversity.

Abiotic and Biotic Factors Affecting the Ingestion Rates of Mixotrophic Nanoflagellates (Haptophyta).

Mixotrophic haptophytes comprise one of several important groups of mixotrophic nanoflagellates in the pelagic environment. This study aimed to investigate if phagotrophy in mixotrophic haptophytes is regulated by light or other factors in the surface (SE) and bottom (BE) of the euphotic zone in the subtropical northwestern Pacific Ocean. We estimated the rates of bacterial ingestion by haptophytes using fluorescently labeled bacteria (FLBs) and fluorescence in situ hybridization. Haptophyte diversity and abundance were also investigated in the same sampling area. The annual mean abundance of haptophytes was 419 ± 85.6 cells mL-1 in both SE and BE. Cells 3-5 µm in size were the dominant group in all haptophytes and accounted for majority of bacteria standing stock removed by haptophytes (53%). Most haptophyte ingestion rates (IRs) were not significantly different between the two layers (average SE ingestion rate: 12.5 ± 2.29 bac Hap-1 h-1; BE: 14.7 ± 3.03 bac Hap-1 h-1). Furthermore, the haptophyte IRs were negatively correlated with nitrate concentrations in the SE and positively correlated with bacterial abundances in the BE, which accounts for the significantly high IRs in August 2012 and 2013. These findings imply that mixotrophic haptophytes in this region had different factors affecting phagotrophy to adapt to the ambient light intensity alterations between SE and BE.

Predator and prey biodiversity relationship and its consequences on marine ecosystem functioning-interplay between nanoflagellates and bacterioplankton.

The importance of biodiversity effects on ecosystem functioning across trophic levels, especially via predatory-prey interactions, is receiving increased recognition. However, this topic has rarely been explored for marine microbes, even though microbial biodiversity contributes significantly to marine ecosystem function and energy flows. Here we examined diversity and biomass of bacteria (prey) and nanoflagellates (predators), as well as their effects on trophic transfer efficiency in the East China Sea. Specifically, we investigated: (i) predator diversity effects on prey biomass and trophic transfer efficiency (using the biomass ratio of predator/prey as a proxy), (ii) prey diversity effects on predator biomass and trophic transfer efficiency, and (iii) the relationship between predator and prey diversity. We found higher prey diversity enhanced both diversity and biomass of predators, as well as trophic transfer efficiency, which may arise from more balanced diet and/or enhanced niche complementarity owing to higher prey diversity. By contrast, no clear effect was detected for predator diversity on prey biomass and transfer efficiency. Notably, we found prey diversity effects on predator-prey interactions; whereas, we found no significant diversity effect on biomass within the same trophic level. Our findings highlight the importance of considering multi-trophic biodiversity effects on ecosystem functioning in natural ecosystems.

Evaluation of the Relationship Between the 18S rRNA/rDNA Ratio and Population Growth in the Marine Diatom Skeletonema tropicum via the Application of an Exogenous Nucleic Acid Standard.

Ribosomal RNA (rRNA) has been regarded as a proxy for metabolic activity and population growth in microbes, but the limitations and assumptions of this approach should be better defined, particularly in eukaryotic microalgae. In this study, the 18S rRNA/rDNA ratio of a marine diatom, Skeletonema tropicum, was examined in batch and semi-continuous cultures subjected to low nitrogen and phosphorus treatments at a temperature of 20 °C. In the semi-continuous cultures, the measured 18S rRNA/rDNA ratio ranged from 4.0 × 102 to 5.0 × 103 , and the logarithmic form of this ratio increased linearly with the population growth rate under both low nitrogen and low phosphorus conditions. In batch cultures grown under low nitrogen or low phosphorus conditions, log (rRNA/rDNA) also increased linearly with growth rate when the latter ranged between -0.4 and 1.5 day-1 . The 18S rRNA/rDNA ratios of Skeletonema sampled from in the southern East China Sea were substantially lower than measured from laboratory cultures. Among the field samples, ratios obtained at a coastal station were higher than those obtained farther offshore. These results imply higher growth rate at the coastal station, but the influences of other factors, such as cell size and temperature, cannot be ruled out.

Contrasting the relative importance of species sorting and dispersal limitation in shaping marine bacterial versus protist communities.

A central challenge in microbial ecology is to understand the underlying mechanisms driving community assembly, particularly in the continuum of species sorting and dispersal limitation. However, little is known about the relative importance of species sorting and dispersal limitation in shaping marine microbial communities; especially, how they are related to organism types/traits and water depth. Here, we used variation partitioning and null model analysis to compare mechanisms driving bacterial and protist metacommunity dynamics at the basin scale in the East China Sea, based on MiSeq paired-end sequencing of 16S ribosomal DNA (rDNA) and 18S rDNA, respectively, in surface, deep chlorophyll maximum and bottom layers. Our analyses indicated that protist communities were governed more strongly by species sorting relative to dispersal limitation than were bacterial communities; this pattern was consistent across the three-depth layers, albeit to different degrees. Furthermore, we detected that bacteria exhibited wider habitat niche breadths than protists, whereas, passive dispersal abilities were not appreciably different between them. Our findings support the 'size-plasticity' hypothesis: smaller organisms (bacteria) are less environment filtered than larger organisms (protists), as smaller organisms are more likely to be plastic in metabolic abilities and have greater environmental tolerance.

Community Composition of Photosynthetic Picoeukaryotes in a Subtropical Coastal Ecosystem, with Particular Emphasis on Micromonas.

Photosynthetic picoeukaryotes (PPEs) are important constituents in picoplankton communities in many marine ecosystems. However, little is known about their community composition in the subtropical coastal waters of the Northwestern Pacific Ocean. In order to study their taxonomic composition, this study constructed 18S rRNA gene libraries using flow cytometric sorting during the warm season. The results show that, after diatoms, prasinophyte clones are numerically dominant. Within prasinophytes, Micromonas produced the most common sequences, and included clades II, III, IV, and VI. We are establishing the new Micromonas clade VI based on our phylogenetic analysis. Sequences of this clade have previously been retrieved from the South China Sea and Red Sea, indicating a worldwide distribution, but this is the first study to detect clade VI in the coastal waters of Taiwan. The TSA-FISH results indicated that Micromonas clade VI peaked in the summer (~4 × 102  cells/ml), accounting for one-fifth of Micromonas abundance on average. Overall, Micromonas contributed half of Mamiellophyceae abundance, while Mamiellophyceae contributed 40% of PPE abundance. This study demonstrates the importance of Micromonas within the Mamiellophyceae in a subtropical coastal ecosystem.

Evaluating community-environment relationships along fine to broad taxonomic resolutions reveals evolutionary forces underlying community assembly.

We propose a method for detecting evolutionary forces underlying community assembly by quantifying the strength of community-environment relationships hierarchically along taxonomic ranks. This approach explores the potential role of phylogenetic conservatism on habitat preferences: wherein, phylogenetically related taxa are expected to exhibit similar environmental responses. Thus, when niches are conserved, broader taxonomic classification should not diminish the strength of community-environment relationships and may even yield stronger associations by summarizing occurrences and abundances of ecologically equivalent finely resolved taxa. In contrast, broader taxonomic classification should weaken community-environment relationships when niches are under great divergence (that is, by combining finer taxa with distinct environmental responses). Here, we quantified the strength of community-environment relationships using distance-based redundancy analysis, focusing on soil and seawater prokaryotic communities. We considered eight case studies (covering a variety of sampling scales and sequencing strategies) and found that the variation in community composition explained by environmental factors either increased or remained constant with broadening taxonomic resolution from species to order or even phylum level. These results support the niche conservatism hypothesis and indicate that broadening taxonomic resolution may strengthen niche-related signals by removing uncertainty in quantifying spatiotemporal distributions of finely resolved taxa, reinforcing the current notion of ecological coherence in deep prokaryotic branches.

Prey size diversity hinders biomass trophic transfer and predator size diversity promotes it in planktonic communities.

Body size exerts multiple effects on plankton food-web interactions. However, the influence of size structure on trophic transfer remains poorly quantified in the field. Here, we examine how the size diversity of prey (nano-microplankton) and predators (mesozooplankton) influence trophic transfer efficiency (using biomass ratio as a proxy) in natural marine ecosystems. Our results support previous studies on single trophic levels: transfer efficiency decreases with increasing prey size diversity and is enhanced with greater predator size diversity. We further show that communities with low nano-microplankton size diversity and high mesozooplankton size diversity tend to occur in warmer environments with low nutrient concentrations, thus promoting trophic transfer to higher trophic levels in those conditions. Moreover, we reveal an interactive effect of predator and prey size diversities: the positive effect of predator size diversity becomes influential when prey size diversity is high. Mechanistically, the negative effect of prey size diversity on trophic transfer may be explained by unicellular size-based metabolic constraints as well as trade-offs between growth and predation avoidance with size, whereas increasing predator size diversity may enhance diet niche partitioning and thus promote trophic transfer. These findings provide insights into size-based theories of ecosystem functioning, with implications for ecosystem predictive models.