Particle-attached and free-living bacterial communities in a contrasting marine environment: Victoria Harbor, Hong Kong

Diversity of particle-attached and free-living marine bacteria in Victoria Harbor, Hong Kong, and its adjacent coastal and estuarial environments was investigated using DNA fingerprinting and clone library analysis. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes showed that bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. DNA sequencing of 28 bands from DGGE gel showed Alphaproteobacteria was the most abundant group, followed by the Bacteroidetes, and other Proteobacteria. Bacterial species richness (number of DGGE bands) differed among stations and populations (particle-attached and free-living; bottom and surface). BIOENV analysis indicated that the concentrations of suspended solids were the major contributing parameter for the spatial variation of total bacterial community structure. Samples from representative stations were selected for clone library (548 clones) construction and their phylogenetic distributions were similar to those of sequences from DGGE. Approximately 80% of clones were affiliated to Proteobacteria, Bacteroidetes and Cyanobacteria. The possible influences of dynamic pollution and hydrological conditions in the Victoria Harbor area on the particle-attached and free-living bacterial community structures were discussed.

Particle-attached and free-living bacterial communities in a contrasting marine environment: Victoria Harbor, Hong Kong

Diversity of particle-attached and free-living marine bacteria in Victoria Harbor, Hong Kong, and its adjacent coastal and estuarial environments was investigated using DNA fingerprinting and clone library analysis. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes showed that bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. DNA sequencing of 28 bands from DGGE gel showed Alphaproteobacteria was the most abundant group, followed by the Bacteroidetes, and other Proteobacteria. Bacterial species richness (number of DGGE bands) differed among stations and populations (particle-attached and free-living; bottom and surface). BIOENV analysis indicated that the concentrations of suspended solids were the major contributing parameter for the spatial variation of total bacterial community structure. Samples from representative stations were selected for clone library (548 clones) construction and their phylogenetic distributions were similar to those of sequences from DGGE. Approximately 80% of clones were affiliated to Proteobacteria, Bacteroidetes and Cyanobacteria. The possible influences of dynamic pollution and hydrological conditions in the Victoria Harbor area on the particle-attached and free-living bacterial community structures were discussed.

山东山旺中新世植物重要类群演化生物学与古气候定量重建的研究

　　早在20世纪三十年代，前人发现山旺中新世具有丰富的植物化石，枝叶、果实、花保存完好。1940年我国植物学家胡先骗与美国加州大学钱耐发表研究专著《中国山东山旺中新世植物群》，报道了30科61属84种植物，为最早系统地进行山旺植物研究的奠基之作，至今仍然具有很高的科学价值和国际影响。1946年布朗发表论文涉及山旺部分属种修订工作。1951年斯行健发表文章：评胡先骗与钱耐著“山东中新统植物群”， 对部分属种鉴定提出异议。1978年植物所与南古所老一辈科学家又作了大量补充与修订，完成合著《中国新生代植物》，该书对山旺植物群进行了系统整理，总计43科87属125种植物，奠定了 山旺植物群的基本面貌。此后，山旺植物群陆续有新的类群添加。目前所知山旺植物群主要以温带落叶植物为主混生了部分亚热带常绿和落叶阔叶植物。该植物群不仅种类繁多，数量丰富，而且保存完整，是我国新生代植物群中研究程度最好的代表。 尽管山旺研究已经取得如此丰富的成果，但是在Hu&Chaney (1940)研究山旺植物群之后的长达60年里( 1940-1999)，关于山旺叶、果实与花的研究仅局限于外部的形态描述，缺乏对其进行细胞形态结构和角质层等结构植物学的实验分析研究，从而大大制约了对该植物群性质的理解和类群演化生物学的深入的研究。 近年来我们通过与国际国内同行的广泛合作，运用国际古植物学研究的新技术、新方法，如叶结构分析、角质层分析、细胞形态结构分析等，在光学显微镜与电子显微镜不同层次的观察上，将植物体外部形态学与内部细胞结构研究相结合，开展对山旺植物群的类群研究，取得了重要进展。我们首次成功提取了山旺植物“阿斯羌属”(As tronium)的角质层，并观察到气孔器结构，为该类群的正确修订提供了确凿的生物学证据，建立了化石新属钱耐属(Wang andManchester，2000)。山旺中新世地层中过去报道存在大量的“阿斯羌”花化石，为五个萼片宿存的辐射对称的花，果实球形，一枚，项生。这种生殖结构也普遍见于欧洲大陆的中新世地层和北美大陆美国的始新世地层中。欧洲人Heer在1859年提出它属于分布于东亚的旋花科的飞蛾藤属(Porana)植物; Weyland在1937年认为它应当是热带龙脑香科非洲马达加斯加岛屿特有分布的Mono tes属植物;美国人MacGinitie在1953年认为它属于分布于南美的漆树科的阿斯羌属(Astronium)植物，其确切系统位置众说纷纭。在中国新生代植物一书中，采用了美国人MacGinitie在1953年的观点，称之为阿斯羌花。斯行健先生认为是旋花科的飞蛾藤属植物。 我们调查了密苏里北京植物园标本馆馆藏的南美和非洲标本，以及哈佛大学标本馆的馆藏东亚标本，对上述科属现存植物的花结构进行了全面的详细研究。发现5个萼片宿存的辐射对称的花见于多个不同的科属类群，是一种趋同演化性状，而萼片脉纹与表皮细胞气孔器排列、果实形态与数目等特征在上述不同类群中极易区分。因而逐一排除了属于上述现存类群的可能性。我们研究的山旺与北美的化石标本显示的果实个体发育过程与现存植物苦木科苦木属(Picrasma)的果实发育相似，果实发育早期为5枚分离，后期3-4枚退化，仅1-2枚，通常为1枚球形果实留存，花瓣脱落，萼片宿存。但是，它们二者在果实形态与萼片表皮结构上，仍存在很大差异。现在认为它属于一个绝灭的植物类群，化石的系统位置尚无法放入现存的科属系统，可能接近于苦木科。因此，建立了一个化石新属钱耐属(Chaneya)。目前，该属包含二个种，一个分布于北美始新世地层，另一个分布于东亚中新世地层，欧洲的材料由于缺乏深入研究，尚无法确定归属。钱耐属在北半球第三纪地层中呈东亚北美间断分布。钱耐属的研究，不仅为探讨东亚与北美植物区系的联系和植物类群的分化历史提供了重要的证据(Wang and Manchester，2000)，更为深入探讨山旺化石植物重要类群的结构与演化生物学研究提供了成功的范例。 山旺的榉属(Zelkova)是山旺植物群的一个常见分子。榉属是欧亚大陆分布的植物，现存6种，在欧亚大陆第三纪地层中发现了大量的榉属化石，包括果实与叶连生的枝条。在北美大陆虽然有榉属化石报道，但是由于缺乏果实，仅靠叶的特征难以令人信服，榉属是否存在于北美仍有争议( Manchester，1989;Manchester，个人通讯)。近年基于我们在山旺组发现和采集到大量的榉属离散叶片以及联生有果实和叶的生殖枝条的深入研究，获得了化石叶结构、表皮细胞、表皮毛、气孔器、核果的大量结构生物学信息。通过与奥地利维也纳大学的 Ferguson教授合作，建立了欧亚现存榉属6个种的生物学形态与结构信息参照系，古今对比结果清楚表明山旺榉的叶缘齿式结构及表皮特征组合最接近我国南部分布的Z. schneideriana，而不是Hu and Chaney (1940)建议的广布于东亚的Z serrata，也明显不同于欧亚大陆共有、分布于西亚伊朗和东欧高加索地区的Z. carpinifolia。常见于欧洲第三纪的翁格榉(Z. ungeri)在形态学更接近Z.carpinifolia。因此，山旺榉明显不同于翁格榉，有其独立的种级特征，应建立东亚中新世榉属的一新种一山旺榉(Z shanwangensis sp. nov.)。榉属在欧亚大陆上的种级分化可能至少在中中新世即已出现。 我们1995年报道了山旺中新世地层发现中国特有植物杜仲化石，并利用先进的电子显微镜设备，找到了确凿无疑的原位化石杜仲特殊胶丝结构直接证据。《美国植物学报》1997年发表文章评论这是目前我国唯一可靠的、保存有精美的胶丝细胞结构的杜仲化石。近年我们迸一步采用古今结合的方法，运用自创的“特有种气候分析法”，对中国重要特有植物杜仲开展了多学科综合研究，利用现存杜仲种子春季萌发需要特定温度的生理学研究结果，结合自然分布区的温度分布，定量恢复了山旺中新世古春季温度数值;该项研究工作的论文列入著名国际SCI刊物《美国植物学报》2003年1月第一期首篇文章。美国植物学报评审人评价：这是应用现代植物实验数据解决地质时期古气候学问题的一个极好例子，是经典之说“现代是（认知）过去的钥匙”的例证，更是当前古植物学研究的一个范例。该文作出的重要贡献体现在如下几个方面：1、提供了现代植物生理生态的实验数据;2、提供了古植物学研究需要的解剖学证据（胶丝）;3、更正了古生物地理方面的一个假说（杜仲何时在中国出现）; 4、提出了一个解答古气候学问题的新方法。 最后一点实际上也是最重要的一点。” 油杉属的研究显示该属现代有三个种，生活在东亚中国，越南和老挝。它曾经在北半球广泛分布，以球果、种鳞、具翅的种子、木材等形式保存在新生代第三纪地层中，如已经有报道的北美大陆美国俄勒冈渐新世和中新世，欧洲中部中新世，亚洲日本中新世和上新世，俄罗斯远东渐新世到中新世地层。本次山旺中中新世与美国俄勒冈渐新世Rujada flora发现的油杉，接近现存油杉属的属型种油杉。北美中新世和亚洲日本中新世和上新世报道了接近于现存种铁坚油杉的化石。这些证据反映了东亚与北美第三纪植物区系的联系远比今天密切，油杉在北美中新世末期后、在欧洲早更新世之后和在东亚日本上新世结束，在上述地区消失。油杉今天仅孑遗分布于东亚中国秦岭以南、雅砻江以东，长江以南及台湾、海南岛和越南、老挝等亚热带至热带北缘的低山、丘陵山区。山旺油杉的球果化石具有现存油杉和铁坚油杉复合种的特征，可能代表了兼具中国两种油杉共同特征的一个早期类型，由此也表明油杉和铁坚油杉的物种分化在中国有可能是1500万年以来发生的新近事件。同时，油杉球果（松科）的首次发现，解决了山东山旺中中新世化石产地有无裸子植物这个长达半个世纪的争论(Hu andChaney，1940：斯行健，1951)，进一步确证了山旺中中新世气候为亚热带山地气候类型。

Characterization of TRPC2, an Essential Genetic Component of VNS Chemoreception, Provides Insights into the Evolution of Pheromonal Olfaction in Secondary-Adapted Marine Mammals

Pheromones are chemical cues released and sensed by individuals of the same species, which are of major importance in regulating reproductive and social behaviors of mammals. Generally, they are detected by the vomeronasal system (VNS). Here, we first investigated and compared an essential genetic component of vomeronasal chemoreception, that is, TRPC2 gene, of four marine mammals varying the degree of aquatic specialization and related terrestrial species in order to provide insights into the evolution of pheromonal olfaction in the mammalian transition from land to water. Our results based on sequence characterizations and evolutionary analyses, for the first time, show the evidence for the ancestral impairment of vomeronasal pheromone signal transduction pathway in fully aquatic cetaceans, supporting a reduced or absent dependence on olfaction as a result of the complete adaptation to the marine habitat, whereas the amphibious California sea lion was found to have a putatively functional TRPC2 gene, which is still under strong selective pressures, reflecting the reliance of terrestrial environment on chemical recognition among the semiadapted marine mammals. Interestingly, our study found that, unlike that of the California sea lion, TRPC2 genes of the harbor seal and the river otter, both of which are also semiaquatic, are pseudogenes. Our data suggest that other unknown selective pressures or sensory modalities might have promoted the independent absence of a functional VNS in these two species. In this respect, the evolution of pheromonal olfaction in marine mammals appears to be more complex and confusing than has been previously thought. Our study makes a useful contribution to the current understanding of the evolution of pheromone perception of mammals in response to selective pressures from an aquatic environment.

The stripe-backed weasel Mustela strigidorsa: taxonomy, ecology, distribution and status

1. The stripe-backed weasel Mustela strigidorsa is one of the rarest and least-known mustelids in the world. Its phylogenetic relationships with other Mustela species remain controversial, though several unique morphological features distinguish it from congeners. 2. It probably lives mainly in evergreen forests in hills and mountains, but has also been recorded from plains forest, dense scrub, secondary forest, grassland and farmland. Known sites range in altitude from 90 m to 2500 m. Data are insufficient to distinguish between habitat and altitudes which support populations, and those where only dispersing animals may occur. 3. It has been confirmed from many localities in north-east India, north and central Myanmar, south China, north Thailand, north and central Laos, and north and central Vietnam. Given the limited survey effort, the number of recent records shows that the species is not as rare as hitherto believed. Neither specific nor urgent conservation needs are apparent.

Estimation of the detection probability for Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) with a passive acoustic method

Yangtze finless porpoises were surveyed by using simultaneous visual and acoustical methods from 6 November to 13 December 2006. Two research vessels towed stereo acoustic data loggers, which were used to store the intensity and sound source direction of the high frequency sonar signals produced by finless porpoises at detection ranges up to 300 m on each side of the vessel. Simple stereo beam forming allowed the separation of distinct biosonar sound source, which enabled us to count the number of vocalizing porpoises. Acoustically, 204 porpoises were detected from one vessel and 199 from the other vessel in the same section of the Yangtze River. Visually, 163 and 162 porpoises were detected from two vessels within 300 m of the vessel track. The calculated detection probability using acoustic method was approximately twice that for visual detection for each vessel. The difference in detection probabilities between the two methods was caused by the large number of single individuals that were missed by visual observers. However, the sizes of large groups were underestimated by using the acoustic methods. Acoustic and visual observations complemented each other in the accurate detection of porpoises. The use of simple, relatively inexpensive acoustic monitoring systems should enhance population surveys of free-ranging, echolocating odontocetes. (C) 2008 Acoustical Society of America.

Echolocation click sounds from wild inshore finless porpoise (Neophocaena phocaenoides sunameri) with comparisons to the sonar of riverine N. p. asiaeorientalis

Acoustic signals from wild Neophocaena phocaenoides sunameri were recorded in the waters off Liao-dong-wan Bay located in Bohai Sea, China. Signal analysis shows that N. p. sunameri produced "typical" phocoenid clicks. The peak frequencies f(p), of clicks ranged from. 113 to 131 kHz with an average of 121 +/- 3.78 kHz (n=71). The 3 dB bandwidths Delta f ranged from 10.9 to 25.0 kHz with an average of 17.5 +/- 3.30 kHz. The signal durations At ranged from 56 to 109 mu s with an average 80 +/- 11.49 mu s. The number of cycles N, ranged from 7 to 13 with an average of 9 +/- 1.48. With increasing peak frequency there was a faint tendency of decrease in bandwidth, which implies a nonconstant value of f(p)/Delta f. On occasion there were some click trains with faint click energy presenting below 70 kHz, however, it was possibly introduced by interference effect from multiple pulses structures. The acoustic parameters of the clicks were compared between the investigated population and a riverine population of finless porpoise. (c) 2007 Acoustical Society of America.

Population genetic structure of Carchesium polypinum (Ciliophora : Peritrichia) in four chinese lakes inferred from ISSR fingerprinting: High diversity but low differentiation

Although the peritrichous ciliate Carchesium polypinum is common in freshwater, its population genetic structure is largely unknown. We used inter-simple sequence repeat (ISSR) fingerprinting to analyze the genetic structure of 48 different isolates of the species from four lakes in Wuhan, central China. Using eight polymorphic primers, 81 discernible DNA fragments were detected, among which 76 (93.83%) were polymorphic, indicating high genetic diversity at the isolate level. Further, Nei's gene diversity (h) and Shannon's Information index (I) between the different isolates both revealed a remarkable genetic diversity, higher than previously indicated by their morphology. At the same time, substantial gene flow was found. So the main factors responsible for the high level of diversity within populations are probably due to conjugation (sexual reproduction) and wide distribution of swarmers. Analysis of molecular variance (AMOVA) showed that there was low genetic differentiation among the four populations probably due to common ancestry and flooding events. The cluster analysis and principal component analysis (PCA) suggested that genotypes isolated from the same lake displayed a higher genetic similarity than those from different lakes. Both analyses separated C. polypinum isolates into subgroups according to the geographical locations. However, there is only a weak positive correlation between the genetic distance and geographical distance, suggesting a minor effect of geographical distance on the distribution of genetic diversity between populations of C. polypinum at the local level. In conclusion, our studies clearly demonstrated that a single morphospecies may harbor high levels of genetic diversity, and that the degree of resolution offered by morphology as a marker for measuring distribution patterns of genetically distinct entities is too low.

Nonconstant quality of auditory filters in the porpoises, Phocoena phocoena and Neophocaena phocaenoides (Cetacea, Phocoenidae)

Simultaneous tone-tone masking in conjunction with the envelope-following response (EFR) recording was used to obtain tuning curves in porpoises Phocoena phocoena and Neophocaena phocaenoides asiaeorientalis. The EFR was evoked by amplitude-modulated probes with a modulation rate of 1000 Hz and carrier frequencies from 22.5 to 140 kHz. Equivalent rectangular quality Q(ERB) of the obtained tuning curves varied from 8.3-8.6 at lower (22.5-32 kHz) probe frequencies to 44.8-47.4 at high (128-140 kHz) frequencies. The QERB dependence on probe frequency could be approximated by regression lines with a slope of 0.83 to 0.86 in log-log scale., which corresponded to almost frequency-proportional quality and almost constant bandwidth of 34 kHz. Thus, the frequency representation in the porpoise auditory system is much closer to a constant-bandwidth rather that to a constant-quality manner. (c) 2006 Acoustical Society of America.

NK-lysin of channel catfish: Gene triplication, sequence variation, and expression analysis

Antimicrobial peptides (AMPs) are important components of the host innate immune response against microbial invasion. In addition to the previously known four classes of antimicrobial peptides, a fifth class of antimicrobial peptides has been recently identified to include NK-lysins that have a globular three-dimensional structure and are larger with 74-78 amino acid residues. NK-lysin has been shown to harbor antimicrobial activities against a wide spectrum of microorganisms including bacteria, fungi, protozoa, and parasites. To date, NK-lysin genes have been reported from only a limited number of organisms. We previously identified a NK-lysin cDNA in channel catfish. Here we report the identification of two noveltypes of NK-lysin transcripts in channel catfish. Altogether, three distinct NK-lysin transcripts exist in channel catfish. In this work, their encoding genes were identified, sequenced, and characterized. We provide strong evidence that the catfish NK-lysin gene is tripled in the same genomic neighborhood. All three catfish NK-lysin genes are present in the same genomic region and are tightly linked on the same chromosome, as the same BAC clones harbor all three copies of the NK-lysin genes. All three NK-lysin genes are expressed, but exhibit distinct expression profiles in various tissues. In spite of the existence of a single copy of NK-lysin gene in the human genome, and only a single hit from the pufferfish,genome, there are two tripled clusters of NK-lysin genes on chromosome 17 of zebrafish in addition to one more copy on its chromosome 5. The similarity in the genomic arrangement of the tripled NK-lysin genes in channel catfish and zebrafish suggest similar evolution of NK-lysin genes. (c) 2005 Elsevier Ltd. All rights reserved.

Echolocation signals of the free-ranging Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientialis)

This paper describes the high-frequency echolocation signals from free-ranging Yangtze finless porpoise in the Tian-e-zhou Baiji National Natural Reserve in Hubei Province, China. Signal analysis showed that the Yangtze finless porpoise clicks are typical high-frequency narrow-band (relative width of the frequency spectrum Q=6.6 &PLUSMN; 1.56, N=548) ultrasonic pulses. The peak frequencies of the typical clicks range from 87 to 145 kHz with an average of 125 &PLUSMN; 6.92 kHz. The durations range from 30 to 122 μ s with an average of 68 &PLUSMN; 14.12 μ s. The characteristics of the signals are similar to those of other members of the Phocoenidae as well as the distantly related delphinids, Cephalorhynchus spp. Comparison of these signals to those of the baiji (Lipotes vexillifer), who occupies habitat similar to that of the Yangtze finless porpoise, showed that the peak frequencies of clicks produced by the Yangtze finless porpoise are remarkably higher than those produced by the baiji. Difference in peak frequency between the two species is probably linked to the different size of prefer-red prey fish. Clear double-pulse and multi-pulse reverberation structures of clicks are noticed, and there is no indication of any low-frequency (< 70 kHz) components during the recording period. © 2005 Acoustical Society of America.

Diving behaviour of freshwater finless porpoises (Neophocaena phocaenoides) in an oxbow of the Yangtze River, China

Dive-depth and swim-speed of a juvenile and an adult free-ranging, Yangtze finless porpoises (Neophocaena phocaenoides) were observed using velocity-time-depth recorders in an oxbow of the Yangtze River. In total, 8222 individual dives were recorded over 59 hours. Two dive types, deep-dive (greater than or equal to 2.7 m) and shallow-dive. were recognized. Horizontal travel distances of two finless pot-poises in a day were 94.4 km and 90.3 km, which were longer than those of oceanic relative species (harbor porpoises, Phocoena phocoena). Although the shallow water limited the maximum dive-depth, dive-duration, and bottom-time of finless porpoises were similar to the harbor porpoises. A sudden drop of swim-speed below 0.25 m s(-1) was frequently observed nearby the maximum dive-depth. This seemed to indicate "turning, around" behaviour, possibly during prey pursuit. (C) 2002 Published by Elsevier science Ltd on behalf of International Council for the Exploration of the Sea.

Fine-scale vertical distribution of bacteria in the East Pacific deep-sea sediments determined via 16S rRNA gene T-RFLP and clone library analyses

Although the deep-sea sediments harbor diverse and novel bacteria with important ecological and environmental functions, a comprehensive view of their community characteristics is still lacking, considering the vast area and volume of the deep-sea sedimentary environments. Sediment bacteria vertical distribution and community structure were studied of the E272 site in the East Pacific Ocean with the molecular methods of 16S rRNA gene T-RFLP (terminal restriction fragment length polymorphism) and clone library analyses. Layered distribution of the bacterial assemblages was detected by both methods, indicating that the shallow sediments (40 cm in depth) harbored a diverse and distinct bacterial composition with fine-scale spatial heterogeneity. Substantial bacterial diversity was detected and nine major bacterial lineages were obtained, including Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Nitrospirae, Planctomycetes, Proteobacteria, and the candidate divisions OP8 and TM6. Three subdivisions of the Proteobacteria presented in our libraries, including the alpha-, gamma- and delta-Proteobacteria. Most of our sequences have low similarity with known bacterial 16S rRNA genes, indicating that these sequences may represent as-yet-uncultivated novel bacteria. Most of our sequences were related to the GenBank nearest neighboring sequences retrieved from marine sediments, especially from deep-sea methane seep, gas hydrate or mud volcano environments. Several sequences were related to the sequences recovered from the deep-sea hydrothermal vent or basalt glasses-bearing sediments, indicating that our deep-sea sampling site might be influenced to certain degree by the nearby hydrothermal field of the East Pacific Rise at 13A degrees N.

Bacterial roles in the formation of high-molecular-weight dissolved organic matter in estuarine and coastal waters: Evidence from lipids and the compound-specific isotopic ratios

High-molecular-weight dissolved organic matter (HMW-DOM, > 1,000 Daltons) is actively involved in the global biogeochemical cycling of many elements, but its carbon sources and detailed formation pathways are still not well understood. In this study, we measured bulk stable carbon and nitrogen isotopic ratios, lipid composition, and compound-specific carbon isotopic ratios of HMW-DOM samples collected from four U.S. estuaries (Boston Harbor/Massachusetts Bay, Delaware/Chesapeake Bay, San Diego Bay, and San Francisco Bay). Analytical results show (1) a fraction of HMW-DOM (lipid associated) in estuarine and coastal waters is derived from bacteria and phytoplankton; (2) this fraction of HMW-DOM is formed by various release processes of bacterial membrane components and bacterial reworking of phytoplankton-derived material; (3) this fraction of HMW-DOM is generally present in all samples from different coastal systems despite variable organic matter inputs and environmental conditions, suggesting an important bacterial role in HMW-DOM formation.