Temporal changes in community composition of heterotrophic bacteria during in situ iron enrichment in the western subarctic Pacific (SEEDS-II)

Little is known about the effects of iron enrichment in high-nitrate low-chlorophyll (HNLC) waters on the community composition of heterotrophic bacteria, which are crucial to nutrient recycling and microbial food webs. Using denaturing gradient gel electrophoresis (DGGE) of 16S rDNA fragments, we investigated the heterotrophic eubacterial community composition in surface waters during an in situ iron-enrichment experiment (SEEDS-II) in the western subarctic Pacific in the summer of 2004. DGGE fingerprints representing the community composition of eubacteria differed inside and outside the iron-enriched patch. Sequencing of DGGE bands revealed that at least five phylotypes of α-proteobacteria including Roseobacter, Cytophaga-Flavobacteria-Bacteroides (CFB), γ-proteobacteria, and Actinobacteria occurred in almost all samples from the iron-enriched patch. Diatoms did not bloom during SEEDS-II, but the eubacterial composition in the iron-enriched patch was similar to that in diatom blooms observed previously. Although dissolved organic carbon (DOC) accumulation was not detected in surface waters during SEEDS-II, growth of the Roseobacter clade might have been particularly stimulated after iron additions. Two identified phylotypes of CFB were closely related to the genus Saprospira, whose algicidal activity might degrade the phytoplankton assemblages increased by iron enrichment. These results suggest that the responses of heterotrophic bacteria to iron enrichment could differ among phylotypes during SEEDS-II.     

Trophic efficiency of the planktonic food web in a coastal ecosystem dominated by Phaeocystis colonies

The trophic efficiency of the planktonic food web in the Phaeocystis-dominated ecosystem of the Belgian coastal waters was inferred from the analysis of the carbon flow network of the planktonic system subdivided into its different trophodynamic groups. A carbon budget was constructed on the basis of process-level field experiments conducted during the spring bloom period of 1998. Biomass and major metabolic activities of auto- and heterotrophic planktonic communities (primary production, bacterial production, nanoproto-, micro- and mesozooplankton feeding activities) were determined in nine field assemblages collected during spring at reference station 330. In 1998, the phytoplankton spring flowering was characterised by a moderate diatom bloom followed by a massive Phaeocystis colony bloom. Phaeocystis colonies, contributing 70% to the net primary production, escaped the linear food chain while the early spring diatom production supplied 74% of the mesozooplankton carbon uptake. The rest of mesozooplankton food requirement was, at the time of the Phaeocystis colony bloom, partially fulfilled by microzooplankton. Only one-third of the microzooplankton production, however, was controlled by mesozooplankton grazing pressure. Ungrazed Phaeocystis colonies were stimulating the establishment of a very active microbial network. On the one hand, the release of free-living cells from ungrazed colonies has been shown to stimulate the growth of microzooplankton, which was controlling 97% of the nanophytoplankton production. On the other hand, the disruption of ungrazed Phaeocystis colonies supplied the water column with large amounts of dissolved organic matter available for planktonic bacteria. The budget calculation suggests that ungrazed colonies contributed up to 60% to the bacterial carbon demand, while alternative sources (exudation, zooplankton egestion and lysis of other organisms) provided some 30% of bacterial carbon requirements. This suggests that the spring carbon demand of planktonic bacteria was satisfied largely by autogenic production. The trophic efficiency was defined as the ratio between mesozooplankton grazing on a given source and food production. In spite of its major contribution to mesozooplankton feeding, the trophic efficiency of the linear food chain, restricted to the grazing on diatoms, represented only 5.6% of the available net primary production. The trophic efficiency of the microbial food chain, the ratio between mesozooplankton grazing on microzooplankton and the resource inflow (the bacterial carbon demand plus the nanophytoplankton production) amounted to only 1.6%. These low trophic efficiencies together with the potential contribution of ungrazed Phaeocystis-derived production to the bacterial carbon demand suggest that during spring 1998 most of the Phaeocystis-derived production in the Belgian coastal area was remineralised in the water column.     

针胞藻海洋卡盾藻(Chattonella marina)和硅藻中肋骨条藻(Skeletonema costatum)藻际细菌的分离、鉴定及影响研究

对实验室培养的两株海洋微藻海洋卡盾藻和中肋骨条藻的可培养细菌进行了分离鉴定,分别在海洋卡盾藻和硅藻中肋骨条藻不同生长时期分离得到48株和34株可培养细菌,去除重复序列后得到12株不同的菌株。这些菌株分属α-变形杆菌纲、γ-变形杆菌纲、拟杆菌和放线菌。α-变形杆菌纲在两种藻的藻际环境中占据优势,其中红杆菌科是最为常见的科,中肋骨条藻的藻际细菌比海洋卡盾藻更为多样化。海洋卡盾藻生长后期细菌密度大幅度上升,可能导致了藻细胞的衰亡。中肋骨条藻相关细菌中有一株对旋链角毛藻的生长具有明显抑制作用,而大部分海洋卡盾藻相关细菌对角毛藻的生长具有抑制作用。本研究结果说明海洋卡盾藻的藻际细菌也许对其与硅藻的种间竞争中具有一定作用。     

Basin-scale distribution of prokaryotic phylotypes in the epipelagic layer of the Central South Pacific Ocean during austral summer

In the present study, we used catalyzed reporter deposition-fluorescence in situ hybridization to quantify the abundance of five bacterial (Alphaproteobacteria, SAR11, Gammaproteobacteria, SAR86, and Bacteroidetes) and two archaeal (Crenarchaeota and Euryarchaeota) phylotypes in the epipelagic layer (0–200 m) of the Central South Pacific Ocean along 170°W from 0° to 40°S. We found that the distribution patterns of these phylotypes differed from each other. All phylotypes except Gammaproteobacteria were particularly abundant at the surface water of the equatorial region, whereas Gammaproteobacteria was relatively abundant in the area from the southern part of the South Pacific Ocean. SAR11, affiliated with Alphaproteobacteria was the dominant phylotype at all depths, throughout the study area. The abundance of SAR11 significantly increased with chlorophyll a concentration, suggesting that phytoplankton could affect their distribution pattern. There was a positive correlation between Bacteroidetes abundance and water temperature, suggesting that the temperature gradient could be a critical factor determining their distribution in the South Pacific Ocean. Crenarchaeota and Euryarchaeota were more abundant at the equatorial region than in other study areas. Euryarchaeota abundance significantly decreased with depth, and increased with chlorophyll a concentration. This suggests that there was ecological interaction between Euryarchaeota and phytoplankton in the equatorial surface. Our data indicate that distinct hydrographic properties such as seawater temperature, salinity, and the concentrations of chlorophyll a and nutrients can principally control the basin-scale distribution of different prokaryotic phylotypes in the epipelagic layer of the Central South Pacific Ocean.     

The response of microbial communities to diverse organic matter sources in the Arctic Ocean

The response of Arctic microbial communities to a variety of natural organic matter substrates, including peat, ice algae and ice-rafted debris was examined using bacterial regrowth experiments and compared to unamended controls. Bacterial growth and production were followed together with the phylogenetic composition using length-heterogeneity polymerase chain reaction (LH-PCR), and 16S rRNA gene cloning and sequencing. Intact phospholipids (IPLs) and fatty acids evaluated the relationship between lipids and bacterial community structure and the impact of varied organic substrates on microbial lipid synthesis. Differential responses to organic matter sources were observed, with ice algae supporting both higher bacterial growth and production than terrestrial-derived peat. In spite of disparate growth kinetics, the community composition remained similar in all amended incubations as was confirmed by automated ribosomal intergenic spacer analysis (ARISA). Gammaproteobacteria dominated the initial incubations, whereas in extended incubations with terrestrial peat Alphaproteobacteria dominated; in particular Sulfitobacter phylotypes closely related (>99%) to an Arctic sea-ice-associated member of the Roseobacter clade (ARK10278). Arctic bacterioplankton preferentially synthesized two phospholipids, phosphatidylethanolamine (PE) and phosphatidylglygerol (PG), with 18:0n, 18:1Δ11, 16:0n and 16:1Δ9 as the primary fatty acids. Overall, results show that organic matter source plays an important role in structuring bacterioplankton community composition, with similar IPL and fatty acid lipid distributions observed among phylogenetically distinct bacteria.     

Long-term changes in the seasonality of selected diatoms related to grazers and environmental conditions

The decoupling of trophic interactions could be one of the severe consequences of climate warming in aquatic systems. The timing of phytoplankton blooms, in particular, can affect competition within the plankton community as well as food-web interactions with zooplankton and fish. Using long-term data from Helgoland Roads in the southern North Sea, we examine diatom seasonality, using three representative diatom species combined with environmental and copepod time series over the last four decades. The long-term annual abundances of Guinardia delicatula, Thalassionema nitzschioides and Odontella aurita exhibited interannual variations and dissimilar cyclic patterns during the time period under study (1962-2008). Of the three diatoms, G. delicatula showed a significant trend towards earlier bloom timings for 1962-2008 and a later decline of its abundance over time was found. Grazing and water transparency explained most of the bloom timing fluctuations of the diatoms considered. The annual timing of occurrences of each diatom species was correlated with their preceding concentrations. Earlier bloom timings occurred when autumn/winter concentrations were higher than average and later bloom timings occurred when autumn/winter concentrations were lower than average. Different environmental and predation variables related to the diatom bloom timings were found suggesting that climate warming might not affect the onset of the blooms of the three diatom species in the same manner. The results of the multiple linear regression analyses showed that the timings of decline of the three diatoms were mainly correlated with decreasing nutrient concentrations. Sunshine duration could prolong the duration of the blooms of T. nitzschioides and O. aurita provided that enough nutrients were available. In the case of G. delicatula, however, sunshine duration was negatively correlated with its end of the growth period. G. delicatula and T. nitzschioides showed later decreases in abundances under warmer spring and summer temperatures. Such species specific differences in the sensitivity to the forcing variables could lead to shifts in community structure and could ultimately have wider implications to the overall ecosystem health of the North Sea.     

Temporal dynamics of dissolved organic carbon (DOC) produced in a microcosm with red tide forming algae <Emphasis Type="Italic">Chattonella marina</Emphasis> and its associated bacteria

Acute and severe hypoxia associated with harmful algal bloom has become one of the major causes for the environmental deterioration of coastal areas. Although it is generally thought that a large part of the dissolved oxygen consumption at a bloom site is initiated by heterotrophic bacteria that attack organic matter derived from dead or dying algal cells, precise microbial processes leading to the hypoxia are yet to be examined. Here we show temporal dynamics of extracellular dissolved organic carbon (DOC) of the red tide forming raphidophyte Chattonella marina and bacterial populations associating with the algae under laboratory conditions. During the growth of non-axenic strains of C. marina, we monitored abundance of algae, associated bacteria, and DOC in the culture media. Bacterial cell abundance increased in response to the increase in DOC both at the beginning and the late log phase of the algal growth. Flow cytometric analysis revealed that transient increase in the percentage of respiratory-active bacterial cells also coincided with the timing of the increase in bacterial abundance and DOC. These results strongly suggest that DOC released from growing C. marina fuels respiration and growth of planktonic bacteria surrounding the algae. This has implications for the role of DOC released from C. marina bloom before the collapse in mediating interactions between neighboring algae and bacterial assemblage which may eventually lead to algal bloom-associated hypoxia.     

Population dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific

We characterized the community composition of phytoplankton in the western subarctic Pacific from the pre-bloom to the decline phase of the spring bloom with special reference to decreases in the silicic acid concentration in surface waters as an index for diatom bloom development. Furthermore, responses of heterotrophic bacteria and viruses to the spring bloom were also concomitantly investigated. Under pre-bloom conditions when nutrients were abundant but the surface mixed layer depth was relatively deep, chlorophyll (Chl) a concentrations were consistently low and green algae (chlorophytes and prasinophytes), cryptophytes, and diatoms were predominant in the phytoplankton assemblages as estimated by algal pigment signatures. Together with the shallowing of the mixed layer depth and the decrease in silicic acid concentration, diatoms bloomed remarkably in the Oyashio region, though the magnitude of the bloom in the Kuroshio-Oyashio transition (hereafter Transition) region was relatively small. A total of 77 diatom species were identified, with the bloom-forming diatoms mainly consisting of Thalassiosira, Chaetoceros, and Fragilariopsis species. It has become evident that the carotenoid fucoxanthin can serve as a strong indicator of the diatom carbon biomass during the spring diatom bloom. Differences in the species richness of diatoms among stations generally enabled us to separate the Oyashio bloom stations from the Transition and the Oyashio pre-bloom stations. Relatively high values of the Shannon-Wiener index for the diatom species were also maintained during the Oyashio bloom, indicating that a wide variety of species then shared dominance. In the decline phase of the Oyashio bloom when surface nutrient concentrations decreased, senescent diatom cells increased, as inferred from the levels of chlorophyllide a. Although the cell density of heterotrophic bacteria changed little with the development of the diatom bloom, viral abundance increased toward the end of the bloom, suggesting an increased likelihood of mortality among organisms including diatoms resulting from viral infection. This is the first report on the microbial trophodynamics, including viruses, during the spring diatom bloom in the western subarctic Pacific.     

Mass development of the planktonic diatom Proboscia alata over the bering sea shelf in the summer season

During most of the vegetation season from late May to early September, the large-sized diatom alga Proboscia alata forms local patches with high abundances and biomasses in different oceanographic domains of the eastern Bering Sea shelf. The average abundance and biomass of the species in these patches amounts to 0.7 × 10⁶ cells/l and 5 g WW/m³, respectively, for the layer of 0–25 m, while the corresponding estimates for the layer of the maximal species concentrations are 4.0 × 10⁶ cells/l and 38 g WW/m³ (1.6 g C/m³). These levels of abundance and biomass are typical of the spring diatom bloom in the region. The outbursts of P. alata mass development are an important element of carbon cycling in the pelagic zone of the shelf area in the summer season. The paradox of the P. alata summertime blooms over the middle shelf lies in their occurrences against the background of the sharp seasonal pycnocline and the deficiency in nutrients in the upper mixed layer. The duration of the outbursts in the P. alata development is about two weeks and the size of the patches with high abundances can be as large as 200 km across. Degradation of the P. alata summertime outbursts may occur during 4–5 days. The rapid sinking of the cells through the seasonal pycnocline results in an intense transport of organic matter to the bottom sediments. One of the possible factors responsible for the rapid degradation of the blooms is the affect on the population by ectoparasitic flagellates. At the terminal stages of the P. alata blooms, the share of infected cells may reach 70–99%.     

Dynamique Spatio-temporelle du Bactérioplancton d'une aire marine Méditerranéenne perturbée par l'effluent urbain de l'agglomération marseillaise

The sewage originated by the city of Marseilles (more than 1 M inhabitants) is entering the sea water at surface level in the calanque of Cortiou (6–8 km east of the city). In the 6 km ² basin concerned with the polluted flux, bacterial communities demonstrate some behavorial analogies with other plankton. In spite of organic and mineral enrichments, both bacterial and phytoplanktonic densities are similar to those of the oligotrophic surrounding sea water. As for zooplanktonic species, which are able to resist in polluted environments, bacteria show capabilities to grow in various cultural conditions. No effect of the annual temperature cycle is noted on bacterial numbers. But, considering the bacterial community dynamics, the mechanisms leading to these similarities are quite different for bacteria. Heterotrophic bacteria mineralize the exogenous organic matter far from the ‘point source’, as demonstrated by the good correlation between N-NH₄ and viable counts (r=0·98). The fluctuations of the numbers of viable heterotrophs and total coliforms (incubated at 37°C) are similar (r=0·98 for n=60) showinga low selective effect of the temperature and the capability for the same bacteria to grow both on selective or not selective medium. The biomass production resulting is shown by high values of the frequency of dividing cells and is due to heterotrophic bacteria. Bacterial production is more important in the more polluted part of the basin (N-NE), but, the bacterial density is regulated by factors varying in relation to the distance to the source of sewage. Near the mouth of the outlet sedimentation, dispersion, zooplanktonic grazing of particles with attached bacteria and bacterial predators grazing on free-living bacteria are the major causes of bacterial disappearance. In the west and south areas, where pollutants and organic material are diluted, the amount of substrate necessary for energy maintenance also plays a role. On the other hand, in these areas, no attempt was made to detect viable but not active cells in direct counts.     

Importance of seagrass as a carbon source for heterotrophic bacteria in a subtropical estuary (Florida Bay)

A stable carbon isotope approach was taken to identify potential organic matter sources incorporated into biomass by the heterotrophic bacterial community of Florida Bay, a subtropical estuary with a recent history of seagrass loss and phytoplankton blooms. To gain a more complete understanding of bacterial carbon cycling in seagrass estuaries, this study focused on the importance of seagrass-derived organic matter to pelagic, seagrass epiphytic, and sediment surface bacteria. Particulate organic matter (POM), seagrass epiphytic, seagrass (Thalassia testudinum) leaf, and sediment surface samples were collected from four Florida Bay locations with historically different organic matter inputs, macrophyte densities, and primary productivities. Bulk (observed and those reported previously) and compound-specific bacterial fatty acid δ¹³C values were used to determine important carbon sources to the estuary and benthic and pelagic heterotrophic bacteria. The δ¹³C values of T. testudinum green leaves with epiphytes removed ranged from −9.9 to −6.9‰. Thalassia testudinum δ¹³C values were significant more enriched in ¹³C than POM, epiphytic, and sediment samples, which ranged from −16.4 to −13.5, −16.2 to −9.6, and −16.7 to −11.0‰, respectively. Bacterial fatty acid δ¹³C values (measured for br14:0, 15:0, i15:0, a15:0, br17:0, and 17:0) ranged from −25.5 to −8.2‰. Assuming a −3‰ carbon source fractionation from fatty acid to whole bacteria, pelagic, epiphytic, and sediment bacterial δ¹³C values were generally more depleted in ¹³C than T. testudinum δ¹³C values, more enriched in ¹³C than reported δ¹³C values for mangroves, and similar to reported δ¹³C values for algae. IsoSource mixing model results indicated that organic matter derived from T. testudinum was incorporated by both benthic and pelagic bacterial communities, where 13–67% of bacterial δ¹³C values could arise from consumption of seagrass-derived organic matter. The IsoSource model, however, failed to discriminate clearly the fraction of algal (0–86%) and mangrove (0–42%) organic matter incorporated by bacterial communities. These results indicate that pelagic, epiphytic, and sediment surface bacteria consumed organic matter from a variety of sources. Bacterial communities incorporated consistently seagrass-derived organic matter, the dominant macrophyte in Florida Bay, but seagrass δ¹³C values alone could not account fully for bacterial δ¹³C values.     

Silicon dynamics within an intense open-ocean diatom bloom in the Pacific sector of the Southern Ocean

An intense diatom bloom developed within a strong meridional silicic acid gradient across the Antarctic Polar Front at 61°S, 170°W following stratification of the water column in late October/early November 1997. The region of high diatom biomass and the silicic acid gradient propogated southward across the Seasonal Ice Zone through time, with the maximum diatom biomass tracking the center of the silicic acid gradient. High diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of January 1998 (ca. 70 d) driving the gradient over 500 km to the south of its original position at the Polar Front. The bloom consumed 30 to >40 μM Si(OH)₄ in the euphotic zone between about 60 and 66°S leaving near surface concentrations <2.5 μM and occasionally <1.0 μM in its wake. Integrated biogenic silica concentrations within the bloom averaged 410 mmol Si m⁻² (range 162–793 mmol Si m⁻²). Average integrated silica production on two consecutive cruises in December 1997 and January 1998 that sampled the bloom while it was well developed were 27.5±6.9 and 22.6±20 mmol Si m⁻² d⁻¹, respectively. Those levels of siliceous biomass and silica production are similar in magnitude to those reported for ice-edge diatom blooms in the Ross Sea, Antarctica, which is considered to be among the most productive regions in the Southern Ocean. Net silica production (production minus dissolution) in surface waters during the bloom was 16–21 mmol Si m⁻² d⁻¹, which is sufficient for diatom growth to be the cause of the southward displacement of the silicic acid gradient. A strong seasonal change in silica dissolution : silica production rate ratios was observed. Integrated silica dissolution rates in the upper 100–150 m during the low biomass period before stratification averaged 64% of integrated production. During the bloom integrated dissolution rates averaged only 23% of integrated silica production, making 77% of the opal produced available for export to depth. The bloom ended in late January apparently due to a mixing event. Dissolution : production rate ratios increased to an average of 0.67 during that period indicating a return to a predominantly regenerative system.Our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around Antarctica also occur in the deep ocean near the Polar Front. The bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of Antarctica implying a role for such blooms in the formation of those sediments. Comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the Southern Ocean, which have been corrected for sediment focusing, indicate a burial efficiency of 4.6% for biogenic silica. That efficiency is considerably lower than previous estimates for the Southern Ocean.     

Bacterial community succession in response to dissolved organic matter released from live jellyfish

Jellyfish blooms have increased worldwide, and the outbreaks of jellyfish population not only affect the food web structures via voracious predation but also play an important role in the dynamics of nutrients and oxygen in planktonic food webs. However, it remains unclear whether specific carbon compounds released through jellyfish metabolic processes have the potential to shape bacterial community composition. Therefore, in this study, we aimed to investigate the compositional succession of the bacterioplankton community in response to the dissolved organic matter (DOM) released by the live Scyphomedusae Cyanea lamarckii and Chrysaora hysoscella collected from Helgoland Roads of the North Sea. The bacterial community was significantly stimulated by the DOM released form live jellyfish and different dominant phylotypes were observed for these two Scyphomedusae species. Furthermore, the bacterial community structures in the different DOM sources, jellyfish-incubated media, Kabeltonne seawater, and artificial seawater (DOM-free) were significantly different, as revealed by automated ribosomal intergenic spacer analysis fingerprints. Catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) revealed a rapid species-specific shift in bacterial community composition. Gammaproteobacteria dominated the community instead of the Bacteroidetes community for C. lamarckii, whereas Gammaproteobacteria and Bacteroidetes dominated the community for C. hysoscella. The significant differences in the bacterial community composition and succession indicate that the components of the DOM released by jellyfish might differ with jellyfish species.     

Benthic-pelagic coupling and export of organic carbon from a tropical Atlantic continental shelf—Sierra Leone

An approximate carbon budget for the Sierra Leone River Estuary and adjacent inner continental shelf indicates that phytoplankton production is balanced by grazers only in the wet season estuary; on the continental shelf, and in the dry season estuary, phytoplankton production exceeds demand of consumer populations by 70–90%. Regional production is dominated by the dry season estuary where diatom blooms support large population of the phytophagous clupeid fish Ethmalosa fimbriata. It is suggested that as well as what is used in bacterial respiration, not quantified in the study, important amounts of inshore organic carbon production are available for export, or burial, or both, either in inshore mudbanks or at the shelf edge by slumping. This carbon budget for a tropical continental shelf having neither coral-reefs nor coastal upwelling appears to correspond with what is emerging as a generic distinction between shelves and open ocean ecosystems: surplus production on shelves, balance between producers and consumers in the open ocean.     

Diel fluctuations in dissolved free amino acids and monosaccharides in Chesapeake Bay dinoflagellate blooms

The accumulation of phytoplankton biomass in recurring summer dinoflagellate blooms of Chesapeake Bay is accompanied by large pools of dissolved organic matter (DOM). Two fractions of the DOM, free amino acids (DFAA) and monosaccharides (MONO), were measured at 3 h intervals in mixed species dinoflagellate blooms (Katodinium rotundatum, Gymnodinium spp.) and related to productivity, biomass and photoperiod. Peak chlorophyll levels for the three blooms were 28, 65 and 938 μg1⁻¹. In general, DFAA and MONO concentrations increased with increasing biomass of bloom-forming species, reaching 203 and 844 μg1⁻¹. MONO appeared to accumulate during the day while there was no consistent pattern for DFAA. The accumulations of DFAA and MONO in blooms indicate that bloom production might stimulate microheterotrophy, thereby enhancing carbon and nutrient cycling in bloom-impacted regions.     

Factors controlling the temporal variation of fatty acids in piculate matter during a phytoplankton bloom in a marine mesocosm

Fatty acids present in suspended particles were examined following the onset of a phytoplankton bloom in an experimental marine mesocosm set up in Patricia Bay, Saanich Inlet, British Columbia, Canada. The predominantly diatom bloom, triggered by the addition of nutrients at the commencement of the experiment, was succeeded by several other phytoplankton forms, which in turn led to changes in particulate chl a, and organic carbon, and nitrogen within the experimental enclosure. An examination of individual fatty acids indicated that the variation of individual fatty acids can be classified into three groups, although the variation pattern of individual fatty acids differed from one another. An attempt was made to establish the factors controlling particulate fatty acid concentrations by principal component analysis. After Varimax rotation, six factors were extracted, of which four factors contributed to 83% of the data matrix. These were mainly dependent on the source of the particles i.e., diatoms, dinoflagellates, and bacteria.Fatty acid concentrations obtained by analysis of different lipid class fractions indicated that the production of fatty acids was distinctly related to the growth stage of the phytoplankton bloom. Fatty acids in triglycerides peaked during stationary phase, whereas fatty acids in polar lipids were high during the stage of an increase of phytoplankton biomass. However, fatty acid composition in both lipid classes changed under the influence of species succession of the phytoplankton population. This strongly suggested that species succession of the phytoplankton exerted the most significant control on fatty acid composition of particulate matter.     

米氏凯伦藻(Km02)共培养细菌群落的研究

米氏凯伦藻作为我国沿岸典型的有毒赤潮甲藻,近年来赤潮频度越来越密,赤潮规模越来越大,持续时间也越来越长,造成了严重的经济损失,并且威胁到海洋生态系统的健康。众多研究表明藻际细菌可能是影响赤潮生消与有毒藻产毒的关键生物影响因子。为揭示米氏凯伦藻共培养菌群的物种多样性信息,本文通过高通量测序技术手段,分析了我国沿岸产米氏凯伦藻(Km02)共培养菌群的种类、丰度、多样性及系统发育信息。结果显示,米氏凯伦藻(Km02)共培养菌群共有40个种与已有数据库匹配,隶属于4个门(Phylum),11个纲(Class),21个目(Order),26个科(Family),37个属(Genus)。其中优势菌属有8个(5%),分别为Balneola(20.28%)、 Marinobacter(19.25%)、Rhodobacteraceae科下的未知属(13.15%)、Alteromonas (8.88%)、Methylophaga (8.17%)、Thalassospira(6.24%)、Reichenbachiella (5.92%)及Mesoflavibacter (5.41%)。研究结果表明我国沿岸产米氏凯伦藻(Km02)含有生物多样性较高的共培养菌群,且具有探寻未知海洋细菌的潜力,同时该研究结果也为未来更进一步研究细菌群落对米氏凯伦藻赤潮暴发与消亡,以及产毒特性等机制的研究提供了实验方法与奠定了理论基础。     

The impact of changing climate on phenology,productivity, and benthic–pelagic coupling in Narragansett Bay

The timing and magnitude of phytoplankton blooms have changed markedly in Narragansett Bay, RI (USA) over the last half century. The traditional winter–spring bloom has decreased or, in many years, disappeared. Relatively short, often intense, diatom blooms have become common in spring, summer, and fall replacing the summer flagellate blooms of the past. The annual and summer mean abundance (cell counts) and biomass (chl a) of phytoplankton appear to have decreased based on almost 50 years of biweekly monitoring by others at a mid bay station. These changes have been related to warming of the water, especially during winter, and to increased cloudiness. A significant decline in the winter wind speed may also have played a role. The changes in the phenology of the phytoplankton and the oligotrophication of the bay appear to have decreased greatly the quantity and (perhaps) quality of the organic matter being deposited on the bottom of the bay. This decline has resulted in a very much reduced benthic metabolism as reflected in oxygen uptake, nutrient regeneration, and the magnitude and direction of the net flux of N₂ gas. Based on many decades of standard weekly trawls carried out by the Graduate School of Oceanography, the winter biomass of bottom feeding epibenthic animals has also declined sharply at the mid bay station. After decades of relatively constant anthropogenic nitrogen loading (and declining phosphorus loading), the fertilization of the bay will soon be reduced during May–October due to implementation of advanced wastewater treatment. This is intended to produce an oligotrophication of the urban Providence River estuary and the Upper Bay. The anticipated decline in the productivity of the upper bay region will probably decrease summer hypoxia in that area. However, it may have unanticipated consequences for secondary production in the mid and lower bay where climate-induced oligotrophication has already much weakened the historically strong benthic–pelagic coupling.     

Export and mesopelagic particle flux during a North Atlantic spring diatom bloom

Spring diatom blooms are important for sequestering atmospheric CO₂ below the permanent thermocline in the form of particulate organic carbon (POC). We measured downward POC flux during a sub-polar North Atlantic spring bloom at 100 m using thorium-234 (²³⁴Th) disequilibria, and below 100 m using neutrally buoyant drifting sediment traps. The cruise followed a Lagrangian float, and a pronounced diatom bloom occurred in a 600 km² area around the float. Particle flux was low during the first three weeks of the bloom, between 10 and 30 mg POC m⁻² d⁻¹. Then, nearly 20 days after the bloom had started, export as diagnosed from ²³⁴Th rose to 360-620 mg POC m⁻² d⁻¹, co-incident with silicate depletion in the surface mixed layer. Sediment traps at 600 and 750 m depth collected 160 and 150 mg POC m⁻² d⁻¹, with a settled volume of particles of 1000-1500 mL m⁻² d⁻¹. This implies that 25-43% of the 100 m POC export sank below 750 m. The sinking particles were ungrazed diatom aggregates that contained transparent exopolymer particles (TEP). We conclude that diatom blooms can lead to substantial particle export that is transferred efficiently through the mesopelagic. We also present an improved method of calibrating the Alcian Blue solution against Gum Xanthan for TEP measurements.     

Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations

The export of organic matter from the oceanic euphotic zone is a critical process in the global biogeochemical cycling of bioelements (C, N, P, Si). Much of this export occurs in the form of sinking particles, which rain down into the unlit waters of the deep sea. Classical models of oceanic production and export balance this gravitational loss of particulate bioelements with an upward flux of dissolved nutrients, and they describe reasonably well those areas of the ocean where deep winter mixing occurs. The surface waters of the North Pacific Subtropical Gyre (NPSG), however, are strongly stratified and chronically nutrient-depleted, especially in summer. Nevertheless, there is ample evidence that blooms of phytoplankton and subsequent pulses of particle export occur during the height of summer stratification in these waters, especially to the northeast of the Hawaiian Islands. These blooms impact regional bioelemental cycling and act as a food source to the deep-sea benthos. We review here numerous published observations of these events in the NPSG, and present new data collected at Station ALOHA (22.75°N, 158°W) during the first 176 cruises of the Hawaii Ocean Time-series program (1988-2005), along with results from transect cruises conducted in the region in 1996 and 2005. We suggest that the summer phytoplankton bloom can be considered a frequent, perhaps annual feature in the northeastern NPSG, and that its perceived stochastic nature is a manifestation of chronic undersampling in time and space. The bloom is typically dominated by only a few genera of large diatoms and the cyanobacterium Trichodesmium. It appears to be consistently supported by dinitrogen fixation, but the fate of the organic matter produced during the summer depends critically on the species composition of the responsible diazotrophs. We estimate that the summer bloom is responsible for up to 38% of N₂ fixation and up to 18% of N-based new production annually at Station ALOHA. We hypothesize that the spatial distribution, timing and magnitude of the bloom may be determined largely by the physical and biological processes controlling new phosphorus delivery into the euphotic zone during the summer and the preceding winter.