Globosa Bloom Research Articles

Comparative transcriptomics reveals colony formation mechanism of a harmful algal bloom species Phaeocystis globosa

Phaeocystis globosa is a major causative agent of harmful algal blooms in the global ocean, featuring a complex polymorphic life cycle alternating between free-living solitary cells and colonial cells. Colony is the dominant morphotype during P. globosa bloom. However, the underlying mechanism of colony formation is poorly understood. Here, we comprehensively compared global transcriptomes of P. globosa cells at four distinctive colony formation stages: free-living solitary cells, two cell-, four cell- and multi-cell colonies, under low (20 °C) and high (32 °C) temperatures, and characterized the genes involved in colony formation. Glycosaminoglycan (GAG) synthesis was enhanced while its degradation was decreased during colony formation, resulting in the accumulation of GAGs that are an essential substrate of the colony matrix. Nitrogen metabolism and glutamine synthesis were remarkably increased in the colonial cells, which provided precursors for GAG synthesis. Furthermore, cell defense and motility were down-regulated in the colonial cells, thereby conserving energy for GAG synthesis. Notably, high temperature led to decreased synthesis and increased degradation of GAGs, resulting in insufficient substrates to form the colony. Our study indicates that GAGs accumulation is critical for colony formation of P. globosa, but high temperature inhibits GAGs' accumulation and colony formation.

Pseudo-nitzschia sp. diversity and seasonality in the southern North Sea, domoic acid levels and associated phytoplankton communities

The diversity, toxicity and seasonality of Pseudo-nitzschia sp. were investigated from February to November 2012 in the southern Bight of the North Sea (SBNS) along the French coast. The identification of Pseudo-nitzschia species in this area was addressed for the first time in this study. Our results revealed a low species richness (3 distinct species) in association with moderate (102 pg mL−1) to high (263 pg mL−1) domoic acid (DA) levels in autumn and spring, respectively.Pseudo-nitzschia succession corresponded to the dominance of P. delicatissima in April–May (86% of total diatoms) as a co-occurring species of the Phaeocystis globosa bloom. Following the Phaeocystis bloom (May–September), P. pungens dominated markedly over P. fraudulenta and P. delicatissima and was the only species present in autumn, although at low abundance (<1000 cell L−1). The results of this study support the idea that Pseudo-nitzschia seasonality in the SBNS relies principally on temperature and nutrient availability (DIN and silicates), which, in turn, depend on locally fluctuating environmental conditions (rainfalls and winds). This study highlights the potential for the SBNS to be a potential risk area in regard to the possible impacts of DA on marine resources and the DA transfer through marine food webs. This is of particular concern since DA concentration in seawater was not systematically correlated to potentially toxic Pseudo-nitzschia abundance.

Complementary Metaproteomic Approaches to Assess the Bacterioplankton Response toward a Phytoplankton Spring Bloom in the Southern North Sea.

Annually recurring phytoplankton spring blooms are characteristic of temperate coastal shelf seas. During these blooms, environmental conditions, including nutrient availability, differ considerably from non-bloom conditions, affecting the entire ecosystem including the bacterioplankton. Accordingly, the emerging ecological niches during bloom transition are occupied by different bacterial populations, with Roseobacter RCA cluster and SAR92 clade members exhibiting high metabolic activity during bloom events. In this study, the functional response of the ambient bacterial community toward a Phaeocystis globosa bloom in the southern North Sea was studied using metaproteomic approaches. In contrast to other metaproteomic studies of marine bacterial communities, this is the first study comparing two different cell lysis and protein preparation methods [using trifluoroethanol (TFE) and in-solution digest as well as bead beating and SDS-based solubilization and in-gel digest (BB GeLC)]. In addition, two different mass spectrometric techniques (ESI-iontrap MS and MALDI-TOF MS) were used for peptide analysis. A total of 585 different proteins were identified, 296 of which were only detected using the TFE and 191 by the BB GeLC method, demonstrating the complementarity of these sample preparation methods. Furthermore, 158 proteins of the TFE cell lysis samples were exclusively detected by ESI-iontrap MS while 105 were only detected using MALDI-TOF MS, underpinning the value of using two different ionization and mass analysis methods. Notably, 12% of the detected proteins represent predicted integral membrane proteins, including the difficult to detect rhodopsin, indicating a considerable coverage of membrane proteins by this approach. This comprehensive approach verified previous metaproteomic studies of marine bacterioplankton, e.g., detection of many transport-related proteins (17% of the detected proteins). In addition, new insights into e.g., carbon and nitrogen metabolism were obtained. For instance, the C1 pathway was more prominent outside the bloom and different strategies for glucose metabolism seem to be applied under the studied conditions. Furthermore, a higher number of nitrogen assimilating proteins were present under non-bloom conditions, reflecting the competition for this limited macro nutrient under oligotrophic conditions. Overall, application of different sample preparation techniques as well as MS methods facilitated a more holistic picture of the marine bacterioplankton response to changing environmental conditions.

Spatio-temporal patterns in phytoplankton assemblages in inshore–offshore gradients using flow cytometry: A case study in the eastern English Channel

A pulse-shape recording flow cytometer (CytoSense©) was applied to the monitoring of changes in phytoplankton distribution along an inshore–offshore transect across the eastern English Channel (EEC), on 13 occasions during the main productive period of the year. Amongst the eight phytoplankton groups discriminated, picophytoplankton (picoeukaryotes and Synechococcus spp.) and Phaeocystis globosa nanoflagellates were the main contributors to total phytoplankton abundance, while Diatoms-like, Coccolithophores, and Cryptophytes represented each one less than 5%. High spatial resolution revealed important changes on relatively short distances. Moreover, a general decrease of Diatoms-like, P. globosa haploid cells, Coccolithophores, and picoeukaryote abundance was evidenced from inshore to offshore waters, associated with an increase of Synechococcus spp. abundance. Seasonal variability accounted for 71% of phytoplankton abundance changes. Compared to previous studies in the area the CytoSense allowed highlighting new players during the winter–spring–summer phytoplankton succession: (i) high abundance of Synechococcus spp. and picoeukaryotes I in winter and of Synechococcus spp. also in the summer, (ii) a transient abundance peak of picoeukaryotes II, and (iii) high abundance of Coccolithophores and Cryptophytes during the wax of P. globosa bloom and in the summer. The relationships between environmental variables and phytoplankton assemblages indicated that nutrients and the daily light intensity were the most important parameters in structuring the winter–spring–summer transitions.

Planktonic community structure during a harmful bloom of Phaeocystis globosa in a subtropical bay, with special reference to the ciliate assemblages.

Planktonic community structure was investigated during outbreak of harmful Phaeocystis globosa bloom in a subtropical bay, the Maowei Sea, South China Sea. The phytoplankton assemblage was numerically dominated by colonial P. globosa, with its abundance ranging from 1.23 × 10(8) to 11.12 × 10(8) cells m(-3) and contributing nearly 90 % to the total abundance. Totally 66 mesozooplankton (>169 µm) and 19 ciliates species were recorded, with the densities ranged from 169 to 1633 ind m(-3) and 74 to 1118 cells L(-1), respectively. The dominant species for mesozooplankton were Copepoda (larvae), Bestiola sinicus, B. amoyensis, Macrura (larvae) and Acartia spinicauda, respectively. The ciliate assemblage was numerically dominated by Codonella rapa, Strombidium globosaneum and Mesodinium rubrum. During the bloom, P. globosa seemed to be negatively affected by the nutrient phosphate significantly (p < 0.05). However, no correlation between P. globosa and ciliate assemblage was detected, but P. globosa was negatively correlated with total biomass of mesozooplankton and abundance of B. sinicus (p < 0.05), suggesting that P. globosa was uncoupled from the grazing by both ciliates and mesozooplankton when appearing as colonies form. On the other hand, both positive and negative correlations among the dominant groups of mesozooplankton and ciliates were observed (p < 0.05) which possibly indicated that the predation of mesozooplankton upon ciliates might be strengthened during the Phaeocystis bloom and the complex effect also varied from species to species.

Winter–Summer Succession of Unicellular Eukaryotes in a Meso-eutrophic Coastal System

The objective of this study was to explore the succession of planktonic unicellular eukaryotes by means of 18S rRNA gene tag pyrosequencing in the eastern English Channel (EEC) during the winter to summer transition. The 59 most representative (>0.1%, representing altogether 95% of total reads), unique operational taxonomic units (OTUs) from all samples belonged to 18 known high-level taxonomic groups and 1 unaffiliated clade. The five most abundant OTUs (69.2% of total reads) belonged to Dinophyceae, Cercozoa, Haptophyceae, marine alveolate group I, and Fungi. Cluster and network analysis between samples distinguished the winter, the pre-bloom, the Phaeocystis globosa bloom and the post-bloom early summer conditions. The OTUs-based network revealed that P. globosa showed a relatively low number of connections-most of them negative-with all other OTUs. Fungi were linked to all major taxonomic groups, except Dinophyceae. Cercozoa mostly co-occurred with the Fungi, the Bacillariophyceae and several of the miscellaneous OTUs. This study provided a more detailed exploration into the planktonic succession pattern of the EEC due to its increased depth of taxonomic sampling over previous efforts based on classical monitoring observations. Data analysis implied that the food web concept in a coastal system based on predator-prey (e.g. grazer-phytoplankton) relationships is just a part of the ecological picture; and those organisms exploiting a variety of strategies, such as saprotrophy and parasitism, are persistent and abundant members of the community.

Do transparent exopolymeric particles (TEP) derived from Phaeocystis globosa bloom impact the physiological performances of European sea bass juveniles

A mesocosm experiment (1m3 enclosures) was carried out over 28days to evaluate the effect of Transparent Exopolymer Particles (TEP) derived from decaying Phaeocystis globosa colonies on the physiological performance and mortality of European sea bass juveniles (35mm in length and 113days old). Accordingly, sea bass juveniles were exposed to high TEP concentrations derived from both a culture of decaying P. globosa colonies (7634.27±3514.41μgXGeqL−1) and freshly sea foam (932.54±341.21μgXGeqL−1). During the experiment, environmental parameters (pH, dissolved oxygen, temperature and salinity) were stable, similar among treatments and not limiting for growth. Only TEP concentration varied between treatments. Fish mortality rate was relatively low (0 to 2%d−1) and mostly occurred during the first week when TEP concentrations were relatively low. Despite high and varying TEP concentrations, juvenile sea bass physiological performances (growth in length and weight, Fulton condition index and RNA–DNA ratio) did not differ between replicates and treatments. In addition, juvenile sea bass increased both in size by 7.2mm (growth rate of 0.26mmd−1) and weight by 708mg (growth rate of 25.29mgd−1) during the 28days of the experiment. Altogether, these results showed clearly that TEP derived from decaying P. globosa bloom and foam accumulation have no negative effect on juvenile sea bass physiological performances and survival if water dissolved oxygen is maintained high. Despite being one of the most recurrent phytoplankton blooms recorded in the northwest European shelf seas and forms large blooms in the coastal waters during spring, we suggest that Phaeocystis bloom has no deleterious effect on marine coastal fish aquaculture activities.

Microzooplankton herbivory during the diatom– Phaeocystis spring succession in the eastern English Channel

The dilution technique was used to investigate microzooplankton grazing and phytoplankton growth in the eastern English Channel during the diatom– Phaeocystis spring succession from January 2009 to June 2009. Four periods were defined based on phytoplankton composition: Periods 1, 2 and 4 composed of distinct diatom communities of small (5–20 μm length) to larger cells (20–120 μm) in colonies; Period 3 characterized by the Phaeocystis globosa bloom. Dilution experiments were conducted before, during and after the P. globosa bloom. Microzooplankton carbon consumption (from 18.1 to 360.9 μg C L − 1 d − 1 ) often equalled or exceeded phytoplankton production (from 1.7 to 129.0 μg C L − 1 d − 1 ) in particular at the end of the P. globosa bloom when microzooplankton grazed on previously formed phytoplankton biomass. Results of size-fractionated dilution experiments, conducted with distinct grazer communities, suggested different roles for ciliates and dinoflagellates. Ciliates appeared to be very efficient grazers of small diatoms (5–10 μm) and P. globosa free cells, whereas dinoflagellates grazed on both larger diatoms (> 10 μm; P1) and small P. globosa colonies. Ciliates and dinoflagellates did not seem to compete for food resources, as they were oriented towards different phytoplankton size classes.

Succession of primary producers and micrograzers in a coastal ecosystem dominated by Phaeocystis globosa blooms

The community structures and succession of phytoplankton, protozooplankton and copepods were studied from February 2007 to July 2009 in a coastal area of the eastern English Channel subject to Phaeocystis globosa blooms. While diatom blooms preceded P. globosa blooms each year, the community structure and stock of heterotrophic protists appeared to be related to the dominant P. globosa life cycle stages. In 2007, the dominance of large colonies (>100 µm, up to 316 µg C L−1), which resulted in a high biomass of healthy free cells (up to 132 µg C L−1), accompanied high spirotrich ciliate stocks (up to 58 µg C L−1) and high abundances of the copepods Acartia clausi and Temora longicornis (up to 11 ind. L−1). In 2008, the bloom which lasted a shorter period of time was dominated by large colonies (up to 328 µg C L−1) and fewer free cells (up to 98 µg C L−1). This corresponded with a lower abundance of grazers, with stocks of heterotrophic protists and copepods 1.6 times and 2.2 times lower, respectively. In 2009, the P. globosa bloom was again dominated by large colonies and <100 µm diatoms. This corresponded to a dominance of heterotrophic dinoflagellates among the protists (62% of the total heterotrophic protist biomass) and Acartia clausi (55% of the copepod abundance). Overall, heterotrophic dinoflagellates appeared to be likely the most important group of phytoplankton grazers.

Temporal changes of major bacterial groups and bacterial heterotrophic activity during a Phaeocystis globosa bloom in the eastern English Channel

The temporal changes in major bacterial groups and bulk heterotrophic activity were determined during the growth, senescence and post-bloom phases of a Phaeocystis globosa bloom, at a coastal site in the eastern English Channel. Cell-specific exoenzymatic activities were highest dur- ing the growth period of P. globosa, while bacterial abundance and bacterial heterotrophic produc- tion peaked during the senescence of the P. globosa bloom. Bacteroidetes and Gammaproteobacteria were the most important bacterial groups during the growth period of P. globosa, contributing between 13 and 47% of bulk bacterial abundance and leucine incorporation. At the end of the phyto- plankton growth period, Gammaproteobacteria were the most important contributors to bacterial heterotrophic production, accounting for 68% of bulk leucine incorporation. During the senescent phase, 36 and 29% of bulk leucine incorporation were attributable to Bacteroidetes and Alphapro- teobacteria, respectively. Finally, after the disappearance of the bloom, Gamma- and Alphapro- teobacteria dominated by the Roseobacter clade were responsible for 33 and 43% of bulk leucine incorporation, respectively. The relative abundance of the bacterial groups showed little variability between consecutive dates during most of the study period. The contributions of different bacterial groups to bulk abundance and leucine incorporation were correlated with exo-proteolytic and -glu- cosidic activities and with particulate organic carbon, suggesting at least some specificity of these bacterial groups with respect to their metabolic properties in the environment.

Impact of the Phaeocystis globosa spring bloom on the intertidal benthic compartment in the eastern English Channel: A synthesis

From 1999 to 2005, studies carried out in the frame of regional and national French programs aimed to determine whether the Phaeocystis globosa bloom affected the intertidal benthic communities of the French coast of the eastern English Channel in terms of composition and/or functioning. Study sites were chosen to cover most of the typical shore types encountered on this coast (a rocky shore, an exposed sandy beach and a small estuary). Both the presence of active Phaeocystis cells and their degradation product (foam) did have a significant impact on the studied shores. The primary production and growth rates of the kelp Saccharina latissima decreased during the bloom because of a shortage of light and nutrient for the macroalgae. On sandy sediments, the benthic metabolism (community respiration and community primary production), as well as the nitrification rate, were enhanced during foam deposits, in relation with the presence of bacteria and active pelagic cells within the decaying colonies. In estuarine sediments, the most impressive impact was the formation of a crust at the sediment surface due to drying foam. This led to anoxic conditions in the surface sediment and resulted in a high mortality among the benthic community. Some organisms also tended to migrate upward and were then directly accessible to the higher trophic level represented by birds. Phaeocystis then created a shortcut in the estuarine trophic network. Most of these modifications lasted shortly and all the systems considered came back to their regular properties and activities a few weeks after the end of the bloom, except for the most impacted estuarine area.

Mesoscale and microscale spatial variability of bacteria and viruses during a Phaeocystis globosa bloom in the Eastern English Channel

Sampling was conducted within inshore and offshore sites, characterized by highly dissimilar hydrodynamic and hydrobiological conditions, in the Eastern English Channel. The eutrophic inshore site was dominated by the influence of a dense bloom of the Prymnesiophyceae phytoplankton species Phaeocystis globosa, while the offshore site was characterized by more oceanic conditions. Within each site the microscale distributions of chlorophyll a and several flow cytometrically-defined subpopulations of heterotrophic bacteria and viruses were measured at a spatial resolution of 5 cm. The inshore site was characterized by comparatively high levels of microscale spatial variability, with concentrations of chlorophyll a, heterotrophic bacteria, and viruses varying by 8, 11 and 3.5-fold respectively across distances of several centimeters. Within the offshore site, microscale distributions of chlorophyll a and bacteria were markedly less variable than within the inshore site, although viruses exhibited slightly higher levels of heterogeneity. Significant mesoscale variability was also observed when mean microbial parameters were compared between the inshore and offshore sites. However, when the extent of change (max/min and coefficient of variation) was compared between meso- and microscales, the variability observed at the microscale, particularly in the inshore site, was substantially greater. This pattern suggests that microscale processes associated with Phaeocystis globosa bloom dynamics can generate heterogeneity amongst microbial communities to a greater degree than large scale oceanographic discontinuities.

Microbial dynamics in autotrophic and heterotrophic seawater mesocosms. I. Effect of phytoplankton on the microbial loop

The effect of phytoplankton on the other compartments of a simple microbial loop con- sisting of dissolved matter, bacteria and nanoflagellates was investigated in three 1000 l mesocosms. These mesocosms were inoculated with a natural bacterial community and (1) no other additions (No Addition tank), (2) Phaeocystis globosa (Phaeocystis tank), or (3) 2 diatom species (Diatom tank). Dur- ing the 20 d experiment, autotrophic activity was negligible in the No Addition tank. In contrast a small P. globosa bloom developed in the Phaeocystis tank and a large diatom bloom dominated the Diatom tank. In this paper we describe the experiment, the changes in chlorophyll a and hetero- trophic nanoflagellate concentrations, as well as the cycling of nitrogen, phosphorus, and silica. Then we provide a synthesis of the structure and functioning of the microbial loops in these 3 systems using cluster analysis, a statistical pattern recognition tool. The goal was to test the hypothesis that differ- ences in the resident phytoplankton populations would be reflected in (1) the composition and con- centration of dissolved organic matter, (2) the composition of the bacterial community, (3) the food web, and (4) the cycling of elements and organic matter. In all 3 mesocosms, nitrate and silicic acid remained abundant. Orthophosphate was preferred by diatoms, whereas Phaeocystis appeared to prefer dissolved organic phosphorus. The hypothesis that phytoplankton composition shapes the structure and functioning of the microbial loop was partially supported: 6 d after inoculation each mesocosm exhibited a distinct organic matter signature. After 10 to 12 d, concentrations of hetero- trophic nanoflagellates were high enough to exert significant grazing pressure in all 3 mesocosms. A parallel shift in bacterial community composition was visible in all mesocosms at this time, possibly reflecting grazing pressure. The food-web structure developed divergently in the 3 mesocosms dur- ing the second half of the experiment. Differences in biochemical cycling between mesocosms were predominantly driven by the large quantitative differences in autotrophs.

A Phaeocystis globosa bloom associated with upwelling in the subtropical South Atlantic Bight

We observed an unusual, subtropical bloom of Phaeocystis globosa Scherffel during a strong upwelling event and confirmed its identity using morphological, physiological and genetic traits. This low-latitude bloom of P. globosa colonies occurred on the South Atlantic Bight continental shelf during the summer of 2003. Maximum chlorophyll a concentration measured during this subsurface bloom was 11.37 μg L at 31 m depth. Divers reported abundant flocculent material below the steep thermocline, and this material was identified as Phaeocystis sp. using microscopy. Using morphological and physiological traits as well as the sequence of the 18S small subunit ribosomal RNA gene (GenBank Accession # EF100712), this phytoplankton was identified as P globosa. Interactions of physical and biological conditions may restrict low-latitude P globosa blooms to strong upwelling events.

Variability and classification of remote sensing reflectance spectra in the eastern English Channel and southern North Sea

During spring and summer 2004, intensive field bio-optical campaigns were conducted in the eastern English Channel and southern North Sea to assess the mechanisms regulating the ocean color variability in a complex coastal environment. The bio-optical properties of the sampled waters span a wide range of variability, due to the various biogeochemical and physical processes occurring in this area. In-water hyperspectral remote sensing reflectances ( R rs) were acquired simultaneously with measurements of optically significant parameters at 93 stations. An empirical orthogonal function (EOF) analysis indicates that 74% of the total variance of R rs is partly explained by particulate backscattering ( b bp), while particulate and dissolved absorption only explain 15% of the ocean color variability. These results confirm, for the first time from in situ backscattering measurements, previous studies performed in other coastal environments. Whereas the amplitude factors of the first EOF mode are well correlated ( r = 0.75) with the particulate backscattering coefficient ( b bp), the highest correlation ( r = 0.83) is found with the particulate backscattering ratio ( b bp/ b p). This result highlights the fundamental role of the nature of the bulk particulate assemblage in the ocean color variability. An unsupervised hierarchical cluster analysis applied to our data set of normalized R rs spectra, leads to five spectrally distinct classes. We show that the class-specific mean R rs spectra significantly differ from one another by their bio-optical properties. Three classes particularly stand out: one class corresponds to a Phaeocystis globosa bloom situation, whereas the two others are associated with water masses dominated by mineral and non-living particles, respectively. Among the different bio-optical parameters, the particulate backscattering ratio, the chlorophyll concentration, and the particulate organic carbon to chlorophyll ratio, are the most class-specific ones. These different results are very encouraging for the inversion of bio-optical parameters from class-specific algorithms.

Abundance and activity of major groups of prokaryotic plankton in the coastal North Sea during spring and summer

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 45:237-246 (2006) - doi:10.3354/ame045237 Abundance and activity of major groups of prokaryotic plankton in the coastal North Sea during spring and summer Anne-Carlijn Alderkamp1,3,*, Eva Sintes2, Gerhard J. Herndl2 1Department of Marine Biology, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, 9759 AA Haren, The Netherlands 2Department of Biological Oceanography, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands 3Present address: Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305-2215, USA *Email: alderkam@stanford.edu ABSTRACT: The dynamics of the abundance and activity of selected heterotrophic prokaryotic groups were determined in the coastal North Sea during a coastal spring bloom dominated by the prymnesiophyte Phaeocystis globosa and in the subsequent spring and summer season using catalyzed reporter deposition-fluorescence in situ hybridization combined with microautoradiography (MICRO-CARD-FISH). Both Crenarchea and Euryarchaea were detected throughout the study period, albeit never at levels that exceeded 2% of the total prokaryotic abundance, corresponding to a maximum abundance of 2.8 × 108 Archaea l–1. Euryarchaea were generally more abundant than Crenarchaea. On average, 21% of the Crenarchaea and 35% of the Euryarchaea were taking up leucine throughout the study period. Members of the Bacteroidetes were abundant during the P. globosa bloom period. At the senescent stage of the bloom, Bacteroidetes comprised up to 63% of the prokaryotic community. The abundances of members of the Roseobacter clade as well as the SAR86 cluster were low during the phytoplankton spring bloom period (mean 2% each), but increased during August. On average, 51% of the Roseobacter, 38% of the Bacteroidetes and 39% of the SAR86 cluster were taking up leucine. The percentages of active Bacteria increased during the decline of the P. globosa spring bloom. The Bacteroidetes cluster showed the strongest increase, indicating that members of this cluster are likely to play a major role in the degradation of organic matter produced in the P. globosa spring bloom. KEY WORDS: Marine Archaea · Bacteroidetes · Roseobacter · North Sea · CARD-FISH · Microautoradiography · Bacterial succession · Phaeocystis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 45, No. 3. Online publication date: December 21, 2006 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2006 Inter-Research.

Phytoplankton community structure and primary production in small intertidal estuarine-bay ecosystem (eastern English Channel, France)

From May 2002 to October 2003, a fortnightly sampling programme was conducted in a restricted macrotidal ecosystem in the English Channel, the Baie des Veys (France). Three sets of data were obtained: (1) physico-chemical parameters, (2) phytoplankton community structure illustrated by species composition, biovolume and diversity, and (3) primary production and photosynthetic parameters via P versus E curves. The aim of this study was to investigate the temporal variations of primary production and photosynthetic parameters in this bay and to highlight the potential links with phytoplankton community structure. The highest level of daily depth-integrated primary production Pz (0.02–1.43 g C m−2 d−1) and the highest maximum photosynthetic rate PB max (0.39–8.48 mg C mg chl a−1 h−1) and maximum light utilization coefficient αB [0.002–0.119 mg C mg chl a−1 h−1 (μmol photons m−2 s−1)] were measured from July to September. Species succession was determined based on biomass data obtained from cell density and biovolume measurements. The bay was dominated by 11 diatoms throughout the year. However, a Phaeocystis globosa bloom (up to 25 mg chl a m−3, 2.5 × 106 cells l−1) was observed each year during the spring diatom bloom, but timing and intensity varied interannually. Annual variation of primary production was due to nutrient limitation, light climate and water temperature. The seasonal pattern of microalgal succession, with regular changes in composition, biovolume and diversity, influenced the physico-chemical and biological characteristics of the environment (especially nutrient stocks in the bay) and thus primary production. Consequently, investigation of phytoplankton community structure is important for developing the understanding of ecosystem functioning, as it plays a major role in the dynamics of primary production.

Coastal bacterial viability and production in the eastern English Channel: A case study during a Phaeocystis globosa bloom

Heterotrophic bacterial standing stocks (total and viable cells) and production were determined in the coastal surface waters of the eastern English Channel, during different stages of a phytoplankton succession. Two coastal zones of variable freshwater influence were surveyed within the ‘coastal flow system’ (Wimereux and Somme Bay) where massive and recurrent Phaeocystis globosa blooms take place in spring. The proportion of intact (MEM+) cells, assessed by the LIVE/DEAD® BacLight™ (L/D) method, varied from 15 to 94% at the two coastal stations studied (median of 46%). MEM+ and total (DAPI) cell counts were significantly correlated over the study period, whereas the higher proportion of MEM+ cells did not correspond to an elevated bacterial cell production (BP). Low levels of living (potentially active) cells were nevertheless responsible for the high productivity levels within the bacterial community when the P. globosa bloom declined. Our study revealed that the bacterial carbon production/primary production ratios (BCP/PP) showed broad variations (7 to 111%) within each site, going from low values (7–16%) when the bloom was the most productive, to higher values (61–111%) at the end of the bloom. This suggested (i) a temporal uncoupling between bacteria and phytoplankton throughout the bloom duration and (ii) a drastic change of the amount of PP potentially processed by the bacterial community among high and low productive periods. The BCP increase after the decline of the P. globosa bloom implies that, at this time, a large part of the phytoplankton-derived organic matter (OM) was remineralised via the bacterial heterotrophic production. With respect to the L/D results, this bacterial remineralisation was due to a small yet productive total cell fraction.

Biologically induced modification of seawater viscosity in the Eastern English Channel during a Phaeocystis globosa spring bloom

To identify the potential relationship between Pheaocystis globosa bloom conditions and seawater properties, a hydrobiological survey was performed in the inshore waters of the Eastern English Channel over the course of the phytoplankton spring bloom. Chlorophyll concentration, auto- and hetero/mixotrophic composition of protists and standing stock, and seawater viscosity were measured weekly from March to June 2004. The decline of the bloom is characterized by a massive foam formation in the turbulent surf zone. Before foam formation, seawater viscosity significantly increased, showing a significant positive correlation with chlorophyll concentration. In contrast, after foam formation this correlation was negative, seawater viscosity kept increasing despite a sharp decrease in chlorophyll concentrations. No significant correlation has been found between seawater viscosity and the composition of the phytoplankton assemblages observed during the survey. However, significant positive correlations have been found between seawater viscosity and both the size and the abundance of P. globosa colonies. From the correlation patterns observed between chlorophyll concentration and seawater viscosity, we suggest that the rheological properties of seawater are mainly driven by extracellular materials associated with colony formation and maintenance rather than by cell composition and standing stock.

A mesocosm study of Phaeocystis globosa population dynamics : I. Regulatory role of viruses in bloom control

The regulatory role of viruses on population dynamics of the prymnesiophyte Phaeocystis globosa was studied during a mesocosm experiment in relation to growth and loss by microzooplankton grazing and cell lysis. The mesocosms were conducted under varying light conditions (20 and 150 μmol photons m −2 s −1) and nutrient regime (inorganic nitrogen to phosphorus ratios of 4, 16 and 44). Overall, viruses infecting P. globosa (PgV) were found to be an important cause of cell lysis (30–100% of total lysis) and a significant loss factor (7–67% of total loss). We demonstrate that the morphology of P. globosa cells (solitary versus colonial) differently regulated viral control of P. globosa bloom formation. Reduced irradiance (20 μmol photons m −2 s −1) was provided for 11 days to select for the solitary cell morphotype. Viruses were able to restrict P. globosa bloom formation even after irradiance became saturating again (150 μmol photons m −2 s −1). Saturating light conditions from the start of the experiment allowed colony formation and because the colony-morphotype acted as a mechanism reducing viral infection bloom formation succeeded. Nutrient depletion, however, affected specifically the colonies that disintegrated while releasing single cells. Virus infection of these solitary cells resulted in the termination of the bloom. The nature of phytoplankton growth-limiting nutrient (nitrate and/or orthophosphate) did not seem to noticeably affect the level of viral control.