Fereidoun Rassoulzadegan

PLANKTON AND NUTRIENT DYNAMICS IN MARINE WATERS

Abstract Biological oceanographers generally distinguish between two contrasting trophic pathways in the pelagic environment, i.e. the herbivorous and the microbial food webs. The former goes from large phytoplankton and zooplankton to fish, whereas the latter comprises small eukaryotic algae and cyanobacteria as well as heterotrophic bacteria and protozoa. The present paper describes a continuum of trophic pathways, between systems dominated by the herbivorous food web and those dominated by the microbial loop (i.e. almost closed system of heterotrophic bacteria and zooflagel-late grazers, the latter releasing dissolved organic matter used as substrate by the bacteria). It is proposed that the continuum goes from the herbivorous web (or chain) to a “multivorous food web”, to the microbial web, and finally the microbial loop. Characteristics of the various pathways maybe summarized as a series of interconnected ratios. It is hypothesized that systems dominated by the herbivorous food web or the microbial ...

Conceptual models for the biogeochemical role of the photic zone microbial food web, with particular reference to the Mediterranean Sea

Abstract Observations are reviewed which indicate that not only phytoplankton, but also heterotrophic bacteria are P-limited during those seasons when there is stratification in the Mediterranean. It is discussed how these observations fit into a general concept of a size-structured food chain where the structure is a result of combined top-down control from size-selective predators and size-dependent competition for phosphate. It is argued that, conceptually, labile DOC and silicate have symmetrical roles in potentially controlling the flux of P through the ‘microbial’ and ‘classical’ sides, respectively, of this food web. The resulting concept provides a model linking C, P, and Si fluxes to the size spectrum of biogenic particles in the photic zone.

Phosphate transfer between microbial size-fractions in Villefranche bay (N. W. Mediterranean Sea), France in autumn 1992

Abstract Turnover time of orthophosphate, uptake of phosphate into particulate size-fractions (0.2-1, 1-5, 5-10, > 10 µm), and subsequent release from size-fractions were examined using 32P, in samples from surface waters of Villefranche Bay during the autumnal erosion of the thermocline. Turnover time of orthophosphate increased from 1.6 h in early October to 58 h in December. Throughout the study period, total uptake was dominated (50-68%) by the smallest size-fraction (0.2-1 µm) presumably corresponding to auto- and heterotrophic bacteria as it contained little chlorophyll a. Among the size-fractions > 1 µm, representing eucaryotic phytoplankton uptake, the erosion of the thermocline was accompanied by a shift from the total dominance of uptake by the 1-5 µm fraction (98%) toward an increasing contribution from the > 10 µm fraction (˜ 0 to 50%). In cold chase experiments, release of 32P from the size-fractions which dominated uptake (0.2-1 µm & 1-5 µm), was relatively slow (≤ 1% h-1) indicating long tu...

Release of dissolved amino acids by flagellates and ciliates grazing on bacteria

Abstract Release of amino acids was examined in the laboratory in the form of dissolved primary amine (DPA) by two marine planktonic protozoa (the oligotrichous ciliate, Strombidium sulcatum and the aplastidic flagellate Pseudobodo sp.) grazing on bacteria. DPA release rates were high (19–25 × 10 −6 and 1.8–2.3 × 10 −6 μmol DPA cell −1 h −1 for flagellates and ciliates, respectively) during the exponential phase, when the ingestion rates were maximum. Release rates were lower during the other growth phases. The release of DPA accounted for 10 % (flagellates) and 16 % (ciliates) of the total nitrogen ingested. Our data suggest that the release of DPA by protozoa could play an important role in supporting bacterial and consequently autotrophic pico- and nanoplankton growth, especially in oligotrophic waters, where the release of phytoplanktonic dissolved organic matter is low.

Role of micro- and nanozooplankton in marine food webs

Besides distinguishing between ‘zooplankton’ and ‘phytoplankton’, one of the more useful ways for oceanographers to classify planktonic organisms is by size (e.g. Sieburth et al. , 1978). Of the free-living protozoa in the marine plankton, most fall into the ‘microplankton’ (20–200 μm, including many of the sarcodinans and ciliates as well as a number of larval metazoa and the larger phytoplankton) and ‘nanoplankton’ (2–20 μm, including the smaller ciliates and most of the heterotrophic flagellates as well as most of the phytoplankton).

Association of marine viral and bacterial communities with reference black carbon particles under experimental conditions: an analysis with scanning electron, epifluorescence and confocal laser scanning microscopy

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass, constitutes a significant fraction of the marine organic carbon pool. However, little is known about the possible interactions of BC and marine microorganisms. Here, we report the results of experiments using a standard reference BC material in high concentrations to investigate basic principles of the dynamics of natural bacterial and viral communities with BC particles. We assessed the attachment of viral and bacterial communities using scanning electron, epifluorescence and confocal laser scanning microscopy and shifts in bacterial community composition using 16S rRNA gene denaturing gradient gel electrophoresis (DGGE). In 24-h time-course experiments, BC particles showed a strong potential for absorbing viruses and bacteria. Total viral abundance was reduced, whereas total bacterial abundance was stimulated in the BC treatments. Viral and bacterial abundance on BC particles increased with particle size, whereas the abundances of BC-associated viruses and bacteria per square micrometer surface area decreased significantly with BC particle size. DGGE results suggested that BC has the potential to change bacterial community structure and favour phylotypes related to Glaciecola sp. Our study indicates that BC could influence processes mediated by bacteria and viruses in marine ecosystems.

Inorganic nutrient control of dissolved organic carbon (DOC) dynamics in NW Mediterranean waters: An experimental approach

Abstract In order to understand the microbial processes controlling seasonal DOC accumulation in NW Mediterranean Sea surface waters, nutrient addition experiments were conducted. The dynamics of DOC as well as the response of heterotrophic bacteria in terms of abundance, activity, and community composition were investigated in October, January, March, and July of 2006–2007. During the summer stratified period, the addition of phosphorus (at 0.1 and 1 µM final concentrations), as well as nitrogen and phosphorus (at 10 µM N and 1 µM P) yielded marked decreases in DOC concentrations. Bacterial DOC consumption was 5.6 µM C per day in the low phosphorus treatment, 8.6 µM C per day in the high phosphorus treatment, and 8 µM C per day in the nitrogen plus phosphorus treatment. Most, but not all experiments, showed a clear decrease in the DOC concentrations when amended with phosphorus, even when in-situ concentrations were low. The addition of phosphorus also enhanced bacterial production and caused changes in the community composition of the bacterioplankton. The phosphorus limitation of bacterial activity and the changes in the relative abundance of specific phylotypes upon relieving nutrient limitation might explain the low DOC consumption rates during the summer stratified period.

Elemental composition of individual pico- and nano-sized marine detrital particles in the northwestern Mediterranean Sea

The elemental composition of individual < 10 μm detrital particles from Mediterranean surface waters was analysed using a Transmission Electron Microscope (TEM) equipped with an energy dispersive X-Ray microanalyser. Results show that carbon and phosphorus content per detritus volume are much higher in pico-detrital particles < 2 μm (42 kg C m−3 and 1 kg P m−3) than in 5–10 μm detrital particles (20 kg C m−3 and 0.1 kg P m−3). The C:N:P atomic ratios for different sized fractions of the detrital particles were found to be 82:10:1 for < 2 μm particles, 120:29:1 for 2–5 μm particles and 308:37:1 for 5–10 μm particles. The average ratio for all size classes of detrital particles (< 10 μm) was 132:23:1. The differences in elementary compositions of the detrital particles studied here suggest that the different size fractions probably have different origins. The role and origins of < 10 μm detrital particles within the biogeochemical cycles are discussed.

Interactive effects of viral and bacterial production on marine bacterial diversity.

A general model of species diversity predicts that the latter is maximized when productivity and disturbance are balanced. Based on this model, we hypothesized that the response of bacterial diversity to the ratio of viral to bacterial production (VP/BP) would be dome-shaped. In order to test this hypothesis, we obtained data on changes in bacterial communities (determined by terminal restriction fragment length polymorphism of 16S rRNA gene) along a wide VP/BP gradient (more than two orders of magnitude), using seawater incubations from NW Mediterranean surface waters, i.e., control and treatments with additions of phosphate, viruses, or both. In December, one dominant Operational Taxonomic Unit accounted for the major fraction of total amplified DNA in the phosphate addition treatment (75±20%, ± S.D.), but its contribution was low in the phosphate and virus addition treatment (23±19%), indicating that viruses prevented the prevalence of taxa that were competitively superior in phosphate-replete conditions. In contrast, in February, the single taxon predominance in the community was held in the phosphate addition treatment even with addition of viruses. We observed statistically robust dome-shaped response patterns of bacterial diversity to VP/BP, with significantly high bacterial diversity at intermediate VP/BP. This was consistent with our model-based hypothesis, indicating that bacterial production and viral-induced mortality interactively affect bacterial diversity in seawater.

Pigment dynamics associated with the grazing of a ciliate and a flagellate feeding on a cyanobacterium

Abstract The egestion of particulate material as well as pigment degradation during microzooplankton grazing on phytoplankton are poorly known processes. In an attempt to evaluate these processes, changes in pigment concentrations within various size fractions were monitored in batch cultures of an assemblage of a pelagic ciliate ( Strombidium sulcatum ) and a heterotrophic flagellate ( Paraphysomonas sp.) feeding on a cyanobacterium ( Synechococcus sp.) over a 10-day period. Chlorophyll a , carotenoids and phaeopigments were not found in the 0.1–0.7 μm fraction while the pigments originally in the 0.7–3.0 μm fraction (prey) were transferred into the > 3.0 μm size fraction (predator). During this transfer, the carotenoids (zeaxanthin and β-carotene) were not degraded significantly. In contrast, chlorophyll a was degraded into phaeophytin-like compounds which accounted for almost 100% of the recorded phaeopigments. The destruction of chlorophyll a varied with time ranging from 4% (day 3) to almost 100 % (end of the experiment) and this destruction was inversely related to micro-grazer ingestion rates. Microscopic examinations of samples did not reveal any large egested particles > 3.0 μm, suggesting that phaeopigments and carotenoids measured in this size fraction were accumulated inside the protozoa. Zeaxanthin was very stable even when it was within the mico-grazer.