168 research outputs found
The role of mixotrophic protists in the biological carbon pump
No full textThe traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic – combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure affects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies
One-dimensional spin-polarized surface states: A comparison of Bi(112) with other vicinal bismuth surfaces
No full textVicinal surfaces of bismuth are unique test beds for investigating one-dimensional (1D) spin-polarized surface states that may one day be used in spintronic devices. In this paper, such states have been observed for the (112) surface when measured using angle- and spin-resolved photoemission spectroscopy, and also when calculated using a tight-binding model and with density functional theory. The surface states appear as elongated Dirac-cones which are 1D and almost dispersionless in the ky direction, but disperse with energy in the orthogonal kx direction to form two ×-like features centered at the ky line through Γ ̄. Unlike many materials considered for spintronic applications, their 1D nature suggests that conductivity and spin-transport properties are highly dependent on direction. The spin polarization of the surface states is mainly in plane and parallel to the 1D states, but there are signs of a tilted out-of-plane spin-vector component for one of the features. The Bi(112) surface states resemble those found for other vicinal surfaces of bismuth, strongly indicating that their existence and general properties are robust properties of vicinal bismuth surfaces. Furthermore, differences in the details of the states, particularly related to their spin polarization, suggest that the electronic band structure may be engineered simply by precise cutting and polishing of the crystal
Effect of P and N addition to oligotrophic Eastern Mediterranean waters influenced by near-shore waters: A microcosm experiment
No full textInternational audiencePhosphate (P), nitrate (N) or P+N added in a microcosm experiment to oligotrophic waters of the Eastern Mediterranean influenced by near-shore waters triggered a range of responses in the autotrophic and heterotrophic compartments of the system. Chlorophyll a increased in all treatments, including the no-addition control, implying that nutrients became available also from internal sources (recycling). Larger and faster biomass increase as well as a larger P utilization took place in the P+N treatments. Diatoms bloomed in the P+N treatments whereas coccolithophores bloomed following the addition of P ultimately reaching N-limitation. Bacterial activity responded with a transient peak to both low P-alone and N-alone additions (0.01 and 1 mu M, respectively). For reasons not well understood, no such response was observed at higher P-alone additions (0.05 and 0.5 mu M), whereas at the two highest P+N additions the positive response was delayed. We therefore were unable to conclude conclusively on bacteria] limitation. In most cases, the increase in bacterial activity was not matched by an increase in abundance, suggesting a tight top-down control of the biomass. Instead, heterotrophic nanoflagellate and ciliate abundances increased in all treatments. A slightly elevated orthophosphate turnover-time (T-t) (32h) in the initial waters did not give a clear indication of P-limitation, although the system could absorb the lowest P-addition (0.01 mu M) without increase in T-t N alone lead to a reduction in T-t as would be expected in an N-limited system consuming existing surplus P after N-addition. The response of the near-shore influenced system used in this study was in accord with the `classical' response to nutrient introduction-increase in chlorophyll a and in large size phytoplankton. In contrast, in the ultraoligotrophic Cyprus Eddy [Krom, Thingstad, Carbo, Drakopoulos, Fileman, Flaten, Groom, Herut, Kitides, Kress, Law, Liddicoact, Mantoura, Pasternak, Pitta, Polychronaki, Psarra, Rassoulzadegan, Skjoldal, Spyres, Tanaka, Tselepides, Wassmann, Wexels-Riser, Woodward, Zodiatis, Zohary, 2005. Overview of the CYCLOPS P addition lagrangian experiment in the Eastern Mediterranean. Deep-Sea Research II, this volume.], the short T, (< 4h) indicated P-limitation, the combined addition of P and N (as ammonium) induced a bloom of picocyanobacteria [Zohary, Herut, Krom, Mantoura, Pitta, Psarra, Rasssoulzadegan, Stambler, Tanaka, Thingstad, Woodward, 2005. P-limited bacteria but N&P co-limited phytoplankton in the Eastern Mediterranean-a microcosm experiment. Deep-Sea Research II, this volume.] and the in situ P alone addition led to a decrease in chlorophyll. (c) 2005 Elsevier Ltd. All rights reserved
NUTRIENT LIMITATIONS, MICROBIAL FOOD WEBS, AND BIOLOGICAL C-PUMPS - SUGGESTED INTERACTIONS IN A P-LIMITED MEDITERRANEAN
No full textIn light of evidence suggesting that both phytoplankton and bacteria in the Mediterranean Sea are limited by the availability of phosphorus rather than of nitrogen, and that most of the P in the photic zone during summer stratification exists as dissolved organic compounds (DOP), we address the question of how these observations may interact with the 'biological pump' transporting carbon to deep waters. From theoretical considerations, the C storage via sinking particles should function better in a P- than in an N-limited system. It is argued, however, that the microbial food web during summer stratification has a net accumulation of dissolved organic carbon (DOG) and DOP. The limited data available suggest a high DOC:DOP value which would make downwards transport of DOC with winter deep water formation a potentially effective mechanism in C sequestration from the atmosphere. Part of the DOC accumulating in the photic zone appears to be readily biodegradable. This is in conflict with a simple model of phytoplankton-bacterial competition for phosphate since phytoplankton, as an inferior competitor, would be expected to be reduced in biomass until autochthonous production of organic C falls to a level where bacteria become C-limited. The conflict is resolved by including microzooplankton grazing as a controlling factor of bacterial biomass
Microbial degradation of <i>Phaeocystis</i> material in the water column
No full textObservational evidence shows that the large amounts of mucilaginous substances produced by blooms of Phaeocystis colonies largely resist rapid microbial degradation in surface waters of most Phaeocystis-dominated ecosystems. In this paper the biodegradability of Phaeocystis colony-derived material is analysed with respect to current knowledge and novel data on the chemical nature of Phaeocystis material in relationship with specific bacterial enzymatic activities. Particular emphasis is given to the chemical nature of Phaeocystis colony matrix which constitutes more than 80% of total colony biomass at maximum development. This analysis gives evidence of the potential biodegradability of this mucilaginous material made of nutrient-deprived polysaccharides. Other factors controlling microbial degradation as the production of antibacterial substances by Phaeocystis colonies, cold temperature and lack of inorganic nitrogen and phosphate are further considered. It is concluded that nutrient limitation currently observed at the senescent stage of Phaeocystis blooms might well explain the low biodegradability of Phaeocystis material. However the lack of bacteria attached to colonies during the exponential phase of Phaeocystis bloom development are not clearly understood and needs further investigations
Conceptual models for the biogeochemical role of the photic zone microbial food web, with particular reference to the Mediterranean Sea
No full textInternational Conference on Progress in Oceanography of the Mediterranean Sea, ROME, ITALY, NOV 17-19, 1997International audienceObservations 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 site-structured food chain where: the structure is a result of combined top-down control from sits-selective predators and site-dependent competition for phosphate. It is argued that, conceptually, labile DOC and silicate have symmetrical roles in potentially controlling the Aux 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. (C) 1999 Elsevier Science Ltd. All rights reserved
Does small scale turbulence interfere competition for nutrients between diatoms and bacteria?
No full textSymposium GLOBEC-IMBER España celebrado del 28-30 marzo de 2007 en Valencia.-- 1 pageIn aquatic systems phytoplankton and bacteria compete for inorganic nitrogen and phosphorus. It has been shown that the outcome of this competition will be in favour of diatoms when silicate (Si) is in excess and in favour of bacteria when Si is depleted [Havskum et al, 2003; Thingstad et al 2007]. We postulate that turbulence could change this competition interaction. Shear derived from turbulence increases nutrient flux to large cells, thus it should increase the phytoplankton competition capability for nutrients and reduce the organic matter utilisation by bacteria. Then, when Si is in excess diatom growth should be further enhanced by turbulence. We evaluated the effect of turbulence on the competition between bacteria and diatoms in experiments with natural plankton communities enclosed in microcosms. The response of plankton to turbulence versus still conditions was evaluated in four different nutrient conditions: C (glucose addition), Si (silicate addition), CSi (glucose and silicate addition), and B (no addition). In general, turbulence increased both heterotrophic and autotrophic biomass under all nutrient addition conditions, and enhanced the diatom competition for nutrients when Si was available. However turbulence did not change the ratio between autotrophic and heterotrophic biomass. We discuss the discrepancy between these results and previous data in terms of the level and frequency of nutrient additions, trophic interactions and initial conditions.
References
HAVSKUM, H, TF THINGSTAD, R SCHAREK, F PETERS, E BERDALET, MM SALA, M ALCARAZ , JCBANSGHOLT, UL ZWEIFEL, Å HAGSTRÖM, M PEREZ AND JR DOLAN (2003) Silicate and labile DOC interfere in structuring food web via algal-bacteria competition for mineral nutrients: Results of a mesocosm experiment. Limnol. Oceanogr. 48(1): 129-140.
THINGSTAD TF, H HAVSKUM, UL ZWEIFEL, E BERDALET, MM SALA, F PETERS, M ALCARAZ, R SCHAREK, M PEREZ, S JACQUET, GA FONNES FLATEN, JR DOLAN, C MARRASE, F RASSOULZADEGAN, ÅHAGSTRØM, D VAULOT (2007). Ability of a minimum microbial food web model to reproduce response patterns observed in mesocosms manipulated with N and P, glucose and Si. Journal of Marine systems. 64: 15-3
How nested and monogamous infection networks in host-phage communities come to be
No full textabstract: We show that a chemostat community of bacteria and bacteriophage in which bacteria compete for a single nutrient and for which the bipartite infection network is perfectly nested is permanent, a.k.a. uniformly persistent, provided that bacteria that are superior competitors for nutrient devote the least effort to defence against infection and the virus that are the most efficient at infecting host have the smallest host range. This confirms an earlier work of Jover et al. (J. Theor. Biol. 332:65–77, 2013) who raised the issue of whether nested infection networks are permanent. In addition, we provide sufficient conditions that a bacteria-phage community of arbitrary size with nested infection network can arise through a succession of permanent subcommunties each with a nested infection network by the successive addition of one new population. The same permanence results hold for the monogamous infection network considered by Thingstad (Limnol Oceanogr 45:1320–1328, 2000) but without the trade-offs.This is the authors' final accepted manuscript. The final publication is available at http://dx.doi.org/10.1007/s12080-014-0236-
Analyzing the trophic link between the mesopelagic microbial loop and zooplankton from observed depth profiles of bacteria and protozoa
No full textIt is widely recognized that organic carbon exported to the ocean aphotic layer is significantly consumed by heterotrophic organisms such as bacteria and zooplankton in the mesopelagic layer. However, very little is known for the trophic link between bacteria and zooplankton or the function of the microbial loop in this layer. In the northwestern Mediterranean, recent studies have shown that viruses, bacteria, heterotrophic nanoflagellates, and ciliates distribute down to 2000 m with group-specific depth-dependent decreases, and that bacterial production decreases with depth down to 1000 m. Here we show that such data can be analyzed using a simple steady-state food chain model to quantify the carbon flow from bacteria to zooplankton over the mesopelagic layer. The model indicates that bacterial mortality by viruses is similar to or 1.5 times greater than that by heterotrophic nanoflagellates, and that heterotrophic nanoflagellates transfer little of bacterial production to higher trophic levels
The dark portion of the Mediterranean Sea is a bioreactor of organic matter cycling
No full textTotal prokaryotic abundance, prokaryotic heterotrophic production and enzymatic activities were investigated in epi-, meso- and bathypelagic waters along a longitudinal transect covering the entire Mediterranean Sea. The prokaryotic production and enzymatic activities in deep waters were among the highest reported worldwide at similar depths, indicating that the peculiar physico-chemical characteristics of the Mediterranean Sea, characterized by warm temperatures (typically 13°C also at abyssal depths), support high rates of organic carbon degradation and incorporation by prokaryotic assemblages. The higher trophic conditions in the epipelagic waters of the Western basin resulted in significantly higher prokaryotic production and enzymatic activities rates than in the Central-Eastern basin. While all of the variables decreased significantly from epi- to meso- and bathypelagic waters, cell-specific hydrolytic activity and cell-specific carbon production significantly increased. In addition, the deep-water layers were characterised by low half-saturation constants (Km) of all enzymatic activities. These findings suggest that prokaryotic assemblages inhabiting the dark portion of the Mediterranean Sea are able to channel degraded carbon into biomass in a very efficient way, and that prokaryotic assemblages of the deep Mediterranean waters work as a “bioreactor” of organic matter cycling. Since prokaryotic production and enzymatic activities in deep water masses were inversely related with oxygen concentration, we hypothesise a tight link between prokaryotic metabolism and oxygen consumption. As climate change is increasing deep-water temperatures, the predicted positive response of prokaryotic metabolism to temperature increases may accelerate oxygen depletion of deep Mediterranean waters, with cascade consequences on carbon cycling and biogeochemical processes on the entire deep basin
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