162 research outputs found
P-affinity measurements of specific osmotroph populations using cell-sorting flow cytometry
No full textTo elucidate the role that the marine microbes play in global nutrient cycling, it is necessary to recognize
how various phyto- and bacterioplankton groups compete for limiting nutrients. Specific phosphate affinity
describes an organism’s ability to harvest phosphate at low concentrations from the surrounding water. For the
first time, we have taken advantage of cell-sorting flow cytometry in combination with radio-labeled phosphorus
to measure this feature of specific osmotrophic groups in natural communities. Specific phosphate affinities
for Synechococcus spp. and picoeukaryotes were measured using live, unstained cells. The results were always
lower than theoretical calculated maximum values, corresponding well with observations of P-deficiency, or
sub-optimal P supply for the osmotroph community, at the time of investigation. Fixing and staining cells
before flow sorting offers the advantage of better separation of phytoplankton and showed high sorting reproducibility
when applied to nonaxenic Synechococcuscultures. A subsequent investigation of P-leakage from isotopically
labeled, fixed, stained cells in nonaxenic cultures of Synechococcusshowed that it was only slightly larger
than the loss of 17% found when uptake of new label was stopped with adding “cold” phosphate. Possible
applications of the currently developed methodology for population specific P affinity measurements by flow
sorting are discussed
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The Seventh Letter: A Discussion of Myles Burnyeat and Michael Frede, The Pseudo-Platonic Seventh Letter
No full textBurnyeat and Frede argue that the seventh of the letters ascribed to Plato is no more genuine than any other letter purporting to be from a philosopher before Epicurus; that its true author is far too remote from Plato for it to give any reliable evidence about Plato’s thoughts and activities; and that it is to be appreciated as a fine tragedy in prose. This review criticizes some of the details of their arguments, while endorsing and amplifying their conclusion
Fractal Hypothesis of the Pelagic Microbial Ecosystem—Can Simple Ecological Principles Lead to Self-Similar Complexity in the Pelagic Microbial Food Web?
No full textTrophic interactions are highly complex and modern sequencing techniques reveal enormous biodiversity across multiple scales in marine microbial communities. Within the chemically and physically relatively homogeneous pelagic environment, this calls for an explanation beyond spatial and temporal heterogeneity. Based on observations of simple parasite-host and predator-prey interactions occurring at different trophic levels and levels of phylogenetic resolution, we present a theoretical perspective on this enormous biodiversity, discussing in particular self-similar aspects of pelagic microbial food web organization. Fractal methods have been used to describe a variety of natural phenomena, with studies of habitat structures being an application in ecology. In contrast to mathematical fractals where pattern generating rules are readily known, however, identifying mechanisms that lead to natural fractals is not straight-forward. Here we put forward the hypothesis that trophic interactions between pelagic microbes may be organized in a fractal-like manner, with the emergent network resembling the structure of the Sierpinski triangle. We discuss a mechanism that could be underlying the formation of repeated patterns at different trophic levels and discuss how this may help understand characteristic biomass size-spectra that hint at scale-invariant properties of the pelagic environment. If the idea of simple underlying principles leading to a fractal-like organization of the pelagic food web could be formalized, this would extend an ecologists mindset on how biological complexity could be accounted for. It may furthermore benefit ecosystem modeling by facilitating adequate model resolution across multiple scales
Feedback Mechanisms Between Degradation and Primary Production in the Marine Pelagic Environment
No full textHow trophic cascades and photic zone nutrient content interact to generate basin-scale differences in the microbial food web
No full textIn linear food chains, resource and predator control produce positive and negative correlations, respectively, between biomass at adjacent trophic levels. These simple relationships become more complex in food webs that contain alternative food chains of unequal lengths. We have used a “minimum” model for the microbial part of the pelagic food web that has three such food chains connecting free mineral nutrients to copepods: via diatoms, autotrophic flagellates, and heterotrophic bacteria. Trophic cascades from copepods strongly modulates the balance between the three pathways and, therefore, the functionality of the microbial food web in services such as food production for higher trophic levels, DOM degradation, and ocean carbon sequestration. The result is a theoretical framework able to explain, not only apparent conflicts in Arctic mesocosm experiments, but also biogeochemical features of the Mediterranean. Here, the fundamental difference between Arctic and Mediterranean microbial food webs is the way they are predator driven by seasonal migration of large copepods in the Arctic, but resource driven due to the anti-estuarine circulation in the Mediterranean. In this framework, global change effects on microbial ecosystem functions are more like to come indirectly through changes in these drivers than through direct temperature effects on the microbes.publishedVersio
Microbial degradation of Phaeocystis 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. © 1994.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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