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Jan Pawlowski - One of the best experts on this subject based on the ideXlab platform.
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The twilight of sun‐animalcules
The Journal of Eukaryotic Microbiology, 2005Co-Authors: Sergey I. Nikolaev, Cédric Berney, José Fahrni, Alexander P. Mylnikov, Nikolai B. Petrov, Jan PawlowskiAbstract:Molecular sampling of the taxonomic diversity of the living world is nowadays a task of paramount importance. Heliozoa represents one of the major eukaryotic taxa, which remain significantly underrepresented in molecular databases. The term Heliozoa was coined to embrace organisms with a rounded body and stiff pseudopodia. Despite evidences from ultrastructural studies, which conclusively show the polyphyly of selected Heliozoan groups, contemporary morphological systems retain Heliozoa as a monophyletic taxon. From the perspective of reconstructing the true phylogeny of Eukaryota, molecular approaches to analyse relationships within this large protist group are evidently necessary. Phylogenetic analysis of our data shows that the four Heliozoan taxa branch either independently or within different eukaryotic phyla. The actinophryids (Actinosphaerium, Actinophrys) appear as a lineage of stramenopiles, while the desmothoracids (Clathrulina, Hedriocystis) branch within “core Cercozoa”. The position of both groups is strongly supported in all analyses and is congruent with ultrastructure-based taxonomic revisions. The centrohelids (Chlamydaster, Heterophrys, Pterocystis, and Raphidiophrys) do not seem to be related to any particular eukaryotic phylum, in agreement with a recent molecular study. The taxopodid Sticholonche was found to branch between Polycystinea and Acantharea, two classes of radiolarians. Results obtained in this study suggest that the Heliozoan body form cannot be used as a diagnostic argument to unite Heliozoa. Instead, we discriminate between the three heliomorphic taxa of independent origin, Actinophryida, Desmothoracida and Sticholonche, and propose the novel higher rank taxon Centrohelida. The term Heliozoa should thus be used solely in historical context.
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A molecular perspective on the phylogeny of amoeboid protists
The Journal of Eukaryotic Microbiology, 2005Co-Authors: Cédric Berney, Jan PawlowskiAbstract:The origin of amoeboid protists is one of the greatest enigmas of eukaryotic phylogeny. Although it has long been accepted that amoeboid protists are polyphyletic, the lack of molecular data for many amoeboid taxa impeded their placement in the tree of life. We have sequenced the small-subunit ribosomal RNA and actin genes from a broad taxonomic sampling of amoeboid protists, including three lineages of Heliozoans (Actinophryida, Desmothoracida, and Taxopodida) and one lineage of radiolarians (Phaeodarea) for which no molecular data were available yet. Phylogenetic analyses of our data show that (1) all lobose amoebae sequenced to date belong to the recently defined phylum Amoebozoa and (2) Heliozoa and Radiolaria both represent polyphyletic assemblages, as suggested by previous ultrastructural studies. However, although axopodia evolved at least five times independently during the evolution of eukaryotes, our study also reveals that most protists possessing filopodia, reticulopodia, and/or axopodia are closely related, and constitute a new supergroup of amoeboid protists, the Rhizaria.
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The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes
Proceedings of the National Academy of Sciences of the United States of America, 2004Co-Authors: Sergey I. Nikolaev, Cédric Berney, José Fahrni, Alexander P. Mylnikov, Nikolai B. Petrov, Ignacio Bolivar, Stephane Polet, V. V. Aleshin, Jan PawlowskiAbstract:Recent molecular phylogenetic studies revealed the extraordinary diversity of single-celled eukaryotes. However, the proper assessment of this diversity and accurate reconstruction of the eukaryote phylogeny are still impeded by the lack of molecular data for some major groups of easily identifiable and cultivable protists. Among them, amoeboid eukaryotes have been notably absent from molecular phylogenies, despite their diversity, complexity, and abundance. To partly fill this phylogenetic gap, we present here combined small-subunit ribosomal RNA and actin sequence data for the three main groups of “Heliozoa” (Actinophryida, Centrohelida, and Desmothoracida), the Heliozoan-like Sticholonche, and the radiolarian group Polycystinea. Phylogenetic analyses of our sequences demonstrate the polyphyly of Heliozoans, which branch either as an independent eukaryotic lineage (Centrohelida), within stramenopiles (Actinophryida), or among cercozoans (Desmothoracida), in broad agreement with previous ultrastructure-based studies. Our data also provide solid evidence for the existence of the Rhizaria, an emerging supergroup of mainly amoeboid eukaryotes that includes desmothoracid Heliozoans, all radiolarians, Sticholonche, and foraminiferans, as well as various filose and reticulose amoebae and some flagellates.
Thomas Cavalier-smith - One of the best experts on this subject based on the ideXlab platform.
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Oxnerella micra sp. n. (Oxnerellidae fam. n.), a tiny naked centrohelid, and the diversity and evolution of Heliozoa.
Protist, 2012Co-Authors: Thomas Cavalier-smith, Ema E. ChaoAbstract:We describe a new tiny naked centrohelid Heliozoan, Oxnerella micra, and sequenced its 18S and 28S rRNA genes. Its extremely slender axopodia have prominent extrusomes and are normally stretched across the substratum like those of many tiny granofilosean Cercozoa. Phylogenetic analysis of 18S rDNA shows that Oxnerella does not branch within any of the six known centrohelid families but very deeply in the order Pterocystida, between Choanocystidae and Pterocystidae; therefore we place it in a new family, Oxnerellidae. Oxnerella arose from ancestors with siliceous scales by losing them; as independently did Heterophryidae and Marophryidae, which replaced them by organic spicules, and Chlamydaster that is not truly naked but retains a mucilage coat and nests extremely shallowly within Pterocystidae. 28S rDNA has a group I intron. Concatenated Bayesian 18S/28S rRNA phylogeny shows centrohelids weakly as sisters to the naked non-centrohelid Heliozoan Microheliella maris (Microhelida: Heliozoa). The centrohelid Marophrys marina possesses an elongation factor α-like (EFL) protein related to that of Polyplacocystis; Microheliella also has EFL. We also analysed Hsp90 and 18S rDNA sequences from ‘Pinaciophora sp.’ ATCC50355; they must be from a centrohelid, probably misidentified as Pinaciophora, the rDNA sequence branching deeply within Pterocystida. We reclassify two Polyplacocystis, Luffisphaera, Phaeodaria and Rotosphaerida.
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Microheliella maris (Microhelida ord. n.), an ultrastructurally highly distinctive new axopodial protist species and genus, and the unity of phylum Heliozoa.
Protist, 2011Co-Authors: Akinori Yabuki, Ema E. Chao, Ken-ichiro Ishida, Thomas Cavalier-smithAbstract:A new Heliozoan, Microheliella maris, has sufficiently distinctive ultrastructure to merit a new order, Microhelida. Its 18S and 28S rRNA genes were sequenced earlier under the informal name ‘marine microHeliozoan’; we here sequenced its Hsp90 gene. A three-gene tree suggests that it is distantly related to centrohelids and others in chromist subkingdom Hacrobia; but it is too divergent to be placed accurately by few genes. Unlike centrohelids, its central spherical centrosome has two concentric granular shells and a dense core devoid of a trilaminar central disc. Microtubules radiate from the centrosomal shells. Unlike centrohelids, axopodia have only three microtubules, fixed basally by dense plasma membrane anchors, and bear terminal and lateral haptosome-like extrusomes. As in the heliomonad Heliomorpha, the centrosome is embedded in a nuclear cavity, and centrosomal microtubules traverse the nucleus inside cytoplasmic channels. A novel filogranular network interconnects mitochondria, ER, and plasma membrane. The microbody is attached to the nucleus and mitochondrion, which has vermicular tubular cristae. We group Microhelida and Heliomonadida, purged of dissimilar flagellates, as a new tubulicristate class Endohelea within phylum Heliozoa. Previously misassigned GenBank 18S rDNA sequences reveal Microhelida as diverse and ancient. We discuss principles underlying the biogenesis and diversity of axopodial patterns.
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Molecular phylogeny, scale evolution and taxonomy of centrohelid Heliozoa
Molecular Phylogenetics and Evolution, 2007Co-Authors: Thomas Cavalier-smith, Sophie Von Der HeydenAbstract:Heliozoa are ubiquitous, unicellular phagotrophs with slender radiating axopodia for trapping prey. We sequenced 18S rRNA genes from 35 cultured centrohelid Heliozoa (18 studied by electron microscopy) and 28 environmental libraries (18 freshwater, 10 marine), yielding 97 new sequences, this exceeding described species. Phylogenetic analyses show two major groups and that ancestral centrohelids probably had inner plate-like tangential and distinct outer radial silica scales, the latter diverging early into contrasting scale types seen in extant Pterocystis/Choanocystis and Acanthocystis/Raphidiophryidae. Scales were lost at least thrice. Pterocystis is paraphyletic, as was the classical family Acanthocystidae; Heterophrys was polyphyletic. Using scale morphology and rRNA sequences, we establish new families Pterocystidae (Pterocystis, Raineriophrys, Chlamydaster), Marophryidae (type Marophrys (Heterophrys) marina gen. et comb. nov.) and Choanocystidae, new suborders Pterocystina (Pterocystidae, Choanocystidae, Heterophryidae) and Acanthocystina (Acanthocystidae, Raphidiophryidae, Marophryidae), and ten new Pterocystis, Acanthocystis and Choanocystis species. Most clades are exclusively freshwater or exclusively marine; evolutionary transitions between these habitats have been rare.
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Molecular phylogeny of centrohelid Heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss.
Journal of molecular evolution, 2003Co-Authors: Thomas Cavalier-smith, Ema E. Y. ChaoAbstract:Abstract Recent molecular and cellular evidence indicates that eukaryotes comprise three major lineages: the probably ancestrally uniciliate protozoan phylum Amoebozoa; the ancestrally posteriorly uniciliate opisthokont clade (animals, Choanozoa, and fungi); and a very diverse ancestrally biciliate clade, the bikonts—plants, chromalveolates, and excavate and rhizarian Protozoa. As Heliozoa are the only eukaryote phylum not yet placed on molecular sequence trees, we have sequenced the 18S rRNA genes of three centrohelid Heliozoa, Raphidiophrys ambigua, Heterophrys marina, and Chlamydaster sterni, to investigate their phylogenetic position. Phylogenetic analysis by distance and maximum likelihood methods allowing for intersite rate variation and invariable sites confirms that centrohelid Heliozoa are a robust clade that does not fall within any other phyla. In particular, they are decisively very distant from the heterokont pedinellid chromists, at one time thought to be related to Heliozoa, and lack the unique heterokont signature sequence. They also appear not to be specifically related to either Amoebozoa or Radiolaria, with which they have sometimes been classified, so it is desirable to retain Heliozoa as a separate protozoan phylum. Even though centrohelids have no cilia or centrioles, the centrohelid clade branches among the bikont eukaryotes, but there is no strong bootstrap support for any particular position. Distance trees usually place centrohelids as sisters to haptophytes, whereas parsimony puts them as sisters to red algae, but there is no reason to think that either position is correct; both have very low bootstrap support. Quartet puzzling places them with fairly low support as sisters to the apusozoan zooflagellate Ancyromonas. As Ancyromonas is the only other eukaryote that shares the character combination of flat plate-like mitochondrial cristae and kinetocyst-type extrusomes with centrohelids, this position is biologically plausible, but because of weak support and conflict between trees it might not be correct. Irrespective of their precise position, our trees (together with previous evidence that Chlamydaster sterni has the derived dihydrofolate reductase/thymidylate synthetase gene fusion unique to bikonts) indicate that centrohelid Heliozoa are ancestrally derived from a bikont flagellate by the loss of cilia. The centroplast that nucleates their axonemal microtubules is therefore almost certainly homologous with the centrosome of ciliated eukaryotes and should simply be called a centrosome.
Ursula Gaedke - One of the best experts on this subject based on the ideXlab platform.
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Temporal dynamics and growth of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic lake
Freshwater Biology, 2006Co-Authors: Elanor M. Bell, Guntram Weithoff, Ursula GaedkeAbstract:1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001-03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 x 10(3) Heliozoa L-1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single-celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide-size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k(b)) when starved, with low concentrations of single-celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single- celled bacteria (k = 0.22 +/- 0.02 d(-1); k(b) = -0.06 +/- 0.02 d(-1)) and filamentous bacteria (k = 0.52 +/- < 0.01 d(- 1); k(b) = 0.66 d(-1)) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 +/- 0.03 d(-1); k(b) = 0.44 +/- 0.04 d(-1)), the ciliate Oxytricha sp. (k = 0.34 +/- 0.01 d(-1)) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 +/- 0.02-0.32 +/- 0.02 d(-1); maximum k(b) = 0.42 +/- 0.05 d(-1)). The individual- and biomass-based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of 'extreme' environments
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Strong vertical differences in the plankton composition of an extremely acidic lake
Archiv für Hydrobiologie, 2004Co-Authors: Norbert Kamjunke, Ursula Gaedke, Guntram Weithoff, Jörg Tittel, Elanor M. BellAbstract:Vertical differences in food web structure were examined in an extremely acidic, iron-rich mining lake in Germany (Lake 111; pH 2.6, total Fe 150 mg L -1 ) during the period of stratification. We tested whether or not the seasonal variation of the plankton composition is less pronounced than the differences observed over depth. The lake was strongly stratified in summer, and concentrations of dissolved organic carbon and inorganic carbon were consistently low in the epilimnion but high in the hypolimnion. Oxygen concentrations declined in the hypolimnion but were always above 2 mg L -1 . Light attenuation did not change over depth and time and was governed by dissolved ferric iron. The plankton consisted mainly of single-celled and filamentous bacteria, the two mixotrophic flagellates Chlamydomonas sp. and Ochromonas sp., the two rotifer species Elosa worallii and Cephalodella hoodi, and Heliozoa as top predators. We observed very few ciliates and rhizopods, and no heterotrophic flagellates, crustaceans or fish. Ochromonas sp., bacterial filaments, Elosa and Heliozoa dominated in the epilimnion whereas Chlamydomonas sp., single-celled bacteria and Cephalodella dominated in the hypolimnion. Single-celled bacteria were controlled by Ochromonas sp. whereas the lack of large consumers favoured a high proportion of bacterial filaments. The primarily phototrophic Chlamydomas sp. was limited by light and CO 2 and may have been reduced due to grazing by Ochromonas sp. in the epilimnion. The distribution of the primarily phagotrophic Ochromonas sp. and of the animals seemed to be controlled by prey availability. Differences in the plankton composition were much higher between the epilimnion and hypolimnion than within a particular stratum over time. The food web in Lake 111 was extremely species-poor enabling no functional redundancy. This was attributed to the direct exclusion of species by the harsh environmental conditions and presumably enforced by competitive exclusion. The latter was promoted by the low diversity at the first trophic level which, in turn was attributed to relatively stable growth conditions and the independence of resource availability (inorganic carbon and light) from algal density. Ecological theory suggests that low functional redundancy promotes low stability in ecosystem processes which was not supported by our data.
Sophie Von Der Heyden - One of the best experts on this subject based on the ideXlab platform.
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global eukaryote phylogeny combined small and large subunit ribosomal dna trees support monophyly of rhizaria retaria and excavata
Molecular Phylogenetics and Evolution, 2007Co-Authors: David Moreira, Sophie Von Der Heyden, David Bass, Purificacion Lopezgarcia, Ema Chao, Thomas CavaliersmithAbstract:Resolution of the phylogenetic relationships among the major eukaryotic groups is one of the most important problems in evolutionary biology that is still only partially solved. This task was initially addressed using a single marker, the small-subunit ribosomal DNA (SSU rDNA), although in recent years it has been shown that it does not contain enough phylogenetic information to robustly resolve global eukaryotic phylogeny. This has prompted the use of multi-gene analyses, especially in the form of long concatenations of numerous conserved protein sequences. However, this approach is severely limited by the small number of taxa for which such a large number of protein sequences is available today. We have explored the alternative approach of using only two markers but a large taxonomic sampling, by analysing a combination of SSU and large-subunit (LSU) rDNA sequences. This strategy allows also the incorporation of sequences from non-cultivated protists, e.g., Radiozoa (=radiolaria minus Phaeodarea). We provide the first LSU rRNA sequences for Heliozoa, Apusozoa (both Apusomonadida and Ancyromonadida), Cercozoa and Radiozoa. Our Bayesian and maximum likelihood analyses for 91 eukaryotic combined SSU+LSU sequences yielded much stronger support than hitherto for the supergroup Rhizaria (Cercozoa plus Radiozoa plus Foraminifera) and several well-recognised groups and also for other problematic clades, such as the Retaria (Radiozoa plus Foraminifera) and, with more moderate support, the Excavata. Within opisthokonts, the combined tree strongly confirms that the filose amoebae Nuclearia are sisters to Fungi whereas other Choanozoa are sisters to animals. The position of some bikont taxa, notably Heliozoa and Apusozoa, remains unresolved. However, our combined trees suggest a more deeply diverging position for Ancyromonas, and perhaps also Apusomonas, than for other bikonts, suggesting that apusozoan zooflagellates may be central for understanding the early evolution of this huge eukaryotic group. Multiple protein sequences will be needed fully to resolve basal bikont phylogeny. Nonetheless, our results suggest that combined SSU+LSU rDNA phylogenies can help to resolve several ambiguous regions of the eukaryotic tree and identify key taxa for subsequent multi-gene analyses.
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Molecular phylogeny, scale evolution and taxonomy of centrohelid Heliozoa
Molecular Phylogenetics and Evolution, 2007Co-Authors: Thomas Cavalier-smith, Sophie Von Der HeydenAbstract:Heliozoa are ubiquitous, unicellular phagotrophs with slender radiating axopodia for trapping prey. We sequenced 18S rRNA genes from 35 cultured centrohelid Heliozoa (18 studied by electron microscopy) and 28 environmental libraries (18 freshwater, 10 marine), yielding 97 new sequences, this exceeding described species. Phylogenetic analyses show two major groups and that ancestral centrohelids probably had inner plate-like tangential and distinct outer radial silica scales, the latter diverging early into contrasting scale types seen in extant Pterocystis/Choanocystis and Acanthocystis/Raphidiophryidae. Scales were lost at least thrice. Pterocystis is paraphyletic, as was the classical family Acanthocystidae; Heterophrys was polyphyletic. Using scale morphology and rRNA sequences, we establish new families Pterocystidae (Pterocystis, Raineriophrys, Chlamydaster), Marophryidae (type Marophrys (Heterophrys) marina gen. et comb. nov.) and Choanocystidae, new suborders Pterocystina (Pterocystidae, Choanocystidae, Heterophryidae) and Acanthocystina (Acanthocystidae, Raphidiophryidae, Marophryidae), and ten new Pterocystis, Acanthocystis and Choanocystis species. Most clades are exclusively freshwater or exclusively marine; evolutionary transitions between these habitats have been rare.
Guntram Weithoff - One of the best experts on this subject based on the ideXlab platform.
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Temporal dynamics and growth of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic lake
Freshwater Biology, 2006Co-Authors: Elanor M. Bell, Guntram Weithoff, Ursula GaedkeAbstract:1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001-03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 x 10(3) Heliozoa L-1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single-celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide-size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k(b)) when starved, with low concentrations of single-celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single- celled bacteria (k = 0.22 +/- 0.02 d(-1); k(b) = -0.06 +/- 0.02 d(-1)) and filamentous bacteria (k = 0.52 +/- < 0.01 d(- 1); k(b) = 0.66 d(-1)) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 +/- 0.03 d(-1); k(b) = 0.44 +/- 0.04 d(-1)), the ciliate Oxytricha sp. (k = 0.34 +/- 0.01 d(-1)) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 +/- 0.02-0.32 +/- 0.02 d(-1); maximum k(b) = 0.42 +/- 0.05 d(-1)). The individual- and biomass-based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of 'extreme' environments
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Strong vertical differences in the plankton composition of an extremely acidic lake
Archiv für Hydrobiologie, 2004Co-Authors: Norbert Kamjunke, Ursula Gaedke, Guntram Weithoff, Jörg Tittel, Elanor M. BellAbstract:Vertical differences in food web structure were examined in an extremely acidic, iron-rich mining lake in Germany (Lake 111; pH 2.6, total Fe 150 mg L -1 ) during the period of stratification. We tested whether or not the seasonal variation of the plankton composition is less pronounced than the differences observed over depth. The lake was strongly stratified in summer, and concentrations of dissolved organic carbon and inorganic carbon were consistently low in the epilimnion but high in the hypolimnion. Oxygen concentrations declined in the hypolimnion but were always above 2 mg L -1 . Light attenuation did not change over depth and time and was governed by dissolved ferric iron. The plankton consisted mainly of single-celled and filamentous bacteria, the two mixotrophic flagellates Chlamydomonas sp. and Ochromonas sp., the two rotifer species Elosa worallii and Cephalodella hoodi, and Heliozoa as top predators. We observed very few ciliates and rhizopods, and no heterotrophic flagellates, crustaceans or fish. Ochromonas sp., bacterial filaments, Elosa and Heliozoa dominated in the epilimnion whereas Chlamydomonas sp., single-celled bacteria and Cephalodella dominated in the hypolimnion. Single-celled bacteria were controlled by Ochromonas sp. whereas the lack of large consumers favoured a high proportion of bacterial filaments. The primarily phototrophic Chlamydomas sp. was limited by light and CO 2 and may have been reduced due to grazing by Ochromonas sp. in the epilimnion. The distribution of the primarily phagotrophic Ochromonas sp. and of the animals seemed to be controlled by prey availability. Differences in the plankton composition were much higher between the epilimnion and hypolimnion than within a particular stratum over time. The food web in Lake 111 was extremely species-poor enabling no functional redundancy. This was attributed to the direct exclusion of species by the harsh environmental conditions and presumably enforced by competitive exclusion. The latter was promoted by the low diversity at the first trophic level which, in turn was attributed to relatively stable growth conditions and the independence of resource availability (inorganic carbon and light) from algal density. Ecological theory suggests that low functional redundancy promotes low stability in ecosystem processes which was not supported by our data.
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Vertical niche separation of two consumers (Rotatoria) in an extreme habitat.
Oecologia, 2004Co-Authors: Guntram WeithoffAbstract:Herbivore populations are commonly restricted by resource limitation, by predation or a combination of the two. Food supplement experiments are suitable for investigating the extent of food limitation at any given time. The main part of this study was performed in an extremely acidic lake (pH 2.7) where the food web consists of only a few components and potential food sources for herbivores are restricted to two flagellates. Life table experiments proved that Chlamydomonas was a suitable food source whereas Ochromonas was an unsuitable food source. The two flagellates and the two rotifers exhibit a pronounced vertical distribution pattern. In this study, a series of food supplement experiments were performed in order to: (1) quantify and compare potential resource limitation of two primary consumers (Cephalodella hoodi and Elosa worallii, Rotatoria) over time, (2) compare their response at different temperatures, (3) evaluate the effect of having an unsuitable food source alongside a valuable one, (4) estimate the effect of predation on rotifers by Heliozoa, and (5) compare the results with those from other acidic lakes. Additionally, the spatio-temporal population dynamics of both species were observed. The field data confirmed a vertical separation of the two species with E. worallii dominating in the upper water layers, and C. hoodi in the deeper, cooler water layers. The results from the food supplement experiments in which Chlamydomonas served as the supplemented suitable food source showed that the two rotifers were food limited in the epilimnion throughout the season to different extents, with Cephalodella being more severely food limited than Elosa. The experiments at different temperatures provided evidence that Elosa had a higher optimum temperature for growth than Cephalodella. When the unsuitable food algae Ochromonas was added alongside the suitable food source Chlamydomonas, C. hoodi was unaffected but E. worallii was negatively affected. Predation of Heliozoa on rotifers was observed but the total effect on the rotifer dynamics is probably low. The comparison with other lakes showed that resource limitation also occurred in one other lake, although to a lesser extent. Overall, the vertical separation of the two rotifers could be explained by both their differential extent of resource limitation and differential response to temperature.
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Benthic recruitment of zooplankton in an acidic lake
Journal of Experimental Marine Biology and Ecology, 2003Co-Authors: Elanor M. Bell, Guntram WeithoffAbstract:In recent years, most studies of the benthic microbial food web have either been descriptive or were restricted to the measurement of within sediment process rates. Little is known about benthic–pelagic coupling processes such as recruitment. We, therefore, developed an ex situ core incubation procedure to quantify the potential for microbial recruitment from the benthos to the pelagic in an acidic mining lake, Mining Lake 111 (ML 111; pH 2.6), in eastern Germany. Our data suggest that considerable zooplankton recruitment from the benthos takes place. Heliozoan and rhizopod recruitment in both summer and winter sediment cores was highest when they were incubated at 20 °C. Maximum Heliozoan recruitment was 23 (±9 S.E.) individuals cm−2 day−1 (40% initial standing stock daily) in the winter 20 °C incubation. Maximum rhizopod recruitment was 6 (±2 S.E.) individuals cm−2 day−1 in the summer 20 °C incubation. Little or no recruitment was apparent for either taxa when winter cores were incubated at 5 °C, implying a temperature cue. Conversely, the rotifer, Cephalodella hoodi, exhibited a maximum recruitment of 6 (±2 S.E.) individuals cm−2 day−1 during the winter 5 °C incubation, representing 30% of initial standing stock daily, but little recruitment when incubated at 20 °C. Cephalodella may have responded to an increased winter benthic food supply; in situ winter Chl a concentrations in the benthos were 3.4 times higher than those in the summer. The importance of this was reinforced by the poor pelagic food supply available in ML 111. In situ, Heliozoa, rhizopods and Cephalodella were first observed in the epilimnion of ML 111 in spring or early summer, suggesting active or passive recruitment following lateral transport from littoral sediments. Benthic–pelagic coupling via recruitment is potentially important in understanding the pelagic food web in ML 111 and warrants further investigation in this and other aquatic environments.