Catherine Bernard

Microbial diversity and activity in a Danish Fjord with anoxic deep water

Abstract Microbial diversity and activity were studied in a stratified basin of Mariager Fjord, Denmark in August 1994. The basin is about 30 m deep and the lower half of the water column is anoxic and sulphidic. The hydrographical and biological features of the system are described. Based on chemical gradient profiles and measurements of process rates, we found that the relative importance of sulphate reduction, denitrification and methanogenesis in terms of anaerobic terminal mineralisation was about 5:1:0.4. It is possible, however, that methanogenesis is underestimated because an unknown fraction of the methane production escaped by ebullition. It was estimated that 10–15 % of the net primary production is mineralised anaerobically. The mean residence time of methane, sulphide and ammonia beneath the chemocline is within the range 1.6–2.3 yrs. Chemolithotrophic production in the chemocline (sulphide oxidation and nitrification) accounted for about 3% of the net primary production of the system. Methan...

Some free-living flagellates (Protista) from anoxic habitats

Abstract Thirty five species of free-living anaerobic flagellates from freshwater and coastal sediments from Danish and Australian sites are reported. These belong to the genera Ancyromonas, Barthelona n. gen., Bodo, Cafeteria, Carpediemonas, Cercomonas, Chilomastix, Chilomonas, Dimastigella, Goniomonas, Heteromita, Jakoba Mastigamoeba, Monotrichomonas n. gen., Paraphysomonas, Percolomonas, Pseudotrichomonas, Quasibodo n. gen., Rhabdomonas, Rhynchobodo, Rhynchomonas, Salpingoeca, Spumella, Trepomonas and Trimastix. Six new species are described, Barthelona vulgaris, Jakoba incarcerata, Mastigamoeba punctachora, Monotrichomonas carabina, Quasibodo laughtoni and Trimastix inaequalis. The composition of the communities encountered under anoxic conditions overlaps with communities observed in aerobic environments and includes species of heteroloboseids, kinetoplastids, euglenids, jakobids, stramenopiles, cercomonads, cryptomonads and choanoflagellates — all of which are mitochondriate. About half of the flagellates observed lack classical mitochondria, or belong to known amitochondriate groups. These taxa are assignable to the pelobionts, retortamonads, diplomonads, trichomonads, and to the genera Trimastix and Carpediemonas. Some taxa observed during this study have no clear identity and further study is necessary. This work confirms the existence of many poorly understood anaerobic flagellates, the study of which could increase our understanding of the pattern of mitochondrial gain and/or loss among extant eukaryotes, as well as the operation of anoxic ecosystems.

Heterotrophic flagellates from the water column in Port Jackson, Sydney, Australia

Summary An account is given of the diversity of heterotrophic flagellates in Port Jackson (New South Wales, Australia) and is discussed in relation to the geographic patchiness of the marine communities of these organisms. Close to 100 taxa are reported. We report and document 86 species and 51 genera. None of these species are new, although we draw attention to some difficulties with the application of the morphological species concept. Of the species reported here, 50 have been previously recorded from Australia. We find no evidence for an endemic community here — or by extrapolation — elsewhere. However, the matter is contingent upon the validity of the morphological species concept.

The organisation of Mastigamoeba schizophrenia n. sp.: More evidence of ultrastructural idiosyncrasy and simplicity in pelobiont protists

Summary A new species of pelobiont, Mastigamoeba schizophrenia n. sp., is described using light and electron microscopy. Three forms were observed: flagellates, aflagellated amoebae and cysts. This species is distinguished by having adhering paired nuclei. Most cells have a single pair of nuclei but large amoebae may have several pairs. The flagellar apparatus consists of a single basal body, connected to the most anterior nucleus by a cone of microtubules. The basal body is composed of doublets of microtubules. A single ribbonlike microtubular root originates alongside the basal body. Like other pelobionts, the cell has a comparatively simple endomembrane system which includes endoplasmic reticulum but lacks mitochondria, dictyosomes, contractile vacuole and other elements found in many eukaryotic cells. The taxonomy of the pelobionts is discussed, the genus Phreatamoeba is synonymised with Mastigamoeba, and arguments for a deep-branching position for the pelobionts among eukaryotes are discussed.

The ultrastructure and systematic position of the euglenozoon Postgaardi mariagerensis, Fenchel et al

Summary The ultrastructure of Postgaardi mariagerensis is reported for the first time. This biflagellated protist, which is covered by closely and regularly arranged rod-shaped bacteria, was obtained from the sulphidic zone of Burton Lake, Antarctica. There is a large nucleus containing apparently permanently condensed chromatin. The flagella have paraxial rods and emerge from a deep, anteriorly opening pocket. The basal bodies are associated with three asymmetrically arranged microtubular roots. Near the opening of the flagella pocket there is an MTR structure that is continuous with the microtubules of the feeding apparatus. The cell membrane is supported by a corset of evenly spaced and linked parallel microtubules subtended by an electron-dense layer. The cell has batteries of tubular, thick-walled extrusomes with a cruciform internal structure. These ultrastructural features indicate that Postgaardi mariagerensis cannot be regarded as a euglenid (as had been suggested previously) but that it is a member of the Glade Euglenozoa. On current evidence, Posfgaardi cannot be placed in any established subgroup of the Euglenozoa, nor can any subgroup be nominated as a sister taxon to Postgaardi . We therefore suggest that Postgaardi be regarded as Euglenozoa incertae sedis.

The Role of Mixotrophy in Pelagic Environments

This paper reviews the occurrence and ecological importance of mixotrophic flagellates and ciliates in pelagic environments, particularly in marine ecosystems. Mixotrophy is here defined as the combination of photoautotrophic and heterotrophic nutrition in a single individual, often used in the restricted sense of combining photosynthesis and phagotrophy. Mixotrophic protists represent an alternative strategy that allows a shortcut between the traditional food web and the microbial loop. A large number of reports have been published on the ecological importance of mixotrophic flagellates in freshwater, yet only a few studies have been carried out in seawater. In contrast, most of the knowledge of mixotrophic ciliates comes from marine environments. Results from field studies have demonstrated that both mixotrophic flagellates and ciliates are commonly found in many marine environments, and mixotrophic flagellates can dominate the biomass of photoautotrophs and be responsible for the entire grazing of bacteria or protists. Results from laboratory experiments on factors controlling the degree of photoautotrophy/phagotrophy in flagellates are presented. Finally, we present a hypothesis for a growth strategy of bacterivorous mixotrophic flagellates.

The ultrastructure of Trimastix marina Kent 1880 (Eukaryota), an excavate flagellate

Summary The ultrastructure of Trimastix marina is documented for the first time. This protist has four flagella which insert at the head of a longitudinal feeding groove. The basal bodies are elongate and arranged in a cruciate pattern. The anterior and posterior basal bodies lack cartwheels. The posterior flagellum bears two vanes. The anterior flagellum is thickened. The cell has a poorly organised Golgi-like region and small hydrogenosome-like organelles. The flagellar apparatus includes left and right major microtubular roots which support the margins of the groove, and a third, major anterior root associated with a fan of microtubules which supports the dorsal side of the cell. There is also a singlet ‘root’ associated with the groove, a minor anterior root and a suite of non-microtubular structures (including the A, B, C and I fibres). Arrays of microtubules diverge from both left and right roots to support the floor of the groove. Some microtubules from the right array converge to form an inner right root. Halfway down the cell the left root ends, while the right root reduces and associates with a sheet-like striated/dense composite fibre. The groove ends as a cytopharynx, with a ‘tongue’ structure associated with its opening. Trimastix marina is most similar to T. pyriformis, the only previously studied member of Trimastix. Among the other ‘excavate taxa’, Trimastix is most similar to ‘core jakobids’, Malawimonas, Carpediemonas and retortamonads but also shares several unusual features with some heteroloboseids. The complexities and detail of structural similarities amongst various excavate taxa support the ‘excavate hypothesis’ (i.e. that the excavate taxa have a common excavate ancestor) but suggest that understanding the relationships amongst the excavate taxa through morphological data will be a complex task.

An ultrastructural study of a free-living retortamonad, Chilomastix cuspidata (Larsen & Patterson, 1990) n. comb. (Retortamonadida, Protista)

Summary The ultrastructure of a free-living retortamonad, Chilomastix cuspidata, originally described as Percolomonas cuspidata Larsen and Patterson, is reported for the first time. This species has a cosmopolitan distribution and has been reported from marine, brackish and freshwater sites with low levels of oxygen. There are four flagella which are located subapically at the head of a ventral groove. The basal bodies give rise to five roots: two major microtubular roots which support the walls of the ventral groove, two minor microtubular roots and one striated root. The root lying in the right wall of the groove also supports the cytopharyngeal region. An electron-dense “lapel” extends dorsally around the site of flagellar insertion and gives rise to microtubules which support the cell membrane. No mitochondria nor dictyosomes were observed. On the basis of this information, the organism is assigned to Chilomastix. These observations are also discussed in relation to the origin of mitochondria within eukaryotes.

Behavioural responses in oxygen gradients of ciliates from microbial mats

Summary Microaerophilic ciliates which live in the chemocline of marine sediments and of microbial mats orient themselves in O2-gradients where they show a more or less distinct and species-specific preference for an oxygen tension somewhere within the range of 1 to 20% atmospheric saturation. In sufficiently steep O2-gradients some of the species maintain their position within ± 200 μm relative to their preferred pO2-value. Different types of behavioural responses are involved in the orientation in O2-gradients; in steep gradients the “fine tuning” is at least in some cases based on a phobic response which allows the cells to turn 180° when they accidentally leave their optimum zone. This behaviour was studied in detail in the case of the scuticociliate Uronema filificum. In natural sediments ciliates perform diel vertical migrations in response to changes in O2-gradient profiles caused by photosynthetic activity.

Some Microaerobic Ciliates are Facultative Anaerobes

Summary Microaerobic ciliates prefer low oxygen tensions somewhere between 1 and 20% atmospheric saturation, but their tolerance to high values of P02 and to anoxia is variable. Some microaerobic ciliates ( Uronema marinum, U. filificum ) were shown to survive complete anoxia for only a limited time and they were incapable of growth, while other species ( Cyclidium cf. flagellatum, Euplotes sp., E. aberrans, Strombidium sp. and Paranophrys sp.) survived and grew under anoxic and sulphidic conditions at rates which were lower or almost similar to those attained under oxic conditions. These species are thus facultative anaerobes. They all had relatively low aerobic growth rates compared to those reported in the literature for other similarly sized aerobic ciliates. The facultative anaerobes were tolerant to high concentrations of CN − (> 0.5 mM) and S 2− (in two cases up to 5 mM). When grown under anoxic conditions mitochondrial volume as well as volume fraction increased, suggesting that mitochondria play a role in anaerobic energy metabolism.