Microbial food web

Within the past decade there have been significant changes in the perception of the interactions among trophic levels in aquatic environments. The once-traditional phytoplankton-macrozooplankton-fish food web concepts have been revised to more fully accommodate the effects of both prokaryotes and nutritionally diverse protists. For instance, about 40-60% of the carbon fixed by all phytoplankton is thought to pass through the heterotrophic bacterial community (Cole et al. 1988) rather than into zooplankton directly. Furthermore, oligotrophic marine and freshwaters are now characterized by the preponderance of an assemblage of small phytoplankton less than 2 urn in diameter called picoplankton. This assemblage is composed primarily of chroococcoid cyanobacteria such as Synechococcus and Chlorella-like eukaryotes, both of which contribute significantly to overall primary production (Platt et al 1983; Stockner & Antia 1986; Stockner 1988). Because these tiny cells are poorly grazed by macrozooplankton, they contribute little to the traditional food web.

Much of this bacterial and picoplanktonic carbon is transferred to higher trophic levels via diverse protozoa. In a series of seminal papers (Fenchel 1982a, b, c, d), small flagellates < 20 μm in diameter were reported to be efficient bacterivores with relatively high particle ingestion rates. Given the observed densities of these small protozoa in natural systems together with the calculated grazing rates, Fenchel concluded that such protozoa could have a significant impact on natural bacterial abundances.

Introduction

Until fairly recently, most concepts of higher taxa in the Chrysophyceae have been based on some combination of vegetative morphological features and flagellar number, position and behavior. Application of ultrastructural and molecular techniques to chrysophyte systematics research in the last few years has led to major taxonomic changes (Andersen et al. 1993; Moestrup, this volume; Preisig, this volume). However, the small number and limited diversity of species examined critically to date hampers further progress.

One concept that has been particularly difficult to evaluate, for lack of evidence, is that of the order Chrysamoebales, erected to accommodate chrysophytes that have ameboid (‘rhizopodial’) vegetative cells ‘during the greater part of [the] life history’ (Kristiansen 1990: pp. 439, 441). There have been few accounts of general ultrastructure in ameboid chrysophytes (Hibberd 1971, 1976; Hibberd & Chretiennot-Dinet 1979; Grell et al. 1990), and none on the detailed architecture of their flagellar apparatus.

Isolation into unialgal culture of two ameboid chrysophytes from New Zealand freshwaters – Chrysamoeba pyrenoidifera Korshikov and Lagynion delicatulum Skuja – prompted us to investigate their ultrastructure in detail and, in addition, to re-examine the marine species Rhizochromulina marina Hibberd et Chretiennot-Dinet. This paper summarizes the results of these investigations and thereby assesses the validity and status of the Chrysamoebales. Comprehensive accounts of the structure and reproduction of C. pyrenoidifera and L. delicatulum will appear elsewhere.

Introduction

Betaines are quaternary ammonium compounds (QACs) which contain a carboxylic acid group. They may generally be regarded as fully N-methylated amino or imino acids. While the most commonly encountered betaine (glycinebetaine) is structurally, although not biosynthetically, based on the simplest amino acid, a wide range of aliphatic and aromatic betaines have been described (Fig. 1). These include imino and aromatic amino acid betaines, and compounds such as trigonelline (nicotinic acid betaine) and homostachydrine (pipecolic acid betaine). Related compounds such as choline-O-sulphate and the tertiary sulphonium analogue of β-alaninebetaine, 3-dimethylsulphoniopropionate (DMSP), are also considered in this review since they may have similar physiological roles in some plants.

Although the distribution of betaines in plants is imperfectly known, partly because many taxa have not been thoroughly investigated, it is clear that some families normally contain significant amounts (> 10 mol m-3 plant water) of betaines. In lower plants betaines and related compounds may be incorporated into lipids (Sato, 1992). Recent developments in mass and NMR spectroscopy have greatly facilitated the qualitative and quantitative analysis of betaines. Molecular biology has provided useful tools for the study of regulation of key enzymes of betaine biosynthesis. In vitro studies have clarified the role of betaines as compatible solutes in plants subjected to a variety of stresses. Developments in these areas since the review by Wyn Jones and Storey (1981) will be discussed in detail here, and have also been reviewed by Rhodes and Hanson (1993).

Introduction

A major part of our present knowledge on the processes involved in glutathione metabolism originates from research performed by animal and human biochemists and physiologists up to the late 1970s (Meister, 1981; Meister & Anderson, 1983). Until that time interest of plant biochemists and physiologists in research on glutathione was rather low. Apparently, it was assumed that experiments on glutathione metabolism performed with animal tissues had provided answers that are also valid for plants. This situation changed entirely about a decade ago, when it became obvious that glutathione is an important factor in stress physiology of plants (Smith, Polle & Rennenberg, 1990; Rennenberg & Brunold, 1994). Glutathione was found to be an antioxidant, both as a constituent of the chemical defence system and as a substrate of the enzymatic defense system in the cytoplasm and the chloroplasts of the cells (Polle & Rennenberg, 1993, 1994). Glutathione was found to counteract heat, cold and drought stress (Smith et al, 1990). Glutathione was identified as a substrate for the conjugation of xenobiotics (Rennenberg & Lamoureux, 1990) and for the synthesis of phytochelatins, i.e. poly(γ-glutamylcysteinyl)-glycines involved in metal homeostasis and metal detoxification (Rennenberg & Brunold, 1994). Glutathione was found to be involved in signal transduction in plant-pathogen interactions (Rennenberg & Brunold, 1994). Glutathione was found to be a storage form of reduced sulphur, compensating stress from excess sulphur in plants (Rennenberg, 1984; De Kok, 1990).

Introduction

Consideration of the nutrition of chrysophytes from a biochemical or biophysical viewpoint in comparison with that of other algal taxa has become relatively more difficult over the years. This is not necessarily a function of laxness in using presumptive chrysophytes for such work, but rather that many of the more widely used genera are now classified in other major taxa. The most obvious examples are the separation of the class Prymnesiophyceae, with such nutritionally well investigated genera as Emiliania, Isochrysis and Pavlova, and the recognition that Olisthodiscus (Heterosigma), a molecular-genetically and cell-biologically well understood organism, is probably a chloromonad (raphidophyte) (Heywood 1989; Patterson & van Valkenberg 1990). Even fewer data would be available if the chrysophytes sensu stricto were considered in the absence of the recently separated Synurophyceae. However, both of these classes (Chrysophyceae and Synurophyceae) are considered here, as are recently discovered marine picoplanktonic chrysophytes (Anderson 1987; Shapiro et al. 1989; Keller & Rice 1989), giving a range of organisms from marine picoplankton, through the more familiar freshwater unicells and colonies, to the large benthic freshwater Hydrurus.

To be more positive, there is a substantial body of data on various aspects of nutrition in the chrysophytes. We start by considering the evidence as to the ratio of major nutrients found in chrysophytes growing at their maximum specific growth rates, their qualitative allocation to different molecular species, and the mechanism of synthesis of these molecular species from such common intracellular substrates as hexose, ammonium and phosphate.

Introduction

Chrysophytes are known to be characteristic of slightly acid, soft waters with low alkalinity and conductivity, and with moderate or low productivity (Sandgren 1988; Siver & Hamer 1989). These characteristics are typical for Finnish lakes. In addition to the diatoms and cryptophytes, the chrysophytes are important phytoplankton groups in Finnish lakes (e.g. Heinonen 1980; Eloranta 1986a). Most of the studies concerning chrysophytes in Finnish lakes are general phytoplankton surveys, but there are also some floristic studies that concern only chrysophytes (Kristiansen 1964; Eloranta 1985, 1989a, Asmund & Kristiansen 1986; Hällfors & Hällfors 1988). Eloranta (1989b) also studied the ecology of the genus Dinobryon in Finnish lakes. Some chrysophytes have siliceous scales which remain in lake sediments, and have been used since the late 1960s for paleolimnological purposes (Fott 1966; Munch 1980; Smol 1980; Smol, this volume). Some records of scales in sediments of Finnish lakes have also been reported (Battarbee et al. 1980; Tolonen et al. 1986; Christie et al 1988).

This study investigates chrysophyte ecology and seasonality in different areas of Finland, and explores some of the relationships of chrysophyte distribution with environmental factors.

Materials and methods

Phytoplankton collections from 329 lakes located in different parts of Finland are included in this survey. Information for a total of 55 lakes, mostly from coastal areas and northern Finland, was obtained from literature records (Lepistö et al. 1981).

Introduction

At the seventh workshop of the International Association for Phytoplankton Taxonomy and Ecology (IAP) held on 5–15 May 1989 at Konnevesi Research Station, University of Jyväskylä, Finland, phytoplankton was collected in nearby Lake Konnevesi. At that time of year the lake was extremely rich in scaled chrysophytes (Table 14.1) and dominated by different species of Mallomonas and Synura. In the samples investigated, a Mallomonas species similar to M. alpina Pasch. & Ruttn. and M. tonsurata, Teil, was observed. Between 6 and 11 May this Mallomonas species developed stomatocysts (statospores, cysts). Preserved plankton samples were later examined with electron microscopy (EM). The alpina-tonsurata-like Mallomonas was found to be a new species and has been given the name Mallomonas variabilis sp. nov.

M. variabilis had, in fact, been recorded earlier from several Finnish lakes, even from Lake Konnevesi. However, no complete cells, only isolated scales were found; thus not enough information was available to make a new description of the species (Hällfors & Hällfors 1988). This species has also been recorded under the name M. tonsurata fa. in large lakes in northwestern Ontario, central Northwest Territories, and in the Alaskan Toolik Lake area (H. Kling, unpublished data).

Introduction

Polyamines (PAs) are biologically ubiquitous aliphatic nitrogencontaining compounds of low molecular weight and polycationic nature. The diamine putrescine (Put) and the triamine spermidine (Spd) are probably synthesized by all organisms, while eukaryotes contain the tetraamine spermine (Spm) as well (Cohen, 1971).

In plants, PAs are metabolically related to the basic amino acids arginine and ornithine and therefore also to glutamic acid, a key intermediate in nitrogen metabolism (Tiburcio, Kaur-Sawhney & Galston, 1990). The biosynthetic and degradative pathways for the formation of Put, Spd and Spm in plants are now well established; the key enzymes have been characterized, but their regulation at molecular level is still obscure (Tiburcio et al, 1990; Galston & Tiburcio, 1991).

Suggested roles for PA function, and the evidence for these functions, has recently been reviewed (Tiburcio et al. 1993). These include membrane stabilization, free radical scavenging, effects on DNA, RNA and protein synthesis, effects on the activities of RNase, protease and other enzymes, the interaction with ethylene biosynthesis, and effects on second messengers. It was concluded that, in addition to interacting with plant hormones, PAs are able to modulate plant development through (a) fundamental mechanism(s) common to all living organisms (Tiburcio et al, 1993a).

This chapter deals with the study of PA metabolism and function during osmotically induced senescence in Avena sativa L. (oat) leaves and protoplasts.

Introduction

Classification of the chrysophytes sensu lato is presently in a state of flux which increasingly recalls the problems encountered in classification of the green algae. Christensen (1980) included all chrysophytes in a single class Chrysophyceae, with seven orders. A much more restricted concept was suggested by Hibberd (1976) who proposed to include in the Chrysophyceae only species with the Ochromonas type of organization. Hibberd (1986) excluded silicoflagellates, pedinellids and bicosoecids, and his ideas were accepted by Kristiansen (1986, 1990). Subsequently, Andersen (1987), on the basis of pigment and ultrastructural data, suggested separating the Synura group into a class of its own, the Synurophyceae.

Below I first summarize some of the features of the pedinellids, synurophytes and silicoflagellates, as presently known. A classification into three classes is then suggested. It is argued that photoautotrophs such as Ochromonas and its allies in the Ochromonadales are not necessarily the most primitive chrysophytes. It is equally or even more parsimonious, considering the endosymbiosis theory for the origin of chloroplasts, that heterotrophic, apoplastidic species such as members of the Bicosoeca group or, less likely, apoplastidic pedinellids such as Actinomonas and Pteridomonas, may represent the most primitive chrysophytes.

The idea of the Bicosoeca group as a separate class is no longer justified. Members of this group are closely related to the Ochromonadales, differing mainly in the presence of non-silicified resting stages instead of silicified stomatocysts.

Introduction

Biomineralization is the process by which living organisms assemble structures from naturally occurring inorganic compounds. Most groups of living organisms have members that deposit minerals and in many instances the mineralized structures provide skeletal support and protection for softer organic parts. Within the Chrysophyta a diverse range of biomineralized structures are produced (Preisig 1986). The biogenic material most extensively deposited by chrysophytes is silica, which occurs almost universally in stomatocysts and is commonly present in mineralized scales. Other minerals are deposited by members of the Chrysophyceae as well. For instance, iron and manganese mineralized material occurs in the brown-colored stalks of Anthophysa vegetans, the brown-colored loricae of Pseudokephyrion pseudospirale, and the gelatinous holdfasts of Phaeothamnion articulata (Preisig 1986). Calcareous deposits are rare in the Chrysophyta, being limited to the pseudocysts of a few species of the Sarcinochrysidales and the mucilage of some species, such as Celloniella, which form gelatinous colonies (Preisig 1986; see Moestrup, this volume).

This review is concerned with the utilization of silica by chrysophytes. The information will be presented in the overall context of the biogeochemical cycling of silica.

Biogeochemical cycling of silica

Silicon is the second most abundant element in the Earth's crust, accounting for some 28% of its mass. Invariably it occurs in chemical composition with oxygen, as in the crystalline and cryptocrystalline polymers of silica (SiO2). Common naturally-occurring substances in- volving additional elements include: kaolinite, feldspars, micas, and the so-called clay minerals (Stumm & Morgan 1970).

Introduction

The colorless chrysophyte genus Paraphysomonas de Saedeleer includes about 50 taxa. All have typical heterokont flagellation and silicified scales covering the cell body. Only a very few of the larger spine-scales are visible in the light microscope. Electron microscopy is necessary for the identification of most species and subspecies.

Examinations of the freshwater algal flora of the tropics are numerous, and in recent years have started to include the chrysophytes. Cronberg (1989) has summarized all of the literature on scaled chrysophytes from the tropics. However, there has been only one study using electron microscopy from India and two from countries adjacent to India: Bangladesh (Takahashi & Hayakawa 1979) and Sri Lanka (Dürrschmidt & Cronberg 1989).

During a study of the biota of silica-scaled protists in Indian freshwaters, a number of species not previously reported were observed (Saha & Wujek 1990). This paper focusses not only on Paraphysomonas species not previously recorded, adding to the knowledge of their occurrence and distribution, but also presents descriptions of two new species.

Materials and methods

The sites that contained Paraphysomonas taxa are listed in Table 17.1 Sample fixation and preparation were as described in Saha & Wujek (1990); or the specimens were shadowed with platinum/palladium.

Introduction

Although the central role of amino acid biosynthesis in plant metabolism and development is evident, progress in the understanding of the molecular mechanisms by which such pathways are regulated is only recent. To elucidate the genetic regulation of a pathway, mutants have proved to be invaluable in the analysis of microbial systems. The same is true for plants, but again there are only a few pathways for which mutants with defective or altered essential functions are available. However, the improvements in plant mutant isolation at cell and whole plant levels, and in molecular biology have now provided mutants affected in amino acid metabolism as well as amino acid biosynthetic genes or cDNAs (for a review, see Last, 1992).

The branched biosynthetic pathway of aspartate-derived amino acids (lysine, methionine, threonine) and the related pathway leading to isoleucine, valine and leucine have received much interest for various reasons. First, it gives rise to essential amino acids which, if poorly represented as lysine in cereals and methionine in legumes, limit the nutritional quality of crop plants as diet for human beings and monogastric animals. Secondly, three classes of potent herbicides (sulphonylureas, imidazolinones and triazolopyrimidin) kill plants through the inhibition of acetolactate synthase, an enzyme common to the isoleucine and valine pathways. Key regulatory enzymes of the aspartate-derived amino acid biosynthesis would also be suitable targets for efficient herbicides while little affecting the environment.

Introduction

Substantial morphological, biochemical and genetic diversity exists among photosynthetic eukaryotes. Some of the most conspicuous differences involve plastid-based characters. These features, such as the photosynthetic pigment complement, have provided a basis for defining plant and algal taxa. Christensen (1962) classified algae into three divisions according to their pigment composition. In this system, the division chlorophyta includes plants and algae that possess chlorophylls a and b, the Chromophyta are algae that predominantly contain chlorophylls a and c, while the Rhodophyta are defined in part as those algae having primarily chlorophyll a and accessory phycobilin pigments. However, autotrophic eukaryotes evolved in a stepwise manner with the mitochondrial and plastid organelles originating from formerly free-living prokaryotic endosymbionts (Gray & Doolittle 1982). In examining the evolution of these organisms, one must consider the evolution of not only the host, but also its organelles with their prokaryotic ancestors.

By examining cytosolic and nuclear features exclusive of plastid characters, one can infer relationships between extant plants and algae. Such studies have lent support to the validity of the Rhodophyta and Chlorophyta as true phylogenetic assemblages (natural taxa). For example, the complete absence of actual or vestigial flagella in rhodophytes (Gabrielson et al. 1985) and conserved features of karyokinesis and flagellar root structure in chlorophytes (Mattox & Stewart 1984) support the evolutionary cohesiveness of these groups. Furthermore, comparison of nuclearencoded ribosomal RNA (rRNA) supports the model of a monophyletic origin for green and red algae (Perasso et al. 1989; Chapman & Buchheim 1991).

Introduction

Synura petersenii has a worldwide distribution. It is often an important, and frequently the most common, Synura species of the phytoplankton (Kristiansen 1979; Siver 1987; Hällfors & Hällfors 1988; Hickel & Maass 1989). This taxon appears to be pH and temperature indifferent and often occurs under eutrophic conditions (Kristiansen 1975, 1986, 1988; Kies & Berndt 1984; Roijackers 1986).

The first monograph of the genus Synura by Korshikov (1929) was based on scale morphology. Balonov & Kuzmin (1974) later differentiated the genus into three sections: Synura, Petersenianae and Lapponicae. The section Petersenianae is an exception, as the species are distinguished by differences in the construction of elements of scales, whereas the species of the other sections exhibit explicitly different elements (Fott & Ludvik 1957). Several subspecific taxa within this section have been described on the basis of variations in the scales (Siver 1987, 1988); however, it is often difficult to distinguish them from the type for several reasons. First, there is variability among scales of the armour of the same cell (e.g. between body and caudal scales; Fig. 15.4(1, 2)). Second, some taxa are distinguished even though not all scales of the cell armour are significantly different from the type. Third, continuous transitions among subspecific taxa often occur (Siver 1988).