Alderkamp, Anne-Carlijn

English

In this PhD thesis I describe studies relating to the cycling of the algal storage glucan chrysolaminaran. Chrysolaminaran is the most abundant type of storage carbohydrate in marine phytoplankton. I choose it as a model substrate to study factors influencing the cycling of carbohydrates, one of the most important processes in the marine carbon cycle. Carbohydrates comprise an important fraction of the biomass of phytoplankton. Upon release into the environment, they are usually readily degraded by prokaryotes in the microbial loop. Sometimes, however, degradation may be hampered and carbohydrates end up in refractory dissolved organic matter (Benner et al. 1992), in marine particles, and in sediments (Cowie and Hedges 1984). Research into factors hampering the degradation of carbohydrates is complicated by a number of factors, such as 1) the vast diversity of naturally occurring carbohydrates, 2) the low in-situ rates of degradation, and 3) the low concentrations. By studying the cycling of a relatively simple carbohydrate that is structurally well characterized, some of these problems can be circumvented. Moreover, chrysolaminaran is one of the most abundant carbohydrates in the marine system, yet little is known about factors controlling its production and degradation. The aim of this study was to elucidate factors influencing production and degradation rates of carbohydrates in the marine ecosystem by following the steps of the chrysolaminaran cycle. In chapter 1 three research questions were formulated: • How do nutrient concentrations and irradiance levels influence the production of chrysolaminaran by phytoplankton? • What are the characteristics of bacterial enzyme systems involved in degradation of chrysolaminaran? • What is the influence of a massive release of carbohydrates on the abundance and activity of prokaryotes? Some of the principles found by studying this relatively simple carbohydrate provide further insights into the carbohydrate production and degradation patterns observed in marine ecosystems. How do nutrient concentrations and irradiance levels influence the production of chrysolaminaran by phytoplankton? Phaeocystis is a cosmopolitan microalga genus often dominating the phytoplankton population in temperate and polar waters, and known for the production of copious amounts of carbohydrates. It produces two major pools of carbohydrates; extracellular mucopolysaccharides in the colony matrix and storage glucan, of which chrysolaminaran is the main constituent. In chapter 2 the question how nutrient concentrations and irradiance levels influence the production of storage glucan and mucopolysaccharides is addressed. This was studied during a spring bloom of Phaeocystis pouchetii in a mesocosm near Bergen, Norway. Chapter 2 describes a partial precipitation method to separate both pools and study their dynamics in response to variations in nutrients and irradiance. During the growth phase of the bloom the glucan content of the colonies showed a diel cycle, the amplitude of which increased with higher light levels, whereas the mucopolysaccharide content was unaffected. Following nutrient limitation, the bloom reached a stationary phase, during which the carbohydrate to carbon ratio of the colonies increased. This was mainly caused by an increase in glucan content. It was shown that the Phaeocystis colony matrix is built with a relatively small but constant amount of carbohydrates, whereas the glucan contents is highly variable and increases upon nutrient limitation and high light levels. At the end of the bloom glucan contributed up to 60% to the Phaeocystis carbon. Since a major part of Phaeocystis primary production is recycled in the water column by prokaryotes, this suggests that glucan and hence chrysolaminaran is an abundant substrate for prokaryotes after a Phaeocystis bloom. Diatoms are responsible for as much as 40% of the annual marine CO2 uptake (Nelson et al. 1995). Chrysolaminaran is the principal storage glucan of diatoms and therefore represents a major part of their biomass. The effect of excessive light conditions on the production of chrysolaminaran in the marine diatoms Thalassiosira weissflogii and T. antarctica was investigated in chapter 3. This was part of a laboratory study of the effects of high-light acclimation on tolerance to excessive photosynthetically active (PAR) and ultraviolet (UVR) radiation light conditions. Cultures acclimated to either low or high PAR conditions were subjected to simulated surface irradiance (SSI) that mimicked irradiance around noon, including UVR. A number of physiological indicators of stress related to high-light conditions were determined after 30 min SSI and during 120 min recovery in low irradiance. In both T. weissflogii and T. antarctica the chrysolaminaran content of high-light acclimated cells was higher when compared to low-light acclimated cells, consistent with the results of chapter 2. In high-light acclimated cells the pool of xanthophyll pigments was higher and the efficiency of PSII was lower compared to low-light acclimated cells. The SSI treatment caused a decline in the PSII efficiency, coinciding with de-epoxidation of diadinoxanthin. During recovery these processes were reversed, but in low-light acclimated cells reversion was not completed within three hours. This resulted in a reduction in carbohydrate build-up in low-light acclimated cells. In high-light acclimated cells reversion was complete after one hour of recovery. In high-light acclimated T. antarctica the chrysolaminaran content was unaffected by both the SSI treatment and the control treatment, whereas in high-light acclimated T. weissflogii the chrysolaminaran content decreased after both treatments. The incapability to build up chrysolaminaran during the recovery period was probably caused by the reduced efficiency of PSII in high-light-acclimated cells. In T. weissflogii the carbon and energy stored in the chrysolaminaran was likely mobilized during recovery. The UVR component of the SSI treatment had no influence on the chrysolaminaran content of any of the cultures. What are the characteristics of bacterial enzyme systems involved in degradation of chrysolaminaran? This question of the characteristics of bacterial enzyme systems involved in degradation of chrysolaminaran was the base of chapter 4. After a bloom of Phaeocystis globosa in the Dutch coastal North Sea several bacterial strains capable of utilizing laminarin as a sole carbon source were isolated. On average 26% of prokaryotes detected by epifluorescence counts were able to grow in Most Probable Number (MPN) dilution series on laminarin as a sole carbon source. Several bacterial strains were isolated from different dilutions and phylogenetic characterization revealed that they were belonging to different phylogenetic groups. The activity of the laminarin degrading enzyme systems was further characterized in three strains of Vibrio sp. that were able to grow on laminarin as a sole carbon source. At least two types of activity were detected during degradation of laminarin: the release of glucose and the release of glucans larger than glucose. At saturating substrate concentrations the rate of the glucan release exceeded the rate of the glucose release, resulting in accumulation of glucan intermediates during degradation of high concentrations of laminarin. At low concentrations, however, the ratio glucose/ glucan release increased, as a result of a higher substrate affinity of the glucose releasing enzymes than of the glucan releasing enzymes. Therefore, at low substrate concentrations no accumulation of glucan intermediates occurred. The kinetic properties of the laminarin degrading enzyme system may thus explain the observation that bacterial hydrolysis of polymers of aggregates and uptake of low molecular weight compounds are sometimes uncoupled processes, resulting in release of free polymers from particles into the surrounding water mass (Cho and Azam 1988; Smith et al. 1992; 1995; Unanue et al. 1998; Azúa et al. 2003). In aggregates the carbohydrate concentrations are high (Azúa et al. 2003), leading to high substrate concentrations for glucosidases. If the kinetic properties of the laminarin degrading enzymes were exemplary for other hydrolase systems, this would explain the release of polymers from particles. Laminarinase enzymes showed a minimal activity on substrates with similar glucosidic bonds, but a different size, secondary, and /or tertiary structure. In an aqueous environment polymers tend to self-assemble into hydrogels (Chin et al. 1998; Verdugo et al. 2004), thereby altering their tertiary structure and accessibility of the linkages for the enzymes. If the difference in degradation potential of laminarin versus the substrates with a different size and/or structure is exemplary for the difference in degradation potential of “free” polymers and polymers embedded in a gel structure, turnover times may be slowed down from days to years. What is the influence of a massive release of carbohydrates on the abundance and activity of prokaryotes? The effect of a spring bloom of Phaeocystis globosa on the microbial community is investigated in chapter 5 to study what is the influence of a massive release of carbohydrates on the abundance and activity of prokaryotes. MICRO-CARD-FISH analysis was applied to samples collected from the coastal North Sea over the course of a spring and summer season to determine the abundance and activity of different groups of prokaryotes. The abundance, as well as the fraction of active prokaryotes increased during the initial development of the P. globosa bloom. At the peak of the bloom, the prokaryote numbers decreased, but increased again during the decline of the bloom. The activity of the prokaryotes remained high throughout the spring and summer. Bacteria belonging to the Bacteroidetes were dominating the prokaryote community during the wax and wane of the P. globosa bloom. They have been associated with degradation of complex carbohydrates (Kirchman 2002), and therefore are probably involved in degradation of Phaeocystis carbohydrates. Members of the Roseobacter clade always showed the highest fraction of active cells, however, they comprised a minor part of the prokaryote community. Two groups of archaea, Crenarchaea and Euryarchaea, were detected throughout the study period. Their combined contribution was on average 2% of the total prokaryotic community, and the active fraction of archaea was always lower than the fraction of active bacteria. Therefore we conclude that archaea do not play a major role in the biogeochemical cycles of the coastal North Sea during spring and summer. Conclusions: The carbohydrates of Phaeocystis and their degradation in the microbial food web – a review The conclusions from the previous chapters are integrated in chapter 6 to discuss the production of carbohydrates by Phaeocystis and their degradation in the microbial food web. Since chrysolaminaran comprises an important fraction of Phaeocystis biomass, especially at the end of a bloom when nutrients are limited, it is an important component of the biogeochemical cycle following a Phaeocystis bloom. Hence, it should be considered alongside the mucopolysaccharides that have received much more attention so far. Lysis of Phaeocystis cells and deterioration of colonies releases the biomass as dissolved organic matter (DOM) during the wane of a bloom. Laboratory studies have revealed that both mucopolysaccharides and chrysolaminaran are potentially readily degradable by heterotrophic bacteria (Janse 2000b). In particular bacteria belonging to the Bacteroidetes benefit from the carbohydrate rich DOM. Observations of accumulation of DOM and foam after blooms of Phaeocystis (Eberlein et al. 1985; Lancelot et al. 1987a), however, indicate that the DOM may be persistent. One of the reasons put forward is the high C/N and C/P ratio of Phaeocystis organic matter, which may lead to nutrient limitation during microbial degradation, thereby prolonging degradation times. Upon prolonged degradation times carbohydrates can self-assemble into hydrogels (Chin et al. 1998). This may have a profound effect on carbon cycling, since hydrogels provide a vehicle to move DOM up the size spectrum to sizes subject to sedimentation. In addition, it changes the physical nature and microscale structure of the organic matter encountered by bacteria, thereby affecting the degradation potential of the Phaeocystis organic matter

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   University of GroningenCarbohydrate production by phytoplankton and degradation in the marine microbial food webAlderkamp, Anne-CarlijnIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2006Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Alderkamp, A-C. (2006). Carbohydrate production by phytoplankton and degradation in the marinemicrobial food web. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 20-06-2022 Chapter 1   Carbohydrate production by phytoplankton and degradation in the marine microbial food web –  The cycling of chrysolaminaran         THE MARINE CARBON CYCLE  Two-thirds of the earths’ surface is covered by water, and approximately half of the global primary production is produced in the marine ecosystem. The most important process responsible for primary production is oxygenic photosynthesis by phytoplankton (Hedges 1992). Phytoplankton are microscopic plants, mostly algae, which live suspended in the water column. All plants use light, to convert carbon dioxide and water to organic compounds and oxygen in the photosynthesis process. The organic carbon produced by photosynthesis in the sea is respired in the marine food web (Figure 1). In the linear food chain carbon and energy is channelled through algae, to herbivores and on to higher trophic levels. In all these processes dissolved organic carbon (DOC) is released. DOC is respired by heterotrophic prokaryotes and stored as biomass that can re-enter the linear food chain after grazing by protozoans via the “microbial loop” (Azam et al. 1983). The prokaryotes play an important role in the marine carbon cycle, since an estimated 50% of marine primary production is channelled via the microbial loop (Azam 1998). Amongst the prokaryotes, bacteria are the most important consumers of DOC in the marine system (Williams 2000). Archaea have been detected in virtually all marine systems (Olsen 1994; Stein and Simon 1996) and may play a significant role in the carbon cycle in the deep sea (Karner et al. 2001; Herndl et al. 2005). At present, however, their role in the global carbon cycle remains largely unknown.    Chapter 1     CO2 CO2 viruses phyto- plankton archaea gases LMW bacteriaherbivores DOC ecto colloids HMW higher trophic levels protozoans Figure 1. Schematic representation of the marine microbial food web. On the left the “linear food chain” is depicted, where energy and carbon from photosynthesis in phytoplankton is channeled through herbivorous grazers, onto higher trophic levels. On the right the “microbial loop” is depicted, in which dissolved organic carbon (DOC) is respired by prokaryotes and stored as biomass that can re-enter the classic food chain via the protozoans. Although archaea are detected in virtually all marine systems, their role in the marine carbon cycle remains largely unknown. High-molecular-weight (HMW) DOC is cleaved by either “free” extracellular enzymes, or cell-attached ectoenzymes (“ecto”) prior to uptake by bacteria. Energy from the sun fuels phototrophic phytoplankton and prokaryotes. Excessive irradiance and especially the UV component may, however, cause damage to phytoplankton, prokaryotes and viruses. In addition irradiance may alter properties of DOC. At the bottom the flux of sinking particulate organic carbon (sinking POC flux) is depicted. In this way carbon sinks to the deep waters and may eventually become buried in sediments, whereby it is withdrawn from the carbon cycle for prolonged times (adapted from Karl 1994; Delong and Karl 2005). sinking POC flux  (?) 10The chrysolaminaran cycle CARBOHYDRATE PRODUCTION BY PHYTOPLANKTON  The cycling of carbohydrates is a key process in the marine carbon cycle because of the abundance and omnipresence of carbohydrates in the marine ecosystem. Carbohydrates form an important fraction of the organic carbon produced by phytoplankton (Biddanda and Benner 1997; Biersmith and Benner 1998). They are major constituents of marine particles and sediments (Cowie and Hedges 1984), and marine DOC (Benner et al. 1992). Different types of carbohydrates with different functions are produced by phytoplankton. Two major pools are extracellular and storage carbohydrates.  Extracellular carbohydrates are typically large heteropolymers and may form a mucous layer around the algal cells (Guillard and Helleburst 1971; Hoagland et al. 1993). In addition, phytoplankton release a part of the primary production directly as DOC (Nagata 2000; Teira et al. 2001; 2003), a significant part of which typically consists of carbohydrates (Biddanda and Benner 1997; Biersmith and Benner 1998; Aluwihare and Repeta 1999). Excess photosynthesis, e.g. under high light conditions or nutrient limitation, may enhance release of extracellular carbohydrates (e.g. Staats et al. 2000). In this case the contribution of extracellular carbohydrates to the phytoplankton carbon will depend on the nutrient status and light conditions of the algae.  Storage carbohydrates are produced in the light and serve as an internal energy and carbon reserve. At low light conditions, or at night, this reserve can be used for maintaining cell metabolism and protein synthesis (Cuhel et al. 1984; Lancelot and Mathot 1985; Granum and Myklestad 2001). In addition, excess photosynthesis may lead to accumulation of storage carbohydrates (Myklestad and Haug 1972; Myklestad 1988; Janse et al. 1996b). Therefore, the contribution of storage carbohydrates to cellular carbon is highly variable, depending on the time of the day, the light intensity and the nutrient status of the cell.  The storage carbohydrates of most algae are α- or β -glucans. Chrysolaminaran is the most abundant type of storage carbohydrate in marine phytoplankton (Figure 2). It is a ß-1,3-D-glucan with some branching at position 6 and/or 2, typically containing 20-30 glucose residues, corresponding to approximately 4 kDa in size. It is also known as leucosin or laminarin. The latter name refers to the type of glucan found in some macroalgae (e.g. the Phaeophyta), with the same structure but containing mannitol end-groups (Meeuse 1962; Craigie 1974; Myklestad 1978; Painter 1983; Janse et al. 1996b). In diatoms chrysolaminaran is located intracellular in vacuoles (Chiovitti et al. 2004). Because of its presence in phytoplankton all over the world, chrysolaminaran is one of the most abundant carbohydrates on earth.       11Chapter 1 OCH2OHHHOHHOHOHHHOOCH2OHHHOHHOHOHHHOOCH2OHHHOHH OHHHOCH2OCH2OHHHOHH OHHHOOCH2OHHHOHH OHHHOOHHOHH OHHHOOCH2OHHHOHH OHHHOOCH2OHHHOHH OHHOHH Figure 2.  Structure of part of a chrysolaminaran molecule, a ß-1,3-D-glucose backbone with a ß-1,6 branch.    CARBOHYDRATE DEGRADATION BY PROKARYOTES  The carbon of phytoplankton becomes available for prokaryotes as DOC after sloppy feeding and egestion by zooplankton (Strom et al. 1997; Møller et al. 2003), or cell lysis (Van Boekel et al. 1992; Brussaard et al. 1995b; 2005), e.g. as a result of viral infection (Fuhrman 1999; Suttle 2005). Although carbohydrates are usually considered to be labile substrates for prokaryotes, the high concentration of carbohydrates in marine DOC (Benner et al. 1992), marine particles and sediments (Cowie and Hedges 1984), and sediment pore waters (Arnosti and Holmer 1999) demonstrates carbohydrates are not always easily metabolized. Whether a carbohydrate is utilized by a certain prokaryote largely depends on 1) the chemical structure of the carbohydrate and 2) the biochemical capacities of the prokaryote.  1) The chemical structure of carbohydrates displays a remarkable diversity, a consequence of the wide variety of naturally occurring monosaccharides and the different glycosisdic bonds. For example two different hexoses can form 16 different disaccharides; three different hexoses can form 384 different trisaccharides. There are three levels of structural diversity. The primary structure is determined by the type of monosaccharide and their linkage. This leads to a secondary structure in the shape of the polymers (e.g. ß-1,3-linked glucans form helices). Polymers pack into structures that are held together predominantly by hydrogen bonds, determining a tertiary structure (e.g. a loose hydrogel, or a tightly packed network structure like cellulose). Since the enzymes involved in degradation of polymers are selective with respect to their substrate (Arnosti and Repeta 1994; Warren 1996), this diversity requires a similar enzyme diversity. The secondary and tertiary structure may lead to inaccessibility of glucosidic bonds. Therefore, simple, linear polymers consisting of just one monosaccharide, such as storage glucans, are generally believed to be more labile then heteropolymers with complex tertiary structures, such as extracellular mucopolysaccharides or cell wall constituents (Warren 1996).   12The chrysolaminaran cycle Due to the large diversity and low concentrations of carbohydrates in the marine environment, the possibilities of analyzing the structural complexity in marine samples are extremely limited. In addition, the properties of carbohydrates released as DOC into the marine environment are not static, but influenced by abiotic factors. Polymers assemble spontaneously, forming the matrix of hydrogels, supported by ionic interactions such as calcium bridges. In this way soluble polymers end up in particles capable of sedimentation (Chin et al. 1998; Verdugo et al. 2004) (Figure 1). In addition, it alters the secondary and tertiary structure of the carbohydrates. Assembly may, however, be inhibited by ultraviolet (UV) radiation. Moreover, UV radiation may disperse assembled gels and even cleave polymers (Orellana and Verdugo 2003) (Figure 1).  2) The biochemical capacities of the prokaryotes largely depend on the properties of their enzyme systems. Carbohydrates are degraded by glycoside hydrolases; enzymes hydrolyzing the glucosidic bond between two monosaccharides. Since prokaryotes can only transport small molecules (< 600 Da) across their cell membrane (Weiss et al. 1991), larger molecules have to be cleaved by extracellular enzymes prior to uptake. These enzymes may be “ectoenzymes” (Figure 1) that are attached to the prokaryotic cell, either to the cell wall or embedded in exopolymer layers, or “free enzymes” that are released into the environment. Free enzymes may be actively excreted by cells, but may also have been liberated after cell lysis, e.g. as a result of autolysis, viral lysis, or sloppy feeding (Chróst 1991). Ectoenzymes are most abundant in the marine environment (Bochdansky et al. 1995). Very little is known about properties of enzymes of marine prokaryotes, largely due to low amounts of biomass and slow in-situ degradation rates of relevant substrates. Therefore, enzyme characteristics in the marine environment have mainly been determined using small substrate proxies, consisting of a monomer such as glucose linked to a fluorophore such as methylumbelliferyl (MUF), whose fluorescence increases upon hydrolysis (e.g. Martinez et al. 1996; Arrieta and Herndl 2002). While these small substrate proxies are easy to apply and the use of fluorophores ensures the sensitivity that is required detecting low activities in marine samples, these substrates lack the structural properties of macromolecules in solution (Warren 1996).    AIM OF THE THESIS  Given the difficulty of direct structural analysis of marine carbohydrates and their slow degradation rates in-situ, I chose the approach to study the cycling of a well-characterized, relatively simple carbohydrate: chrysolaminaran. Studying the cycling of chrysolaminaran enabled me to circumvent the structural diversity of carbohydrates occurring in the marine ecosystem, yet the structural complexity to mimic complex macromolecules in solution is still present. Therefore, some of the principles found by studying this relatively simple carbohydrate may help to elucidate factors hampering rapid carbohydrate degradation by prokaryotes and better understand carbohydrate cycling in the marine ecosystem. Moreover, chrysolaminaran is one of the most abundant carbohydrates in the marine system, yet little is known about factors controlling its production and degradation. This thesis follows key  13Chapter 1 elements of the chrysolaminaran cycle, specifically its production by phytoplankton and subsequent degradation by prokaryotes.   The main research questions are: • How is the production of chrysolaminaran by phytoplankton influenced by nutrient concentrations and irradiance levels? • What are the characteristics of bacterial enzyme systems involved in degradation of chrysolaminaran? • What is the influence of a massive release of carbohydrates on the abundance and activity of prokaryotes?  Some of the principles found by studying this relatively simple carbohydrate provide further insights in the carbohydrate production and degradation patterns observed in marine ecosystems.    OUTLINE OF THE THESIS  The production of chrysolaminaran was studied in the phytoplankton groups Phaeocystis and diatoms. Both groups are cosmopolitan algae that often dominate the phytoplankton of temperate and polar waters (Nelson et al. 1995; Schoemann et al. 2005). Chrysolaminaran is the principal storage glucan of both groups (Paulsen and Myklestad 1978; Painter 1983; Janse et al. 1996b). Diatoms are extremely ubiquitous both in the marine ecosystem and in fresh water. They may be planktonic, benthic, or growing on sea ice or other organisms, such as seaweeds, animals (from crustaceans to whales). Photosynthesis by marine diatoms generates as much as 40% of the annual carbon production in the sea (Nelson et al 1995).  Phaeocystis has a polymorphic life cycle, comprising various stages of single cells of 3-9 µm in diameter and a colonial stage with cells embedded in a mucous layer usually reaching several mm (Rousseau et al. 1994). This mucous layer consists predominantly of extracellular carbohydrates (Guillard and Helleburst 1971; Solomon et al. 2003), that form a thin, yet strong, gel, surrounding an aqueous lumen (Van Rijssel et al. 1997; Hamm et al. 1999). Colonial Phaeocystis forms massive blooms in temperate and polar waters (Lancelot et al. 1987; Baumann et al. 1994; Mathot et al. 2000; Schoemann et al. 2005). During these blooms organic matter is produced that is rich in carbohydrates (Veldhuis et al. 1986a; Fernández et al. 1992; Janse et al. 1996a) comprising both extracellular mucopolysaccharides and chrysolaminaran.  In chapter 2, a new method is described to separate mucopolysaccharides and chrysolaminaran in organic matter derived from Phaeocystis colonies. This method was applied during a Phaeocystis bloom in a mesocosm experiment near Bergen, Norway. In several experiments, the effect of light intensity and nutrient conditions on the production of both chrysolaminaran and mucopolysaccharides was determined.  The light climate that phytoplankton residing in the water column experience can be highly variable due to wind induced vertical mixing. Thus, phytoplankton may be exposed to short  14The chrysolaminaran cycle periods of excessive high light levels, including UV radiation. This requires specific acclimation to find a balance between protection against photodamage at high-light levels and optimal photosynthesis conditions for sufficient energy for growth at low-light levels (Neale et al. 2003). In chapter 3 the production of storage carbohydrates under excessive light conditions were investigated in an Antarctic and a temperate diatom. This was part of a laboratory study into the effect of acclimation to high-light conditions on tolerance to short periods of excessive light conditions. The characteristics of enzyme systems of selected marine bacteria involved in degradation of chrysolaminaran are described in chapter 4. After a bloom of Phaeocystis in the Dutch coastal North Sea several bacterial strains capable of utilizing laminarin as a sole carbon source were isolated. The laminarin degradation system of three strains of Vibrio was studied in detail in physiological experiments in the laboratory.   In chapter 5 the dynamics in abundance and activity of prokaryotes was determined in the coastal North Sea during a spring and summer season, thus when primary production was dominated by diatoms and Phaeocystis. A combination of Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization with microautoradiography (MICRO-CARD-FISH) was applied to combine quantitative detection of specific phylogenetic groups with their group-specific activity. The results are discussed in relation with the release of carbohydrates produced in a Phaeocystis bloom. Finally, in chapter 6 the current state of the knowledge on the production of carbohydrates by Phaeocystis and their degradation in the microbial food web is reviewed, including the results from the previous chapters.              15Chapter 1       16

Carbohydrate production by phytoplankton and degradation in the marine microbial food web

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Carbohydrate production by phytoplankton and degradation in the marine microbial food web

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