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Phaeocystis Colony Mucus Components and the Importance of Calcium Ions for Colony Stability

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Phaeocystis Colony Mucus Components and the Importance of Calcium Ions for Colony Stability MARINE ECOLOGY PROGRESS SERIES Published October 19 Mar. Ecol. Prog. Ser. NOTE Phaeocystis colony mucus components and the importance of calcium ions for colony stability W. H. M. van Boekel Department of Marine Biology. University of Groningen, PO Box 14.9750 AA Haren. The Netherlands ABSTRACT: The composition and properties of Phaeocystis The alga Phaeocystis sp. is an important component of colony mucus are st~lllargely obscure. In this study some the phytoplankton of several marine ecosystems, such components of the mucus were ident~f~edusing a specific as the North Sea and the Arctic and Antarctic oceans staining technique and the role of Ca2* and other cations as binding agent was investigated. In addition, the effect of Ca2' (Barnard et al. 1984, Cadee & Hegeman 1986, Palrni- concentration on colony formation in batch cultures was sano et al. 1986, Lancelot et al. 1987, Gibson et al. 1990, studied. Colonies of Phaeocystis sp. were stained with alcian Wassmann et al. 1990).Phaeocystis forms colonies con- blue at 2 different pH values. This revealed that the colony sisting of mucus in which cells are randomly distributed. mucus contained both carboxylated and sulfated polymers. Incubation of colonies in medium lacking one or more cations The colonies are spherical or elongated and reach sizes showed that calcium and magnesium ions were essential for of up to 5 mm in diameter, containing over 10 000 cells the gelling of colony mucus, while potasslum ions had no (Rouseau et al. 1990). During bloom situations in the influence. The percentage colony cells formed by Phaeocystis North Sea, when Phaeocystis cell number often exceeds in batch cultures was reduced in medium with calcium con- 50 X 106 1-' (Cadee & Hegeman 1986), colony mucus centrations below 2.5 mm01 I-'. No colonies were formed in % medium with calcium concentrations below 1.5 mm01 1.' may represent 50 or more of total phytoplankton Growth rate was not dependent on calcium concentration. It biomass (Rouseau et al. 1990). The formation of foam is suggested that under natural conditions Phaeocystis colony layers on beaches following Phaeocystis blooms (Batje & firmness and morphology might depend on the composition of Michaelis 1986) suggests that the mucus is not easi1.y mucus polymers. decomposed, either (photo)chemically or bacterially. In the marine environment mucus production is The composition of Phaeocystis mucus is as yet largely known to occur in bacteria (Decho 1990), macroalgae unknown. The research that has been done has yielded (Boney 1981) and several groups of microalgae such as contradictory results. Guillard & Hellebust (1971)found diatoms (Decho 1990),green algae (Crayton 1982) and that the mucus of 2 Phaeocystis strains consisted of the Prymnesiophyceae (Painter 1983).The composition oligo- and polysaccharides of heterogeneous composi- of mucus produced by these groups is often rather tion. Painter (1983)reported some preliminary results of complex, consisting of heteropolymeric chains con- the analysis of an impure bloom of Phaeocystis, showing taining a wide variety of simple sugars, aminosugars, that the mucus might be a very complex, soluble pro- uronic acids, sulfated or phosphated sugars, amino teoglycan. Lancelot et al. (1991) mentioned that the acids, etc. The gelling capacity of mucus depends on the mucus is a polysaccharide, mainly composed of glucose binding of negatively charged groups in the molecule units. Knowledge of mucus composition and properties (mostly carboxyl groups) with cations (mostly Ca2+).In is essential for understanding the role of Phaeocystis in this way ionic bridges are formed between polymer the ecosystem, since it may help to explain the processes strands. The number of ionic bridges formed in a of Phaeocystis colony formation and growth, as well as polymer depends on the number of anionic groups and diffusion inside colonies, aggregation and sedimenta- the steric arrangement of these groups in the molecule tion of colonies and the flux of mucus carbon to the (Kohn et al. 1968). Colony-forming algae such as the microbial foodweb. This study represents a further step Volvocaceae produce mucus with large amounts of in elucidating the composition and properties of the carboxyl and sulfate groups (Crayton 1982). colonial mucus of Phaeocystis. 0 Inter-Research 1992 302 Mar. Ecol. Prog. Ser. 87: 301-305, 1992 Materials and methods. All experiments were per- combination of two of these, or all three were lacking. formed with axenic Phaeocystis sp. (strain K) isolated The omitted ion was replaced by Na' in order to main- from the Dutch Wadden Sea. This strain formed tain approximately the same salinity in the medium. globosa-type colonies (Jahnke 1989). Phaeocystis was Triplicate 5 rnl portions of these media and of complete grown in 1 1 serum bottles incubated on a rolling medium (control) were distributed over sterile, translu- device at 10 "C and a light intensity of 40 pE m-' S-' cent culturing plates containing 25 chambers each. in a 14:10 L:D cycle. The artificial seawater medium Healthy colonies were concentrated from a Phaeo- described by Veldhuis & Admiraal (1987) was used cystis culture growing in standard medium and small for culturing and for all experiments. The medium aliquots (0.1 to 0.25 rnl) of this concentrate were trans- had nutrient concentrations of 4 lmol 1-' Pod3- and ferred to the different media in the chambers. The 70 pm01 1-' NOs-, while the Ca2+ concentration was plates were incubated for 1 wk at 10 "C and low light 3.6 mm01 I-'. Under these conditions Phaeocystis pre- intensity and colonies were checked regularly using dominantly formed large colonies, suitable for the inverted microscopy. experiments described below. The presence of car- The effect of different calcium concentrations on boxylated and sulfated polymers in mucus of living colony formation during growth was studied in batch Phaeocystis colonies was investigated with the use of culture experiments. For these experiments calcium- alcian blue, a cationic copper phthalocyanine dye. free medium was inoculated with Phaeocystis and which is specific for these pcly-anionic groups (Scott et distributed in 200 m1 portions among several 250 m1 al. 1964, Ramus 1977). Differentiation between car- serum bottles. To each bottle a different amount of boxylated and sulfated polysaccharides was possible a CaC12 stock solution was added, to final calcium since the dye binds to both types at pH 2.5, and to concentrations ranging between 0.175 and 7.5 mm01 sulfated polymers only at pH 0.5 (Crayton 1982). Ca 1-l. The bottles were incubated under standard con- Alcian blue does not bind to phosphate groups at these ditions as described above. Growth and colony forma- pH values (Scott et al. 1964).The staining method used tion in each culture was followed in time. Cell counts by Crayton (1982) was adapted for use in seawater were performed according to van Boekel & Veldhuis medium. To obtain the different pH values in the reac- (1990). In all experiments cultures were examined tion mixture 0.1 % (w/v)alcian blue was made in either regularly for bacterial contamination using fluores- 0.5 N HAc (for pH 2.5) or in 0.5 N HCl (for pH 0.5). The cence microscopy after staining of samples with alcian blue stocks were prepared in a solution contain- Hoechst dye no. 33258 (Paul 1982). No bacterial con- ing the major salts of the seawater medium used for tamination was detected during the experiments. culturing Phaeocystis in order to prevent osmotic Results and discussion. Treatment of Phaeocystis sp. shock of the cells during treatment. For staining of colonies with alcian blue at pH 2.5 resulted in a strong colonies the alcian blue stocks were mixed 5 parts to 2 blue staining of the colony mucus (visible as the grey with concentrated colony suspensions in glass tubes. tint of the colony mucus in Fig. 1B compared with the The mixtures were kept at room temperature for at translucent appearance of the mucus in the control least 15 min. Thereafter they were diluted 10-fold colony in Fig. 1A).At pH 0.5 staining of the mucus was with sealvater in order to make the colonies visible. less intense but still evident (in Fig. 1C mainly visible Colonies were picked out with pipettes, transferred to as the darker colony membrane compared with the microscope chambers and observed with an inverted control). These results indicate that Phaeocystis colony microscope. Control experiments showed that alcian mucus contains both carboxylated and sulfated poly- blue did not bind to living single cells of Phaeocystis, mers. Preliminary results of NMR (nuclear magnetic indicating that staining was specific for mucus. Pieces resonance) analysis of purified mucus from Phaeo- of agar gel (a polymer containing both carboxylated cystis cultures and field samples indicate that the and sulfated groups) that were stained using the same mucus is of complex composition. The NMR spectra of method as for colonies showed a strong blue colour at Phaeocystis mucus show no resemblance to spectra both pH values, indicating that the seawater medium of known algal or other polymers (H. Huizing pers. did not affect the staining reaction. Also, Crayton comm.). This is in accordance with the results (1982) found that addition to the reaction mixture of up of Painter (1983), who found hemiester sulfates and to 0.5 M NaCl did not affect binding of alcian blue to residues of galacturonic acid, together with glucos- polymers at both pH values. amine and a number of saccharides, in extracts of a Since cations were expected to have an important natural Phaeocystis bloom. Polymer production also function in the gelling of colony mucus, the effect of occurs in other members of the class Prymnesio- omission of one or more cations from the medium on phyceae, to which Phaeocystis belongs.
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