Sverre Myklestad

Release of extracellular products by phytoplankton with special emphasis on polysaccharides

Organic substances are released from phytoplankton cells during all phases of growth. Carbohydrates, especially polysaccharides, sometimes comprise 80–90% of the total extracellular release. Nutrient status profoundly affects the amount and composition of the exudate. Severe N- and P-limitation favours the release of carbohydrates that do not contain these elements. Increase in the medium NP ratio above the classical Redfield ratio of 16 increases the production of extracellular polysaccharides in laboratory cultures. The type and amount of polysaccharide excreted and the effects of nutrient limitation are often highly species-specific. The production by different species can vary by a factor of up to 50. The rate of release is best specified either as a relative rate of release (PER, percent of total carbon fixation) or as a specific rate. The absolute rate of release for the diatom Chaetoceros affinis was higher in the exponential than in the stationary phase, whereas the PER was 5 times higher in the stationary phase. The mechanisms for excretion in algae are still largely unknown. Both simple and facilitated diffusion may be involved for small molecules and more complex mechanisms for macromolecules. The chemical structures of the extracellular polysaccharides are often species-specific. For example, the marine diatoms C. affinis and C. curvisetus both produce a polysaccharide containing residues of rhamnose, fucose and galactose, but there are major differences in structure. Accumulation of gelatinous masses due to extracellular release by algae was observed in the Adriatic Sea during the summers of 1988–1991. Several species of diatoms and dinoflagellates are implicated, and the jelly is composed of polysaccharides and/or proteoglycans. Blooms of Phaeocystis in the southern part of the North Sea form gelatinous colonies with localised concentrations of up to 7 mg/1. These contain 55–90% of polysaccharidic mucilage.

Production of carbohydrates by the marine diatom Chaetoceros affinis var. willei (Gran) Hustedt. I. Effect of the concentration of nutrients in the culture medium

Abstract The chemical composition of cells of Chaetoceros affinis var. willei (Gran) Hustedt grown in batch culture was markedly influenced by the concentration of nutrients in the medium. In the logarithmic growth phase the content of cellular glucan was relatively low, but in the stationary phase the glucan content showed a rapid increase which seemed to coincide with the depletion of nitrate from the medium. This led to very pronounced variations in the ratio of protein to carbohydrate. This ratio can easily be determined and seems to be a sensitive and convenient parameter for characterizing the physiological state of the diatom cells. In the stationary phase the alga also produced considerable amounts of an extracellular polysaccharide, particularly when grown in a medium with a high proportion of nitrate to phosphate.

Production of carbohydrates by marine planktonic diatoms. I. Comparison of nine different species in culture

Abstract Nine different species of marine diatoms, all but one isolated from the Trondheimsfjord, were grown under the same chemical and physical conditions in dilution/batch culture. One species was grown in a chemostat. Cells were analysed for total cellular carbohydrate, acid-soluble glucan, cellular protein, extracellular polysaccharide as well as alkali-soluble carbohydrate, chlorophyll a, and phosphorus in some cases. All species showed a general tendency to accumulate soluble glucan in the stationary growth phase, so that the ratio of protein to carbohydrate decreased with time and this ratio was, therefore, considered useful as an indicator of the physiological state of the cells for all the species analysed. Glucan accumulation was observed under conditions of low dilution rates in continuous culture. Marked differences have been observed between different species in the production of glucan and extracellular polysaccharide. For stationary phase cultures (12-day) the ratio of extracellular polysaccharide to cellular carbohydrate varied from 0.01 in Skeletonema costatum (Grev.) Cleve to 1.25 in Chaetoceros curvisetus Cleve and the content of glucan was 81.0 % and 24.8 % of the organic dry matter, respectively. It may be concluded that the organic composition of the stationary phase cells of different species of marine diatoms grown under identical conditions differ markedly. Cells harvested in the exponential growth phase were more alike in their chemical composition. Some possible ecological implications of the results are discussed.

A sensitive and rapid method for analysis of dissolved mono- and polysaccharides in seawater

A spectrophotometric method is described for the determination of dissolved mono- and polysaccharides in seawater. It is based upon the well known alkaline ferricyanide reaction, but uses the reagent 2,4,6-tripyridyl-s-triazine (TPTZ) to give a strongly colored complex with the reduced iron. The method has been tested on model carbohydrates and other compounds, and also on natural samples of coastal and oceanic waters. Total carbohydrate content of the natural samples ranged from 5.2 to 25.1 μmol glucose-Cl−1. The coefficient of variation was typically below 6% for values near 17 μmol Cl−1 and approximately 10% for values near 3.5 μmol Cl−1.

Studies on the phytoplankton ecology of the Trondheimsfjord. I. The chemical composition of phytoplankton populations

Abstract Samples of phytoplankton populations from the Trondheimsfjord, collected in 1970 and the first five months of 1971, have been analysed for carbohydrate, protein, lipid, and phosphorus. Lipid was in all cases less than 10% of the organic dry matter. The N P ratio was remarkably constant, but the ratio protein/carbohydrate varied between wide limits. For samples consisting mainly of dinoflagellates, the protein/carbohydrate ratio was always low, due to a large amount of insoluble polysaccharides, probably corresponding to material in the cell walls. For diatoms, the carbohydrates may conveniently be divided into three fractions: 1) an acidsoluble glucan of the β-1, 3-linked type; 2) an alkali-soluble fraction giving a complex mixture of monosaccharides on hydrolysis and, 3) an insoluble glucan. The amounts of acid-soluble glucan varied from 7.7 to 36.5% of organic dry matter and these changes are the main cause of the variation of the protein/carbohydrate ratio of diatom samples. For diatom samples this ratio is a valuable indicator of the physiological state of the population. The variations observed in this study are discussed.

A preliminary study of the use of benthic algae as biological indicators of heavy metal pollution in Sørfjorden, Norway

Concentrations of zinc, copper, lead and cadmium in seawater and in some benthic algae from Sorfjorden are reported. The concentration factors for Fucus vesiculosus and Ascophyllum nodosum are in good agreement with values given by other authors, even though the zinc concentrations are about ten times larger than those found in most previous work. The concentration factors obtained for zinc and cadmium are essentially the same in the whole area; however, for copper and lead the values decrease towards the mouth of the fjord.

Production of carbohydrates by the marine diatom Chaetoceros affinis var. willei (Gran) Hustedt. II. Preliminary investigation of the extracellular polysaccharide

Abstract The isolation of an extracellular polysaccharide from cultures of Chaetoceros affinis var. willei (Gran) Hustedt is described. The polysaccharide behaved as a homogeneous, polyanionic compound in free-boundary electrophoresis at both pH 2 and 7. It contained sulphur, presumably as sulphate half ester groups (8.7% of SO 2 Na), and the following monosaccharides were tentatively identified: rhamnose, fucose, arabinose, and galactose, with the two former constituting 63% of the polysaccharide preparation. The main cellular polysaccharide was a glucan and could be extracted from the cells by dilute acid. The remaining material gave, after hydrolysis, a complex mixture of monosaccharides with rhamnose as the major component. It is concluded that the extracellular polysaccharide is probably excreted from healthy cells.

Dissolved Organic Carbon from Phytoplankton

Dissolved organic carbon in the sea makes up some 700 gigatons according to conservative estimates. Even more important is the labile part, which constitutes more than 50% of newly produced DOC. The source of DOC production in the sea is photosynthetic algae. The major constituent in rapidly growing phytoplankton cells is protein, with a content up to about 50% of organic dry matter; the content of carbohydrate and lipid being somewhat lower. When growth is slow and in stationary phases, the cellular composition will change very markedly and often carbohydrate will be the most prominent constituent.

Studies on the phytoplankton ecology of the trondheimsfjord. III. Dynamics of phytoplankton blooms in relation to environmental factors, bioassay experiments and parameters for the physiological state of the populations

Abstract Quantitative phytoplankton sampling was carried out at weekly intervals at one station in the central part of the Trondheimsfjord and at irregular intervals at one station near Trondheim Harbour during March–October, 1970 and 1971. Various developmental stages of diatom blooms have been observed, which have been related to variations in freshwater discharge, hydrography, nutrients (nitrate, orthophosphate, and reactive silicate in sea water and river water), light, the results of bioassay experiments, parameters for the physiological state of natural phytoplankton populations, and to data on phytoplankton and hydrography collected during 1963–1969. Two spring blooms of diatoms are persistent from year to year in the area. The first one starts in March, triggered by an increase in the incident radiation and culminates in early April. It develops analogously to a batch culture and is nourished mainly by nutrients accumulated during the winter. The second takes place in brackish waters during May–June concomitant with floods in rivers. The magnitude of its populations corresponds to discharge maxima unless disturbed by hydrographical irregularities and heavy grazing by Calanus finmarchicus (Gunnerus). This bloom is analogous to a continuous culture and is nourished by nutrients in entrainment water and to a lesser extent by those in the river water. Furthermore, the unpredictable development of diatom blooms in the autumn seems to follow peaks in the discharge unless prevented by too low salinity and poor incident light. In autumns of little discharge and with turbulence in the upper 5–10 m dinoflagellates predominate. In high salinity waters nitrogen seems generally more limiting than phosphorus for phytoplankton growth The N/P atomic ratio of such waters with no phytoplankton growth was 10–12 in contrast to 13–18 in the phytoplankton. Due to the high N/P ratio of 40–50 in river water, phosphorus was more limiting than nitrogen in some brackish waters. On two occasions trace metals seemed to be the most limiting.

Monthly profiles of DOC, mono- and polysaccharides at two locations in the Trondheimsfjord (Norway) during two years

Abstract Profiles of water samples were collected monthly during 2 years at two locations from the Trondheimsfjord on the Norwegian coast. Concentrations of dissolved organic carbon (DOC) were measured with high temperature catalytic oxidation, and carbohydrates were measured using the 2,4,6-tripyridyl- s -triazine (TPTZ) method. The concentration of DOC below 50 m depth was very similar at both stations and averaged 80 μM C. At the summer maximum, the concentration of DOC in the surface water was 2.3–2.7 times higher than the water below the photic zone at the Inner station, and two times higher at the Central station in the fjord. During autumn the DOC concentration decreased in the surface layer at both locations from mid August until the winter minimum was reached in February–March. Average values of total dissolved carbohydrates at different depths varied from 11–33 μM C, and total carbohydrate-C averaged 19–21% of the DOC in the surface samples, and 15–16% of the DOC in the deeper samples, which indicate that carbohydrates are a significant constituent of DOC in the Trondheimsfjord. Dissolved carbohydrates accumulated in the productive season at both stations and maximum concentrations appeared during spring and summer. The level of monosaccharides was fairly constant at approximately 5–6 μM C below the euphotic zone, whereas polysaccharides showed more varying concentrations at all depths. Carbohydrates and DOC correlated positively.