The algal storage glucan laminarin is one of the most abundant carbon sources for marine prokaryotes. Its degradation was investigated in bacteria isolated during and after a spring phytoplankton bloom in the coastal North Sea. On average, 13% of prokaryotes detected by epifluorescence counts were able to grow in Most Probable Number dilution series on laminarin as sole carbon source. Several bacterial strains were isolated from different dilutions, and phylogenetic characterization revealed that they belonged 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 sole carbon source. At least two types of activity were detected upon degradation of laminarin: release of glucose, and release of glucans larger than glucose. The expression of laminarinase activity was dependent on the presence of the substrate, and was repressed by the presence of glucose. In addition, low levels of activity were expressed under starvation conditions. Laminarinase enzymes showed minimal activity on substrates with similar glucosidic bonds to those of laminarin, but different sizes and secondary and/or tertiary structures. The characteristics found in these enzyme systems may help to elucidate factors hampering rapid carbohydrate degradation by prokaryotes.

The ubiquity and high productivity associated with blooms of colonial Phaeocystis makes it an important contributor to the global carbon cycle. During blooms organic matter that is rich in carbohydrates is produced. We distinguish five different pools of carbohydrates produced by Phaeocystis. Like all plants and algal cells, both solitary and colonial cells produce (1) structural carbohydrates, (hetero) polysaccharides that are mainly part of the cell wall, (2) mono-and oligosaccharides, which are present as intermediates in the synthesis and catabolism of cell components, and (3) intracellular storage glucan. Colonial cells of Phaeocystis excrete (4) mucopolysaccharides, heteropolysaccharides that are the main constituent of the mucous colony matrix and (5) dissolved organic matter (DOM) rich in carbohydrates, which is mainly excreted by colonial cells. In this review the characteristics of these pools are discussed and quantitative data are summarized. During the exponential growth phase, the ratio of carbohydrate-carbon (C) to particulate organic carbon (POC) is approximately 0.1. When nutrients are limited, Phaeocystis blooms reach a stationary growth phase, during which excess energy is stored as carbohydrates. This so-called overflow metabolism increases the ratio of carbohydrate-C to POC to 0.4-0.6 during the stationary phase, leading to an increase in the C/N and C/P ratios of Phaeocystis organic matter. Overflow metabolism can be channeled towards both glucan and mucopolysaccharides. Summarizing the available data reveals that during the stationary phase of a bloom glucan contributes 0-51% to POC, whereas mucopolysaccharides contribute 5-60%. At the end of a bloom, lysis of Phaeocystis cells and deterioration of colonies leads to a massive release of DOM rich in glucan and mucopolysaccharides. Laboratory studies have revealed that this organic matter is potentially readily degradable by heterotrophic bacteria. However, observations in the field of accumulation of DOM and foam indicate that microbial degradation is hampered. The high C/N and C/P ratios of Phaeocystis organic matter may lead to nutrient limitation of microbial degradation, thereby prolonging degradation times. Over time polysaccharides tend to self-assemble into hydrogels. 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

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Growth, mortality and productivity of the hard clam Eurhomalea exalbida from Ushuaia Bay, Beagle Channel, were investigated. The parameters of the von Bertalanffy growth function were estimated to be H l = 74 mm, K = 0.18 y À 1 , t 0 = 0.15 y. Maximum individual production amounted to 2.74 g shell-free wet mass (SFWM) at 49.5 mm shell height. Animals between 40 mm and 70 mm shell height contributed most to overall population somatic production P of 134 g SFWM m À 2 y À 1 . Mean annual biomass B amounted to 1123 g SFWM m À 2 y À 1 . Annual P/B ratio and mortality rate Z were estimated to be 0.12 y À 1 and 0.14 y À 1 , respectively. Slow growth and low turnover make this population less suitable for sustainable commercial exploitation. D

ABSTRACT. Regional and temporal variation in the composition of water-soluble carbohydrates from Phaeocystis colonies sampled in the southern North Sea was small during spring 1994, except for a high variability in the contribution of glucose. Glucose is universally present in storage products of microalgae; the relative constancy of the carbohydrate pattern of the other rnonosaccharides suggests that these are part of the more refractory colony mucus. In all Phaeocystis samples arabinose dominated. followed by xylose (Belgian coast) or galactose and mannose (Dutch coast). Rhamnose, glucuronate and 0-methylated sugars were present in lower amounts. The latter, always present in samples containing Phaeocystis, may be typical for North Sea strains. The sugar patterns we report here differ from those presented in the literature concerning Phaeocystis-derived material, and also from the sugar fingerprint in the preceding diatom bloom. The Phaeocystis mucus apparently behaves as particulate matter since it was retained on filters of over 1 pm. This characteristic together with its refractory nature, typical of 'transparent exopolymer particles' (TEPs), must have consequences for the heterotrophic microbial community in terms of adherence and substrate availability.