We studied the development and decline of the 1990 phytoplankton spring bloom in the Marsdiep area of the North Sea (The Netherlands) with emphasis on the cause of the decline of the Phaeocystis bloom, the role of microbial organisms and the utilization of organic material produced by the algae. At the top of the bloom Phaeocystis was nitrogen limited. The bloom declined through cell lysis. Sinking of colonies and grazing were found to be relatively unimportant as loss factors. Biomass in the microbial foodweb (bacteria and protozooplankton) remained low during the bloom but increased sharply as the bloom started to decline indicating that organic carbon released by the phytoplankton was rapidly utilized in the lnlcrobial foodweb Results suggest that dissolved organic carbon produced by phytoplankton through excretion and lysis was the main source of carbon for the microbial foodweb includ~ng copepods.
In the marine environment, production of dimethylsulfide (DMS) from dissolved dimethylsulfoniopropionate (DMSP,) -an algal osmolyte -is thought to occur mainly through bacterial activity. We have investigated the possibility that phytoplankton cells convert DMSP, into DMS, using axenic batch cultures of Phaeocystis sp. at different growth stages. DMSP, added to the medium was converted enzymatically to DMS by Phaeocystis sp. A culture in the exponential growth phase displayed Michaelis-Menten type kinetics for DMSP, conversion, yielding a n apparent K,,, value for DMSPd of 11.7 f 3.1 FM and a V,,,,, value of 3.05 f 0.48 nmol DMS produced min-' (106 cells)-' DMSP, conversion rates declined during the transition from exponential to stationary growth phase, at least partly due to a diminished overall affinity of the enzyme system(s) involved in DMSP conversion. No evidence was obtained for accumulation of inhibiting substances in the medium. Intracellular DMSP concentrations in Phaeocystis sp. batch cultures increased from 71 mM in exponential-phase cells to ca 150 mM in stationary-phase cells. DMS and DMSP, concentrations in the culture remained very low during the exponential growth phase. DMS production started in early stationary phase. In a senescent culture DMSPd appeared when cell numbers started to decline. DMSP production in this culture continued even when cell numbers declined. In completely lysed batch cultures some 25% of total DMSP remained as DMSPd. The results indicate that Phaeocystis sp. may contribute significantly to DMS production from DMSP, during bloom situations in the field.
Between 30 March and 1 1 May 1990, total copepod abundance and the abundance. b~o m a s s and gut fluorescence of Temora longlcornls were determined and related to the abundance and succession of phytoplankton development in a Dutch tidal inlet Gut plgment values were h~g h e s t In females and lowest In young copepodites, but weight-spec~f~c pigment concentrations were about slrnilar Pigment levels measured In the guts were relatively high at the beginning and end of the penod of investigation when diatoms d o m~n a t e d the phytoplankton community, dnd low d u n n g the Phaeocystls d o m~n a t e d period, when ambient chlorophyll concentrations were h~g h e s t For the latter period, calculated ingestion rates In T longlcornls were low and estimated dally consumption amounted to less than 1 % of the phytoplankton standing stock, s u g g e s w g a negl~gible grazlng Impact on the development of the Phaeoc)/stls bloom In splte of the low grazrng on phytoplankton. T long]-corms biomass increased by one older of magnitude The discrepancy between low giazlng pressuie and copepod development is explained by assumlng that T long~cornls scvltched to heterotrophic food a bloom of ciliates present during the Phaeocystls dominated period
The composition and properties of Phaeocystis colony mucus are st~ll largely obscure. In this study some components of the mucus were i d e n t~f~e d using a specific staining technique and the role of Ca2* and other cations as binding agent was investigated. In addition, the effect of Ca2' concentration on colony formation in batch cultures was studied. Colonies of Phaeocystis sp. were stained with alcian blue at 2 different pH values. This revealed that the colony mucus contained both carboxylated and sulfated polymers. Incubation of colonies in medium lacking one or more cations showed that calcium and magnesium ions were essential for the gelling of colony mucus, while potasslum ions had no influence. The percentage colony cells formed by Phaeocystis in batch cultures was reduced in medium with calcium concentrations below 2.5 mm01 I-'. No colonies were formed in medium with calcium concentrations below 1.5 mm01 1.' Growth rate was not dependent on calcium concentration. It is suggested that under natural conditions Phaeocystis colony firmness and morphology might depend on the composition of mucus polymers.In the marine environment mucus production is known to occur in bacteria (Decho 1990), macroalgae (Boney 1981) and several groups of microalgae such as diatoms (Decho 1990), green algae (Crayton 1982) and the Prymnesiophyceae (Painter 1983). The composition of mucus produced by these groups is often rather complex, consisting of heteropolymeric chains containing a wide variety of simple sugars, aminosugars, uronic acids, sulfated or phosphated sugars, amino acids, etc. The gelling capacity of mucus depends on the binding of negatively charged groups in the molecule (mostly carboxyl groups) with cations (mostly Ca2+). In this way ionic bridges are formed between polymer strands. The number of ionic bridges formed in a polymer depends on the number of anionic groups and the steric arrangement of these groups in the molecule (Kohn et al. 1968). Colony-forming algae such as the Volvocaceae produce mucus with large amounts of carboxyl and sulfate groups (Crayton 1982).The alga Phaeocystis sp. is an important component of the phytoplankton of several marine ecosystems, such as the North Sea and the Arctic and Antarctic oceans (Barnard et al. 1984, Cadee & Hegeman 1986, Palrnisano et al. 1986, Lancelot et al. 1987, Gibson et al. 1990, Wassmann et al. 1990. Phaeocystis forms colonies consisting of mucus in which cells are randomly distributed. The colonies are spherical or elongated and reach sizes of up to 5 mm in diameter, containing over 10 000 cells (Rouseau et al. 1990). During bloom situations in the North Sea, when Phaeocystis cell number often exceeds Mar. Ecol. Prog. Ser. 87: 301-305, 1992 Materials and methods. All experiments were performed with axenic Phaeocystis sp. (strain K) isolated from the Dutch Wadden Sea. This strain formed globosa-type colonies (Jahnke 1989). Phaeocystis was grown in 1 1 serum bottles incubated on a rolling device at 10 "C and a light intensity of 40 pE m-' S-' ...
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