Abstract
The accessory pigment alloxanthin is a well-known taxonomic marker for Crypto- phyceae in natural seawater. The use of alloxanthin to estimate in situ zooplankton grazing and selectivity on Cryptophyceae was studied using 2 dominant copepods of the southern North Sea, Temora longicornis and Centropages hamatus. High-performance liquid chromatography (HPLC) analyses of water samples and of freshly caught copepods, starvation experiments, and feeding experiments (shipboard bottle-incubations) were carried out. Although alloxanthin was always detected in copepod extracts, (1) no correlation was found with alloxanthin in seawater, and (2) no significant grazing on Cryptophyceae was observed. Results of our gut-evacuation experiments showed that at least 78% of the initial alloxanthin content of T. longicornis and C. hamatus remained after 90 min, whereas fucoxanthin and chloropigments decreased rapidly with time. Alloxanthin and astaxanthin esters were the only pigments remaining in the body of T. longicornis after several hours' starvation. It is concluded that most of the detected alloxanthin did not originate from the gut but from the body tissues. Our results suggest that alloxanthin is not suitable as a biomarker for quantitative or qualitative estimates of copepod grazing on Cryptophyceae.
Highlights
In the North Sea, phytoplankton spring blooms consist mainly of Bacillariophyceae and Haptophyceae (Reid et al 1990)
The alloxanthin content was generally higher in Temora longicornis than in Centropages hamatus (Mann-Whitney U-test: U = 45, n1 = 47, n2 = 22, p < 0.001) (Fig. 4)
Temora longicornis and 23 with Centropages hamatus), significant ingestion rates were recorded for dia
Summary
In the North Sea, phytoplankton spring blooms consist mainly of Bacillariophyceae and Haptophyceae (Reid et al 1990). The main reasons are (1) lack of morphologic distinctive features of these algae and (2) damage caused during fixation that renders identification by classical microscopic methods difficult To overcome these problems, a chemotaxonomic approach based on detection of taxon-specific pigments by high-performance liquid chromatography (HPLC) has been used since the 1980s and is commonly employed to distinguish the main algae classes (see review in Jeffrey et al 1999). Using this technique, the abundance and sometimes dominance of Cryptophyceae in natural seawater samples of the North Sea has been documented by the presence of alloxanthin pigment, a specific cryptomonad carotenoid (Gieskes & Kraay 1983, 1984, Brunet et al 1996, Breton et al 2000). The use of carotenoid pigments to study zooplankton grazing and selectivity has been challenged during the last decade because of their possible degradation into unidentified pigments and/or colourless products during
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