Optimal assay conditions for aspartate transcarbamylase (ATCase) activity in mesozooplankton

International audienceThe optimal conditions for storing and assaying mesozooplankton aspartate transcarbamylase (ATCase) (EC 2.1.3.1.) are denned in order to enable the specific activity of this enzyme to be used as an index of mesozooplankton productivity. ATCase activity was found to be stable for more than 2 weeks when stored in liquid nitrogen, with a negligible loss of activity, whatever the state of frozen material (cell-free homogenate or whole cells). As an alternative,-20°C,-90°C freezers and dry ice (-80°C) can give more flexibility for storing and transporting material, provided initial freezing has been carried out at-196°C. ATCase specific activity was stable over a large range of total protein concentrations (2-55 mg ml"') for most mesozooplankton samples, and the enzyme activity was linear between 0.8 and 80 nmol carbamyl aspartate min~' mg~' of total protein. The optimal incubation time (30 min), temperature (35°C) and pH range (9-9.5) for assay of ATCase were similar for Calanus helgolandicus (Crustacea: Copepoda) (G.O.Sars) and three mesozooplankton assemblages sampled off Plymouth (UK) in May, August and November. However, the K m values for both substrates were variable: from 3.9 to 79.1 for aspartate, and from 0.83 to 3.46 for carbamyl phosphate. Such variations are likely to be due to the presence of different ATCase catalytic potential, rather than changes in ATCase assemblage

High abundance of diazotrophic picocyanobacteria (below three micron) in a Southwest Pacific coral lagoon

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Sources of seasonal variability in mesozooplankton aspartate transcarbamylase activity in coastal waters off Plymouth UK

International audienceSome investigators have proposed aspartate transcarbamylase (ATCase) activity as an overall index of mesozooplankton productivity. However, seasonal changes in mesozooplankton species composition have never been investigated as a possible source of variation in ATCase activity. In this study, we investigate mesozooplankton composition in terms of (i) developmental stages, (ii) species and developmental stages body mass and (iii) species composition, and their relationship to ATCase activity. In controlled laboratory conditions, ATCase activity variability was closely related to changes in somatic growth rate of the copepod Calanus helgolandicus, but was not related to changes in nucleic acid concentrations. It can be argued, however, that the activity of this enzyme is partially involved in copepod somatic productivity, and should be a good index of embryogenesis. In addition, changes in ATCase activity were not significantly influenced by variability in mesozooplankton biomass, when investigated both on inter-and intraspecific levels. Finally, when a complete seasonal cycle was investigated at a fixed station off Plymouth (English Channel), ATCase activity was not correlated with the abundance of any mesozooplankton species apart from copepodites and adult C.helgolandicus. Furthermore, ATCase activity measured both on mesozooplankton and female C.helgolandicus was significantly correlated (R 2 = 0.72, n = 33, P < 0.001) throughout the year, apart from April. At that particular time of the year, ATCase activity was in phase with the peak of abundance of copepod eggs and nauplii. It is suggested that mesozooplankton peaks of ATCase activity reflect two periods in the life history of the copepod: embryogenesis and terminal moult. We propose further experiments to test this hypothesis and to promote the development of molecular biomarkers in order to characterize specific zooplankton metabolic processes

ATCase activity RNA : DNA ratio gonad development stage and egg production in the female copepod Calanus helgolandicus

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Application of fluorescent in situ hybridization coupled with tyramide signal amplification (FISH-TSA) to assess eukaryotic picoplankton composition

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