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

Some investigators have proposed the measurement of aspartate transcarbamylase (ATCase) activity as a suitable technique for estimating zooplankton productivity. However, this technique has never been comprehensively evaluated under controlled laboratory conditions nor compared extensively with other productivity techniques in the field. This paper describes such an evaluation, using the reproduction of the calanoid copepod Calanus helgolandicus as an index of germinal growth, an important aspect of zooplankton productivity. ATCase activity and RNA:DNA ratio both increased significantly with increasing gonad maturation. In addition, ATCase activity in females responded immediately to changing food conditions, whilst changes in egg production lagged behind by ≃24 h. Shifting the ATCase data by 24 h revealed a significant and positive correlation of ATCase activity with egg production. However, in well-fed females, ATCase activity assayed at constant temperature was apparently independent of environmental temperature conditions, whilst egg production was temperature-dependent. In the field, ATCase activity was significantly correlated with egg production only in autumn and winter; when a complete seasonal cycle was considered, no correlation was found between the two measurements. These findings suggest that ATCase is involved partially in either germinal growth and/or in other biosynthetic processes of female C. helgolandicus; thus, ATCase activity does not reflect copepod egg-production with any certainty. Received: 4 June 1998 / 3 June 1999     

Isolated bacteriocyte cell suspensions from the hydrothermal-vent tubeworm Riftia pachyptila,a potent tool for cellular physiology in a chemoautotrophic symbiosis

The hydrothermal-vent tubeworm Riftia pachyptila relies entirely on its intracellular chemoautotrophic symbionts to sustain its metabolism. The host must therefore provide them with inorganic metabolites, including carbon. This study describes a tool for studying cell processes occurring in a bacteria-containing cell by the dissociation of trophosome cell types. The physiological assays performed on cell preparations focused on carbon dioxide conversion and transport processes. Trophosome tissue was mechanically dissociated, resulting in cell suspensions enriched in small (7-20 µm) bacteriocytes, which were viable for several hours. In addition, medium-term cell cultures were also attempted. As a start to the understanding of the CO₂ metabolism of these cells, we were interested in evidence of carbonic anhydrase (CA) isoforms, ATPases and chloride exchangers. Variations in intracellular and extracellular pH, and in intracellular concentrations of sodium, potassium and chloride, were followed after addition of selective inhibitors. The data presented here suggest the occurrence of potential cytosolic and membrane-associated carbonic anhydrase isoforms in the bacteriocytes, proton-driven sodium-ATPases and a well represented anion transporter exchanging intracellular chloride against extracellular anions.