Factors affecting the respiration/ETS ratio in marine zooplankton

Respiration (/? ) and electron transport system (ETS) activity were studied in marine zooplankton of different size classes (100-200,200-500 and 500-1000 p.m) in order to determine the factors affecting the variability of the relationship between the two parameters. The R/ETS ratio varied within the range of values observed in the literature. The variability was more pronounced in the smaller size fraction. We found this ratio and primary production to vary in parallel during the development of the late winter bloom in the Canary Island waters. Higher ratios were observed to coincide with higher values of primary production. When the data from different oceanic environments were pooled, we found the ratios to be inversely related to temperature. The R/ETS ratio seemed to reflect the scope of metabolic activity in the zooplankton. Using a flow-through system, we observed short-term respiration rates and ETS activities to be related to the activity level of the organisms. This suggests that variability in the relationship between both measurements could be due to the inadequacy of assessing respiration rates using the classical balance method and the problem of measuring enzymes at VamK. Introduction The assessment of respiration rates in planktonic communities is a prerequisite in estimating energy flow in marine ecosystems. Its measurement in zooplankton provides their minimum energetic requirements. These animals energize the synthesis of new organic compounds by oxidizing the carbohydrates, lipids and proteins in their diet. This rate of oxidation can be calculated from respiration data. However, these measurements are tedious, labour intensive and time consuming. As a result, it is difficult to obtain respiration values at the same data acquisition rate as other modern oceanographic parameters are obtained. This precludes the mesoscale assessment (in both time and space) of oceanic respiration. Partially for this reason, Curl and Sandberg (1961) and Packard (1969) proposed the use of physiologically related enzymatic activities in the cell as an index of respiration. This enzymatic approach was designed to serve as a tool in accessing the vertical distribution of respiration rates, and as a tool for mapping the ocean. In this fashion, the biological effects of mesoscale physical structures could be detected. In addition, this approach would circumvent the problems of stress, starvation, crowding and bacterial growth that are associated with incubation-based respiration measurements. However, the variability in the relationship between enzymatic activities and respiration rates has precluded the use of this approach as a standard respiration method. In spite of this, the electron transport system (ETS) activity is commonly used in biological oceanography to assess plankton metabolism. However, in crustaceans and in other phyla, the respiration (R)/ETS ratios are highly variable. Bamstedt (1979) observed a high variability of both respiration and ETS activity throughout the course of different seasons. He found maximum respiration rates and ETS activity in spring, as expected. However, the variations were not always in phase. In general, the respiration and ETS activity were well correlated, but the © Oxford University Press 239 S.Heinindez-Letia and M.Gomez Table I. Values of the respiration/ETS ratio obtained by different authors for copepods and mixed zooplankton. For comparison, all the ETS values were converted to the method of Kenner and Ahmed (1975) according to the ratios given by Christensen and Packard (1979) Organisms R/ETS Reference Calanoides carinatus 0.65 ± 0.19 (n = 13) Packard el al. (1974) Calanus pacificus (NI) 0.59 ± 0.19 (« = 2) King and Packard (1975) Calanus pacificus (Nil) 0.74 ± 0.07 (n = 2) King and Packard (1975) Calanus pacificus (adults) 0.46 ± 0.06 (n = 48) Owens and King (1975) Acartia tonsa 0.47 ± 0.08 (n = 13) Bamstedt (1980) Acartiaaustralis 0.16 ± 0.02 (n = 4) Ikeda and Skjoldal (1980) Calanus helgolandicus Calanus finmarchicus 0.19 ± 0.02 (n = 10) Hirche (1983) Euchaela norvegica 0.77 ± 0.19 (n = 6) Skjoldal el al. (1984) Antarctic zooplankton 0.57 ± 0.26 (n = 12) Ikeda and Hing Fay (1981) Mixed zooplankton 0.50 ± 0.17 (n = 146) King and Packard (1975) Mixed zooplankton 0.38 ± 0.05 (n = 12) Bidigare et al. (1982) Mixed zooplankton 2.34 ± 0.76in = 5) Alcaraz and Packard (1989) Copepods 0.71 ± 0.40 (n = 6) King and Packard (1975) Copepods 1.39 ± 0.66 (n = 84) Bamstedt (1979) R/ETS ratio showed considerable variation. This demonstrated the weakness in using a single universal ratio to calculate respiration rates from ETS activity values. No conclusions, however, were drawn about the causes of the variability in the relationship between both measurements. From the literature, the R/ETS ratio in zooplanktonic organisms is not constant and at least 3to 4-fold differences are commonly observed (Table I). Packard (1985) also showed the average values of the R/ETS ratio in bacteria, protozoa, phytoand zooplankton to vary 3to 4-fold. Because of this variability, there has been considerable resistance in accepting enzymatic methods for estimating physiological rates. When the enzymatic activity is measured, we are indirectly measuring the amount of enzyme because the activity is measured under substrate saturating conditions, therefore it is the V^. For a determined enzyme concentration, the physiological rate must behave as a Michaelis-Menten function. The physiological rate is measured 'in vivo' with cells and mitochondria exposed to substrate limitation. If the enzymatic activity is only measured at V,^, a high variability in the R/ETS ratio will be observed. Packard and co-workers expected a R/ETS ratio of —0.5 (see Packard, 1985) because ETS is measured at K^ and the organisms would respire at ~50% of their capacity if they were not under standard or active metabolism (sensu Brett, 1964). This is the so-called Michaelis-Menten assumption, which is to say that the R/ETS ratio could depend on the physiological activity of the organisms. Excluding temperature and size, the respiration rates are affected by the level of feeding and locomotory activity, and the so-called specific dynamic action as the effect of the energy cost of protein synthesis (Ki0rboe etal., 1985; Brown and Cameron, 1991). Thus, the respiration rates of zooplankton must be influenced by conditions of quantity and quality of food in the environment, as was stated by Marshall and Orr (1964) several years ago. In this study, we have used respiration