Is the GDH/RNH4+ ratio in the mesozooplankton constant through different oceanic systems?

ZOOPLANKTON METABOLISM: ENZYMATIC AND PHISIOLOGICAL RATES I Institute of Oceanography and Global Change, Biological Oceanography Group, University of Las Palmas de Gran Canaria, Canary Islands, Spain. E-mail address: [email protected]c.es (I) Nitrogen availability is frequently limited in ocean ecosystems. (II) Phytoplankton can use inorganic fixed-nitrogen compounds (NH4+, NO3-, NO2-), as well as some organic compounds (urea, free amino acids). (III) Other pathways of N cycling (Denitrification, Anammox and DNRA) can occur in OMZ and sediments. (Dugdale and Goering, 1967; Zehr and Ward, 2002; Brandes et al., 2007; Francis et al., 2007; Yool et al., 2007). Zehr and Kudela (2011) INTRODUCTION Classical tenets of Nitrogen Cycle (I) NH4+ is an intermeditate product in decomposition of organic matter, which constitutes the most reduced form of N. (II) NH4+ sustains a global average of 80 % of the authotroph’s requeriments (Harrison, 1992), with the mesozooplankton responsible for 12 – 23 % (Hernández-León, 2008). Zehr and Kudela (2011) INTRODUCTION Importance of NH4+ in marine systems RECYCLYING EFFICIENCY Nutritional History (Miller and Roman, 2008; Saba et al., 2009) Temperature (Ikeda, 1985) Trophic Interactions (Glibert, 1998) WATER BOTTLE–INCUBATIONS ENZYMATIC ASSAYS (GDH) Direct measurement. High data acquisition rate. Low data acquisition rate. Measurement of potential NH4+ excretion. Complicated by organism manipulation, Variability in the GDH/RNH4+ ratio. overcrowding and starvation. (Mulling et al., 1975; Ikeda and Skjoldal, 1980; Bidigare, 1983) (Bidigare and King, 1981) INTRODUCTION How to determine NH4+ excretion in zooplankton? Vmax (I) GDH is found in high levels in planktonic crustaceans (Regnault, 1987). Its role in amino acids catabolism agues for its control over a great proportion of NH4+ excretion. Modified from Yuen and Chiew (2010) INTRODUCTION Biochemistry of NH4+ in marine zooplankton (II) Good correlation with NH4+ excretion in several marine zooplankters. INTRODUCTION Biochemistry of NH4+ in marine zooplankton Park et al. (1986) Table I. Correlation coefficients between GDH and RNH4+ calculated in different works. Some factors could affect the relationship between GDH activity and NH4+ excretion!! – Physiology and biochemistry should share scaling exponent (Berges et al., 1993). – Changes in nutritional state lead to increase the variability (Park, 1986; Hernández-León and Torres, 1997; Fernández-Urruzola et al., 2011). Y = a · Wb (Kleiber, 1961) INTRODUCTION Variability in the GDH to NH4+ excretion ratio MATERIAL AND METHODS Location and Sampling MALASPINA 2010: leg 7 June 2011 CAMVALEX I April 2011 SUCCESSION September 2011 MALASPINA 2010: legs 3 - 4 February 2011 MATERIAL AND METHODS Experimental design 150 m sampling SIZE FRACTIONATION Data analysis Laboratory work at institute PHYSIOLOGYCAL ANALYSES ON BOARD (Holmes et al., 1999) Storage in criovials at -80 oC 100 – 500 µm 500 – 1000 µm > 1000 µm GDH Activity (Bidigare and King, 1981) Intracelullar Substrates (Glutamate and NAD+) Protein mass (Lowry, 1951) - Rapid fall of nitrogen release after depletion of the food source (Mayzaud, 1976; Ikeda and Skjodal, 1980). - GDH does not vary significantly with environmental changes, proving its constitutive nature. RESULTS AND DISCUSSION GDH activity to NH4+ excretion relationship RESULTS AND DISCUSSION Intracellular substrate levels as a key factor RNH4+ NAD+ NADP+ Aguiar-González et al. (2012) Roy and Packard (1998) found a decrease in intracellular substrates concentrations with food source limitation. As a concequence, the actual enzimatic rates would also decrease. Time (h) Substrate (µmols) FOOD LIMITATION An enzyme kinetic-based model should predict the in vivo RNH4+ on natural samples of zooplankton from different productivity areas. RESULTS AND DISCUSSION Future work Packard and Gómez (2008) (I) GDH/RNH4+ is not constant in all the marine ecosystems sampled so far. However, the ratios argue that the zooplankton communities are in a healthy physiological state. (II) Starvation causes NH4+ excretion and GDH activity to diverge more than does biomass. (III) Intracellular substrate levels should explain the variability between the physiological and enzimatic rates. As a result, a kinetic-based model would predict in vivo NH4+ excretion rates better than other theories based in biomass, such as the MTE. SUMMARY This research is framed in the EXZOME project (CTM 2008 – 01616/MAR), which is funded by the extinct Spanish Science and Education Ministry. I. Fernandez-Urruzola receives finantial support from the Formation and Perfection of the Researcher Personal Program from the Basque Government. I also thank to MALASPINA 2010 (CSD-20080077) and SUCCESSION projects for inviting me to participate in their cruises. AWKNOLEDGEMENTS ZOOPLANKTON METABOLISM: ENZYMATIC AND PHISIOLOGICAL RATES I Institute of Oceanography and Global Change, Biological Oceanography Group, University of Las Palmas de Gran Canaria, Canary Islands, Spain. E-mail address: [email protected]c.es