NITROGEN METABOLISM OF THE ANTARCTIC BIVALVE LATERNULA ELLIPTICA (KING & BRODERIP) AND ITS POTENTIAL USE AS BIOMARKER

Abstract

The Antarctic marine environment is characterized by the extreme seasonality of the primary production in the water column and the low but stable temperatures. Both are considered the main factors in the adaptative evolution of Antarctic ectothermic organisms. Studies about physiological and biochemical processes of the cold-adapted species revealed the presence of antifreeze glycoproteins in the biological fluids and cold-adapted proteins. The low and stable temperatures have resulted in the appearence of enzymes with high catalytic efficiency and the absence of the thermal stress proteins in some Antarctic fishes. The austral winter promotes a seasonal food shortage, submitting the benthic ectotherms to long periods of starvation. This is particularly true for the organisms that depend on phytoplankton as their primary source of food. The Antarctic marine environment also presents areas of high copper concentrations on the sediment surface as well as cadmium in the water column. The bivalve Laternula elliptica, a circumpolar species, has been proposed as bioindicator for long term monitoring of heavy metals in the shallow waters of Antarctica due to its capacity to accumulate metals, especially cadmium and zinc. Like other Antarctic ectothermic organisms, L. elliptica changes its metabolism from aerobic to anaerobic as a function of temperature, being 6 ºC critical and 9 ºC lethal. This bivalve also shows a marked seasonality in its metabolism, with low oxygen consumption in winter as compared to summer. It is speculated that it enters into a dormancy state during the austral winter as it apparently retracts its siphons below the sediment surface. The apparent supression of the water pumping by the siphons during winter forces L. elliptica to mobilize its energy reserves, using the siphon proteins as its principal source of energy (ratio of oxygen consumption/excreted nitrogen = 3.0). Even during summer, when the high food supply stimulates the bivalve growth, the metabolism is mainly protein based. (O:N ratio = 16). The excretory nitrogen metabolism of this bivalve is typically ammoniotelic, characterized by the excretion of almost 90% of nitrogen in the form of ammonia and 10% as urea. Probably, the urea excreted arises from the hydrolysis of the proteic aminoacid L-arginine by arginase in order to maintain the tissue levels of that aminoacid. In such case, the presence of this enzyme in the kidney tissues may be related to the physiological constraints caused by the retraction of the siphons and the requirements for the excretion of this nitrogen compound during the austral winter. Studies with the renal arginase of this bivalve showed a high metabolic tolerance to the metallic cations Cu, Zn, Fe and Cd, when compared to the arginase behavior of other bivalves such as Dreissena polymorpha. The present work covers the life history of this bivalve, its potential use as a biomarker and its adaptations to the extreme marine environment conditions in Antarctica.