Gannabathula Sree Vani

Nitrogen metabolism of the antarctic bivalve Laternula elliptica (king & broderip) and its potential use as biomarker

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.

Interactions of temperature, salinity and diesel oil on antioxidant defense enzymes of the limpet Nacella concinna

The intertidal and subtidal environments in the Antarctic Peninsula are vulnerable to pollutants, such as diesel oil, a commonly used fuel. Nacella concinna is capable of accumulating polycyclic aromatic hydrocarbons and is a potential biomonitor of diesel oil contamination. This work investigates the interaction of diesel oil, temperature and salinity on the activity of antioxidants enzymes defense of the gills, foot muscle and digestive glands. Upregulation of superoxide dismutase occurred in the three tissues by warming. The foot muscle catalase and the gill glutathione reductase were upregulated only by diesel. The inability to upregulate catalase and glutathione S-transferase in the digestive gland, as well as the increase of lipoperoxidation, suggested that this gland is more susceptible to the deleterious effects from oxidative stress.

Evidence of metabolic microevolution of the limpet Nacella concinna to naturally high heavy metal levels in Antarctica.

The gastropod Nacella concinna is the most conspicuous macroinvertebrate of the intertidal zone of the Antarctic Peninsula and adjacent islands. Naturally high levels of copper and cadmium in coastal marine ecosystems are accumulated in N. concinna tissues. We aimed to study the effects of metal cations on N. concinna arginase in the context of possible adaptive microevolution. Gills and muscle had the highest argininolytic activity, which was concentrated in the cytosol in both tissues. Gills had the highest levels of arginase and may be involved in the systemic control of l-arginine levels. The relatively high argininolytic activity of the N. concinna muscular foot, with KM=25.3±3.4mmolL-1, may be involved in the control of l-arginine levels during phosphagen breakdown. N. concinna arginases showed the following preferences for metal cations: Ni2+>Mn2+>Co2+>Cu2+ in muscle and Mn2+>Cu2+ in gills. Cu2+ activation is a unique characteristic of N. concinna arginases, as copper is a potent arginase inhibitor. Cu2+ partly neutralized N. concinna arginase inhibition by Cd2+, worked synergistically in muscle arginase activation by Co2+ and neutralized muscle arginase activation by Ni2+. Mn2+ was able to activate muscle arginase in the presence of Fe3+ and Pb2+. The selection of arginases that are activated by Cu2+ and resistant to inhibition by Cd2+ in the presence of Cu2+ over evolutionary timescales may have favored N. concinna occupation of copper- and cadmium-rich niches.