Phytoplankton dynamics in a subarctic fjord during the under-ice - open water transition.

The spring blooms of phytoplankton play a key role in the functioning of marine ecosystems in the polar regions. A spring bloom in the Subarctic White Sea was observed in order to determine the effect of ice cover on the distribution, composition, and temporal changes of phytoplankton communities. The obtained results clearly show that in the White Sea, as in other freezing Arctic seas, ice melting and ice removal both play an essential role in the onset and development of spring phytoplankton blooms. This facilitates the release of ice algae and ice-pelagic algae into the water, as well as the rapid development of true planktonic taxa within the euphotic zone. One major peak of algal biomass is associated with ice removal while the other is recorded in the early summer. Comparison of our results with earlier data from 1960s to 1990s indicated strong year-to-year variation in terms of ice removal, the onset of the spring bloom, and the abundance and composition of the dominant phytoplankton taxa.

Mitochondrial performance of a continually growing marine bivalve, Mytilus edulis, depends on body size.

Allometric decline of mass-specific metabolic rate with increasing body size in organisms is a well-documented phenomenon. Despite a long history of research, the mechanistic causes of metabolic scaling with body size remain under debate. Some hypotheses suggest that intrinsic factors such as allometry of cellular and mitochondrial metabolism may contribute to the organismal-level metabolic scaling. The aim of our present study was to determine the metabolic allometry at the mitochondrial level using a continually growing marine ectotherm, the mussel Mytilus edulis, as a model. Mussels from a single cohort that considerably differed in body size were selected, implying faster growth in the larger specimens. We determined the body mass-dependent scaling of the mitochondrial proton leak respiration, respiration in the presence of ADP indicative of the oxidative phosphorylation (OXPHOS), and maximum activity of the mitochondrial electron transport system (ETS) and cytochrome c oxidase (COX). Respiration was measured at normal (15°C), and elevated (27°C) temperatures. The results demonstrated a pronounced allometric increase in both proton leak respiration and OXPHOS activity of mussel mitochondria. Mussels with faster growth (larger body size) showed an increase in OXPHOS rate, proton leak respiration rate, and ETS and COX activity (indicating an overall improved mitochondrial performance) and higher respiratory control ratio (indicating better mitochondrial coupling and potentially lower costs of mitochondrial maintenance at the same OXPHOS capacity) compared with slower growing (smaller) individuals. Our data show that the metabolic allometry at the organismal level cannot be directly explained by mitochondrial functioning.

Does the membrane pacemaker theory of metabolism explain the size dependence of metabolic rate in marine mussels?

According to the membrane pacemaker theory of metabolism (MPT), allometric scaling of metabolic rate in animals is determined by the composition of cellular and mitochondrial membranes, which changes with body size in a predictable manner. MPT has been elaborated from interspecific comparisons in mammals. It projects that the degree of unsaturation of membrane phospholipids decreases in larger organisms, thereby lowering ion permeability of the membranes and making cellular, and thus whole-animal metabolism more efficient. Here, we tested the applicability of the MPT to a marine ectotherm, the mussel Mytilus edulis at the intraspecific level. We determined effects of body mass on whole-organism, tissue and cellular oxygen consumption rates, on heart rate, metabolic enzyme activities and on the lipid composition of membranes. In line with allometric patterns, the organismal functions and processes such as heart rate, whole-animal respiration rate and phospholipid contents showed a mass-dependent decline. However, the allometry of tissue and cellular respiration and activity of metabolic enzymes was poor; fatty acid unsaturation of membrane phospholipids of gill tissue was independent of animal size. It is thus conceivable that most of the metabolic allometry observed at the organismal level is determined by systemic functions. These whole-organism patterns may be supported by energy savings associated with growing cell size but not by structural changes in membranes. Overall, the set of processes contributing to metabolic allometry in ectotherms may differ from that operative in mammals and birds, with a reduced involvement of the mechanisms proposed by the MPT.

Genetic diversity and connectivity within Mytilus spp. in the subarctic and Arctic.

Climate changes in the Arctic are predicted to alter distributions of marine species. However, such changes are difficult to quantify because information on present species distribution and the genetic variation within species is lacking or poorly examined. Blue mussels, Mytilus spp., are ecosystem engineers in the coastal zone globally. To improve knowledge of distribution and genetic structure of the Mytilus edulis complex in the Arctic, we analyzed 81 SNPs in 534 Mytilus spp. individuals sampled at 13 sites to provide baseline data for distribution and genetic variation of Mytilus mussels in the European Arctic. Mytilus edulis was the most abundant species found with a clear genetic split between populations in Greenland and the Eastern Atlantic. Surprisingly, analyses revealed the presence of Mytilus trossulus in high Arctic NW Greenland (77°N) and Mytilus galloprovincialis or their hybrids in SW Greenland, Svalbard, and the Pechora Sea. Furthermore, a high degree of hybridization and introgression between species was observed. Our study highlights the importance of distinguishing between congener species, which can display local adaptation and suggests that information on dispersal routes and barriers is essential for accurate predictions of regional susceptibility to range expansions or invasions of boreal species in the Arctic.

Energy homeostasis as an integrative tool for assessing limits of environmental stress tolerance in aquatic invertebrates.

Energy balance is a fundamental requirement of stress adaptation and tolerance. We explore the links between metabolism, energy balance and stress tolerance using aquatic invertebrates as an example and demonstrate that using key parameters of energy balance (aerobic scope for growth, reproduction and activity; tissue energy status; metabolic rate depression; and compensatory onset of anaerobiosis) can assist in integrating the effects of multiple stressors and their interactions and in predicting the whole-organism and population-level consequences of environmental stress. We argue that limitations of both the amount of available energy and the rates of its acquisition and metabolic conversions result in trade-offs between basal maintenance of a stressed organism and energy costs of fitness-related functions such as reproduction, development and growth and can set limit to the tolerance of a broad range of environmental stressors. The degree of stress-induced disturbance of energy balance delineates transition from moderate stress compatible with population persistence (pejus range) to extreme stress where only time-limited existence is possible (pessimum range). It also determines the predominant adaptive strategy of metabolic responses (energy compensation vs. conservation) that allows an organism to survive the disturbance. We propose that energy-related biomarkers can be used to determine the conditions when these metabolic transitions occur and thus predict ecological consequences of stress exposures. Bioenergetic considerations can also provide common denominator for integrating stress responses and predicting tolerance limits under the environmentally realistic scenarios when multiple and often variable stressors act simultaneously on an organism. Determination of bioenergetic sustainability at the organism's level (or lack thereof) has practical implications. It can help identify the habitats and/or conditions where a population can survive (even if at the cost of reduced reproduction and growth) and those that are incapable of supporting viable populations. Such an approach will assist in explaining and predicting the species' distribution limits in the face of the environmental change and informing the conservation efforts and resource management practices.

Aging reduces reproductive success in mussels Mytilus edulis.

The present study was aimed to determine whether reproductive success constantly increases with age in a relatively short-lived invertebrate with continuous growth - the bivalve mollusc Mytilus edulis or there is an age-related decline such as observed in species with finite growth (mammals, insects, nematodes, etc.). We studied the reproductive output and viability of the offspring during early embryogenesis in females of different sizes and ages, and used allometric approaches to correct for the effects of the body size and to discern pure age-specific effects on these reproductive traits. We have also determined contributions of females of different age and size classes to the total larval pool of a population. Both gonadosomatic index and individual fecundity significantly decreased in the course of aging if the size of the animals was accounted for. The proportion of normally developing embryos declined from almost 100% to 60% in females of 2-10-year-old. We suggest that animals with infinite growth and "slow aging", such as molluscs, undergo senescence, the physiological manifestations of which can be masked by a more pronounced effect of continuously increasing size.

Oxidative stress and expression of chaperones in aging mollusks.

The mechanisms of aging are not well understood in animals with continuous growth such as fish, reptiles, amphibians and numerous invertebrates, including mollusks. We studied the effects of age on oxidative stress, cellular defense mechanisms (including two major antioxidant enzymes, superoxide dismutase (SOD) and catalase), and molecular chaperones in two mollusks--eastern oysters Crassostrea virginica and hard clams Mercenaria mercenaria. In order to detect the age-related changes in these parameters, correction for the effects of size was performed where appropriate to account for growth-related dilution. Fluorescent age pigments accumulated with age in both species. Protein carbonyls did not change with age or size indicating that they are not a good marker of aging in mollusks possibly due to the fast turnover and degradation of oxidized proteins in growing tissues. SOD did not show a compensatory increase with aging in either species, while catalase significantly decreased with age. Mitochondrial heat shock protein (HSP60) decreased with age in mollusks suggesting an age-related decline in mitochondrial chaperone protection. In contrast, changes in cytosolic chaperones were species-specific. HSP70 increased and HSP90 declined with age in clams, whereas in oysters HSP70 expression did not change, and HSP90 increased with aging.