ALBERT

Description: The Arctic represents an extreme habitat for phototrophic algae due to long periods of darkness caused by the polar night (~4 months darkness). Benthic diatoms, which dominate microphytobenthic communities in shallow water regions, can survive this dark period, but the underlying physiological and biochemical mechanisms are not well understood. One of the potential mechanisms for long-term dark survival is the utilisation of stored energy products in combination with a reduced basic metabolism. In recent years, water temperatures in the Arctic increased due to an ongoing global warming. Higher temperatures could enhance the cellular energy requirements for the maintenance metabolism during darkness and, therefore, accelerate the consumption of lipid reserves. In this study, we investigated the macromolecular ratios and the lipid content and composition of Navicula cf. perminuta Grunow, an Arctic benthic diatom isolated from the microphytobenthos of Adventfjorden (Svalbard, Norway), over a dark period of 8 weeks at two different temperatures (0 and 7 °C). The results demonstrate that N. perminuta uses the stored lipid compound triacylglycerol (TAG) during prolonged dark periods, but also the pool of free fatty acids (FFA). Under the enhanced temperature of 7 °C, the lipid resources were used significantly faster than at 0 °C, which could consequently lead to a depletion of this energy reserves before the end of the polar night. On the other hand, the membrane building phospho- and glycolipids remained unchanged during the 8 weeks darkness, indicating still intact thylakoid membranes. These results explain the shorter survival times of polar diatoms with increasing water temperatures during prolonged dark periods.

Description: Organisms in shallow waters at high latitudes are under pressure due to climate change. These areas are typically inhabited by microphytobenthos (MPB) communities, composed mainly of diatoms. Only sparse information is available on the ecophysiology and acclimation processes within MPBs from Arctic regions. The physico-chemical environment and the ecology and ecophysiology of benthic diatoms in Kongsfjorden (Svalbard, Norway) are addressed in this review. MPB biofilms cover extensive areas of sediment. They show high rates of primary production, stabilise sediment surfaces against erosion under hydrodynamic forces,and affect the exchange of oxygen and nutrients across the sediment-water interface. Additionally, this phototrophic community represents a key component in the functioning of the Kongsfjorden trophic web, particularly as a major food source for benthic suspension- or deposit-feeders. MPB in Kongsfjorden is confronted with pronounced seasonal variations in solar radiation, low temperatures, and hyposaline (meltwater) conditions in summer, as well as long periods of ice and snow cover in winter. From the few data available, it seems that these organisms can easily cope with these environmental extremes. The underlying physiological mechanisms that allow growth and photosynthesis to continue under widely varying abiotic parameters, along with vertical migration and heterotrophy, and biochemical features such as a pronounced fatty-acid metabolism and silicate incorporation are discussed. Existing gaps in our knowledge of benthic diatoms in Kongsfjorden, such as the chemical ecology of biotic interactions, need to be filled. In addition, since many of the underlying molecular acclimation mechanisms are poorly understood, modern approaches based on transcriptomics, proteomics, and/or metabolomics, in conjunction with cell biological and biochemical techniques, are urgently needed. Climate change models for the Arctic predict other multifactorial stressors, such as an increase in precipitation and permafrost thawing, with consequences for the shallow-water regions. Both precipitation and permafrost thawing are likely to increase nutrient-enriched, turbid freshwater runoff and may locally counteract the expected increase in coastal radiation availability. So far, complex interactions among factors, as well as the full genetic diversity and physiological plasticity of Arctic benthic diatoms, have only rarely been considered. The limited existing information is described and discussed in this review.

Description: Three benthic diatom taxa Navicula perminuta, Melosira moniliformis and Nanofrustulum shiloi were isolated from sublittoral sandy sediments from the brackish southern Baltic Sea and established as unialgal cultures. Growth rates were determined under controlled conditions at different incubation temperatures (7–27°C), irradiances (10–600 μmol photons m−2 s−1) and salinities (1–50). The diatoms exhibited a wide range of growth tolerance. All of them grew well with growth rates of 0.3–1.5 divisons (μ) d−1 under the given gradients of parameters, indicating a classification as euryhaline and eurythermal species. In accordance with these results, photosynthesis was characterised at optimal with suboptimal growth conditions of temperature and irradiance, using the methodological approach of oxygen production. Maximum oxygen production rates after preincubation under 150 μmol photons m−2 s−1 reached values of 120 to 360 μmol O2 mg chlorophyll a h−1. All three benthic diatoms from the Baltic Sea are physiologically well adapted to the fluctuating environmental conditions in shallow-water habitats without production loss under suboptimal conditions.

Description: Energy availability and local adaptation are major components in mediating the effects of ocean acidification (OA) on marine species. In a long-term study, we investigated the effects of food availability and elevated pCO2 (~ 400, 1000 and 3000 μatm) on growth of newly settled Amphibalanus (Balanus) improvisus to reproduction, and on their offspring. We also compared two different populations, which were presumed to differ in their sensitivity to pCO2 due to differing habitat conditions: Kiel Fjord, Germany (Western Baltic Sea) with naturally strong pCO2 fluctuations, and the Tjärnö Archipelago, Sweden (Skagerrak) with far lower fluctuations. Over 20 weeks, survival, growth, reproduction and shell strength of Kiel barnacles were all unaffected by elevated pCO2, regardless of food availability. Moulting frequency and shell corrosion increased with increasing pCO2 in adults. Larval development and juvenile growth of the F1 generation were tolerant to increased pCO2, irrespective of parental treatment. In contrast, elevated pCO2 had a strong negative impact on survival of Tjärnö barnacles. Specimens from this population were able to withstand moderate levels of elevated pCO2 over 5 weeks when food was plentiful but showed reduced growth under food limitation. Severe levels of elevated pCO2 negatively impacted growth of Tjärnö barnacles in both food treatments. We demonstrate a conspicuously higher tolerance to elevated pCO2 in Kiel barnacles than in Tjärnö barnacles. This tolerance was carried-over from adults to their offspring. Our findings indicate that populations from fluctuating pCO2 environments are more tolerant to elevated pCO2 than populations from more stable pCO2 habitats. We furthermore provide evidence that energy availability can mediate the ability of barnacles to withstand moderate CO2 stress. Considering the high tolerance of Kiel specimens and the possibility to adapt over many generations, near future OA alone does not seem to present a major threat for A. improvisus

Description: Due to a steady increase of carbon dioxide concentrations in the atmosphere, the oceans take up more CO2 causing an increase in seawater pC02 and a decrease in seawater pH and calcite saturation state. Already today, especially in the late summer and autumn, low pHvalues can occur in the surface water of the Kiel Fjord due to the upwelling of CO2 rich bottom water. This is mainly caused by the degradation of phytoplankton blooms additionally increased by eutrophication of the oceans. Assuming that organisms from fluctuating habitats with respect to pCO2 and pH are adapted to - or at least able to cope with these conditions, their tolerance might be reduced when food - and thus energy - is limited. In the present study, freshly settled juvenile barnacles of the species Amphibalanus improvisus were reared for 5 months under experimental conditions. The combined effects of three pCOr (3 80, 1120 and 4000 ppm) and two feeding levels (high and low; 5: 1) on growth, reproduction and calcification, as well as the effects of the pCO2 stress on the settlement success of the offspring generation of the reared barnacles were investigated. Growth and reproduction were not affected by an increase of seawater pCO2, regardless of the diet. In the 4000 μatm pCO2 treatment with seawater undersaturated with respect to calcite, the external organic layer of the barnacle shell (epicuticle) peeled off under the long term exposure to the corrosive water with the underlying calcium carbonate layer starting to dissolve. Barnacles increased the calcification to compensate increasing CaCO3 dissolution. Since energy for successful compensatory calcification was possibly limited in the low food treatment, a decrease in shell strength was observed under the 4000 μatm pCO2 treatment. This reduction in shell strength could lead to an increased susceptibility of barnacles to crushing and drilling predators. The applied pCO2 and food treatments had furthermore no significant effects on the development and the settlement success of the offspring barnacle larvae. These results show that the barnacle Amphibalanus improvisus can cope with predicted future ocean acidification even under limited food conditions. A decrease in shell strength and thus a possible increase in susceptibility of barnacles towards their predators could nevertheless have negative effects on barnacle populations in the future. This highlights on the importance to investigate species interactions in response to multiple environmental stressors such as pC02 and food limitation.