Glacial melting: an overlooked threat to Antarctic krill

Drift Algal Accumulation in Ice Scour Pits Provides an Underestimated Ecological Subsidy in a Novel Antarctic Soft-Sediment Habitat

Ice scouring is one of the strongest agents of disturbance in nearshore environments at high latitudes. In depths, less than 20 m, grounding icebergs reshape the soft-sediment seabed by gouging furrows called ice pits. Large amounts of drift algae (up to 5.6 kg/m²) that would otherwise be transported to deeper water accumulate inside these features, representing an underestimated subsidy. Our work documents the distribution and dimensions of ice pits in Fildes Bay, Antarctica, and evaluates their relationship to the biomass and species composition of algae found within them. It also assesses the rates of deposition and advective loss of algae in the pits. The 17 ice pits found in the study area covered only 4.2% of the seabed but contained 98% of drift algal biomass, i.e., 60 times the density (kg/m²) of the surrounding seabed. Larger ice pits had larger and denser algal accumulations than small pits and had different species compositions. The accumulations were stable over time: experimentally cleared pits regained initial biomass levels after one year, and advective loss was less than 15% annually. Further research is needed to understand the impacts of ice scouring and subsequent algal retention on ecosystem functioning in this rapidly changing polar environment.

Evaluating deep-sea communities' susceptibility to mining plumes using shallow-water data

Increased suspended sediment concentrations (SSC) are a major stressor across aquatic habitats. Here, the literature was synthesized to show that animal responses to increases in relative SSC (test concentration/natural background concentration) were similar in type and negative across different shallow-water (marine, estuarine, freshwater) habitats. Further, animal sensitivities are similar across habitats based on relative SSC and occur starting at low relative SSC increases in all habitats despite differences in natural background SSC. Based upon these similarities in relative SSC sensitivities, deep-sea sensitivity values for acute exposure to increased SSC, where empirical data are almost non-existent, were estimated. Because of the low natural SSC in deep sea environments, very small increases in absolute SSC could result in acute effects. How the methods and results can be used to inform regulatory thresholds are discussed. Because of the large variability in shallow water datasets and differences between deep-sea and shallow-water habitats, deep-sea specific data are needed to verify the estimates and improve their precision. Following the precautionary principle and the results presented here, it is recommended that the threshold for acute plume impacts is set very close to natural background levels.

Can heat shock protein 70 (HSP70) serve as biomarkers in Antarctica for future ocean acidification, warming and salinity stress?

The Antarctic Peninsula is one of the fastest-warming places on Earth. Elevated sea water temperatures cause glacier and sea ice melting. When icebergs melt into the ocean, it “freshens” the saltwater around them, reducing its salinity. The oceans absorb excess anthropogenic carbon dioxide (CO2) causing decline in ocean pH, a process known as ocean acidification. Many marine organisms are specifically affected by ocean warming, freshening and acidification. Due to the sensitivity of Antarctica to global warming, using biomarkers is the best way for scientists to predict more accurately future climate change and provide useful information or ecological risk assessments. The 70-kilodalton (kDa) heat shock protein (HSP70) chaperones have been used as biomarkers of stress in temperate and tropical environments. The induction of the HSP70 genes (Hsp70) that alter intracellular proteins in living organisms is a signal triggered by environmental temperature changes. Induction of Hsp70 has been observed both in eukaryotes and in prokaryotes as response to environmental stressors including increased and decreased temperature, salinity, pH and the combined effects of changes in temperature, acidification and salinity stress. Generally, HSP70s play critical roles in numerous complex processes of metabolism; their synthesis can usually be increased or decreased during stressful conditions. However, there is a question as to whether HSP70s may serve as excellent biomarkers in the Antarctic considering the long residence time of Antarctic organisms in a cold polar environment which appears to have greatly modified the response of heat responding transcriptional systems. This review provides insight into the vital roles of HSP70 that make them ideal candidates as biomarkers for identifying resistance and resilience in response to abiotic stressors associated with climate change, which are the effects of ocean warming, freshening and acidification in Antarctic organisms.

Some Biogeochemical Characteristics of the Trace Element Bioaccumulation in the Benthic Fauna of the Piip Volcano (The Southwestern Bering Sea)

The Piip Volcano is a submarine volcanic edifice occupying the central part of the Volcanologists Massif in the southwestern Bering Sea, with two tops, southern and northern. The minimum depth of the northern top is located at 368 m, and of the southern at 464 m. Active hydrothermal venting occurring at both summits of the volcano supports diverse biological communities, including animals specific for chemosynthetic habitats. In benthic organisms inhabiting the northern and southern tops of the Piip Volcano, for the first time, we examined distribution patterns of the following trace elements: titanium, vanadium, chromium, manganese, iron, nickel, copper, zinc, arsenic, selenium, zirconium, molybdenum, silver, cadmium, antimony, barium, tungsten, lead, bismuth, and uranium. The element contents were quantified by the ICP-MS. Total carbon (TC) and total inorganic carbon (TIC) were determined using a Shimadzu TOC-L-CPN and mineral composition of sediment was determined using the XRD. In the water of the biotope from the northern top, concentrations of Mn, Zn, Ag, Cd, Sb, W, Pb were 2–6 times, and Ba was 50 times higher than those from the southern top. This was attributed to the lower temperature of fluids emanating at the southern top. An abundant population of Calyptogena pacifica (Bivalvia: Vesicomyidae: Pliocardiinae) was found only at the southern top. The main target of most trace elements, such as Fe, V, Cr, Co, Ni, Zn, As, Mo, Ag, Cd, W, Pb, Bi, and U were the soft parts of Calyptogena pacifica (with high TOC content, on average 53.1% in gills and 49.6% in the rest of the body). Gills were characterized by particular high contents (>100 µg g−1 dry w.) of Zn, Cd, Fe, Ni, Cu, and Pb, which can form sulphides or be associated with them. Shells of C. pacifica, as well as Brachiopoda, were depleted in these elements, as well as tissues of the carnivores Paguridae (Crustacea) and Actiniaria (Anthozoa). In suspension feeders from both tops, the lower contents of most elements were detected. Estimation of Biological Concentration Factor (BCF) for most elements varied from 102 to 104, reaching n105 for Ni, Zn, Ag, Cd, and Pb. A significant difference in BCF values between Fe and Mn was revealed.

Characterization of Inducible HSP70 Genes in an Antarctic Yeast, Glaciozyma antarctica PI12, in Response to Thermal Stress

The induction of highly conserved heat shock protein 70 (HSP70) is often related to a cellular response due to harmful stress or adverse life conditions. In this study, we determined the expression of Hsp70 genes in the Antarctic yeast, Glaciozyma antarctica, under different several thermal treatments for several exposure periods. The main aims of the present study were (1) to determine if stress-induced Hsp70 could be used to monitor the exposure of the yeast species G. antarctica to various types of thermal stress; (2) to analyze the structures of the G. antarctica HSP70 proteins using comparative modeling; and (3) to evaluate the relationship between the function and structure of HSP70 in G. antarctica. In this study, we managed to amplify and clone 2 Hsp70 genes from G. antarctica named GaHsp70-1 and GaHsp70-2. The cells of G. antarctica expressed significantly inducible Hsp70 genes after the heat and cold shock treatments. Interestingly, GaHsp70-1 showed 2–6-fold higher expression than GaHsp70-2 after the heat and cold exposure. ATP hydrolysis analysis on both G. antarctica HSP70s proved that these psychrophilic chaperones can perform activities in a wide range of temperatures, such as at 37, 25, 15, and 4 °C. The 3D structures of both HSP70s revealed several interesting findings, such as the substitution of a β-sheet to loop in the N-terminal ATPase binding domain and some modest residue substitutions, which gave the proteins the flexibility to function at low temperatures and retain their functional activity at ambient temperatures. In conclusion, both analyzed HSP70s played important roles in the physiological adaptation of G. antarctica.

Antarctic ascidians under increasing sedimentation: Physiological thresholds and ecosystem hysteresis

Glacier melting sediment inputs affect coastal ecosystems on the Antarctic Peninsula. In Potter Cove (South Shetland Islands, Antarctica), the shift from an "ascidian dominated" to a "mixed" assemblage has been linked to sedimentation. However, in recently described newly ice-free areas ascidians became dominant in spite of total suspended particulate matter (TSPM) concentrations, which are the highest measured in Potter Cove. Here, we compared the gut content and energy reserve of three ascidian species at three stations under different TSPM regimes. All analysed species had a higher gut content with lower %OM at these newly areas. A theoretical relationship between the scope for growth for the targeted ascidians and TSPM explained assemblages' recorded change but failed to explain current ascidians distribution. The results may indicate the existence of a TSPM threshold that allows the spatial coexistence of alternative stable states at benthic Potter Cove system.

Glacial melt disturbance shifts community metabolism of an Antarctic seafloor ecosystem from net autotrophy to heterotrophy

Climate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

Implications for the mesopelagic microbial gardening hypothesis as determined by experimental fragmentation of Antarctic krill fecal pellets

Detritivores need to upgrade their food to increase its nutritional value. One method is to fragment detritus promoting the colonization of nutrient-rich microbes, which consumers then ingest along with the detritus; so-called microbial gardening. Observations and numerical models of the detritus-dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is a fundamental process in the ocean carbon cycle leading to increased respiration of carbon-rich detritus. However, no experimental evidence exists to demonstrate that microbial respiration rates are higher on recently fragmented sinking detrital particles.Using aquaria-reared Antarctic krill fecal pellets, we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 1.9×, but only on brown fecal pellets, formed from the consumption of other pellets. Microbial POC turnover on un- and fragmented green fecal pellets, formed from consuming fresh phytoplankton, was equal. Thus, POC content, fragmentation, and potentially nutritional value together drive POC turnover rates.Mesopelagic microbial gardening could be a risky strategy, as the dominant detrital food source is settling particles; even though fragmentation decreases particle size and sinking rate, it is unlikely that an organism would remain with the particle long enough to nutritionally benefit from attached microbes. We propose "communal gardening" occurs whereby additional mesopelagic organisms nearby or below the site of fragmentation consume the particle and the colonized microbes.To determine how fragmentation impacts the remineralization of sinking carbon-rich detritus and to parameterize microbial gardening in mesopelagic carbon models, three key metrics from further controlled experiments and observations are needed; how particle composition (here, pellet color/krill diet) impacts the response of microbes to the fragmentation of particles; the nutritional benefit to zooplankton from ingesting microbes after fragmentation along with identification of which essential nutrients are being targeted; how both these factors vary between physical (shear) and biological particle fragmentation.

Coastal Bacterial Community Response to Glacier Melting in the Western Antarctic Peninsula

Current warming in the Western Antarctic Peninsula (WAP) has multiple effects on the marine ecosystem, modifying the trophic web and the nutrient regime. In this study, the effect of decreased surface salinity on the marine microbial community as a consequence of freshening from nearby glaciers was investigated in Chile Bay, Greenwich Island, WAP. In the summer of 2016, samples were collected from glacier ice and transects along the bay for 16S rRNA gene sequencing, while in situ dilution experiments were conducted and analyzed using 16S rRNA gene sequencing and metatranscriptomic analysis. The results reveal that certain common seawater genera, such as Polaribacter, Pseudoalteromonas and HTCC2207, responded positively to decreased salinity in both the bay transect and experiments. The relative abundance of these bacteria slightly decreased, but their functional activity was maintained and increased the over time in the dilution experiments. However, while ice bacteria, such as Flavobacterium and Polaromonas, tolerated the increased salinity after mixing with seawater, their gene expression decreased considerably. We suggest that these bacterial taxa could be defined as sentinels of freshening events in the Antarctic coastal system. Furthermore, these results suggest that a significant portion of the microbial community is resilient and can adapt to disturbances, such as freshening due to the warming effect of climate change in Antarctica.

Micro- and mesozooplankton successions in an Antarctic coastal environment during a warm year

The rapid increase in atmospheric temperature detected in the last decades in the Western Antarctic Peninsula was accompanied by a strong glacier retreat and an increase in production of melting water, as well as changes in the sea-ice dynamic. The objective of this study was to analyze the succession of micro- and mesozooplankton during a warm annual cycle (December 2010-December 2011) in an Antarctic coastal environment (Potter Cove). The biomass of zooplankton body size classes was used to predict predator-prey size relationships (i.e., to test bottom-up/top-down control effects) using a Multiple Linear Regression Analysis. The micro- and mesozooplanktonic successions were graphically analyzed to detect the influence of environmental periods (defined by the degree of glacial melting, sea-ice freezing and sea-ice melting) on coupling/uncoupling planktonic biomass curves associated to possible predator-prey size relationship scenarios. At the beginning of the glacial melting, medium and large mesozooplankton (calanoid copepods, Euphausia superba, and Salpa thompsoni) exert a top-down control on Chl-a and microzooplankton. Stratification of the water column benefitted the availability of adequate food-size (Chl-a <20) for large microzooplankton (tintinnids) development observed during fall. High abundance of omnivores mesozooplankton (Oithona similis and furcilia of E. superba) during sea-ice freezing periods would be due to the presence of available heterotrophic food under or within the sea ice. Finally, the increase in microzooplankton abundance in the middle of spring, when sea-ice melting starts, corresponded to small and medium dinoflagellates and ciliates species, which were possibly part of the biota of sea ice. If glacier retreat continues and the duration and thickness of the sea ice layer fluctuates as predicted by climate models, our results predict a future scenario regarding the zooplankton succession in Antarctic coastal environments.

Role of suspension feeders in antarctic pelagic-benthic coupling: Trophic ecology and potential carbon sinks under climate change

Sea-ice and coastal glacier loss in the Western Antarctic Peninsula open new ice-free areas. They allowing primary production and providing new seabed for colonisation, both acting as a negative feedback of climate change. However, the injection of sediment-laden runoff from the melting of land-terminating glaciers may reduce this feedback. Changes in particulate matter will affect nutrition and excretion (faeces stoichiometry and properties) of suspension feeders, reshaping coastal carbon dynamics and pelagic-benthic coupling. Absorption efficiency and biodeposition of Euphausia superba and Cnemidocarpa verrucosa were quantified for different food treatments and varying sediment concentrations. Both species showed high overall absorption efficiency for free-sediment diets, but were negatively affected by sediment addition. High sediment conditions increased krill biodeposition, while it decreased in ascidians. Energy balance estimation indicated high carbon sink potential in ascidians, but it is modulated by food characteristics and negatively affected by sediment inputs in the water column.

Investigation of Sediment-Rich Glacial Meltwater Plumes Using a High-Resolution Multispectral Sensor Mounted on an Unmanned Aerial Vehicle

A Parrot Sequoia+ multispectral camera on a Parrot Bluegrass drone registered in four spectral bands (green, red, red edge (RE), and near-infrared (NIR)) to identify glacial outflow zones and determined the meltwater turbidity values in waters in front of the following Antarctic glaciers: Ecology, Dera Icefall, Zalewski, and Krak on King George Island, Southern Shetlands was used. This process was supported by a Red-Green-Blue (RGB) colour model from a Zenmuse X5 camera on an Inspire 2 quadcopter drone. Additional surface water turbidity measurements were carried out using a Yellow Springs Instruments (YSI) sonde EXO2. From this research, it was apparent that for mapping low-turbidity and medium-turbidity waters (<70 formazinenephelometricunits (FNU)), a red spectral band should be used, since it is insensitive to possible surface ice phenomena and registers the presence of both red and white sediments. High-turbidity plumes with elevated FNU values should be identified through the NIR band. Strong correlation coefficients between the reflectance at particular bands and FNU readings (RGreen = 0.85, RRed = 0.85, REdge = 0.84, and RNIR = 0.83) are shown that multispectral mapping using Unmanned Aerial Vehicles (UAVs) can be successfully usedeven in the unfavourable weather conditions and harsh climate of Antarctica. Lastly, the movement of water masses in Admiralty Bay is briefly discussed and supported by the results from EXO2 measurements.

Antarctic environmental change and biological responses

Antarctica and the surrounding Southern Ocean are facing complex environmental change. Their native biota has adapted to the region's extreme conditions over many millions of years. This unique biota is now challenged by environmental change and the direct impacts of human activity. The terrestrial biota is characterized by considerable physiological and ecological flexibility and is expected to show increases in productivity, population sizes and ranges of individual species, and community complexity. However, the establishment of non-native organisms in both terrestrial and marine ecosystems may present an even greater threat than climate change itself. In the marine environment, much more limited response flexibility means that even small levels of warming are threatening. Changing sea ice has large impacts on ecosystem processes, while ocean acidification and coastal freshening are expected to have major impacts.

Anatomy of a glacial meltwater discharge event in an Antarctic cove

Glacial meltwater discharge from Antarctica is a key influence on the marine environment, impacting ocean circulation, sea level and productivity of the pelagic and benthic ecosystems. The responses elicited depend strongly on the characteristics of the meltwater releases, including timing, spatial structure and geochemical composition. Here we use isotopic tracers to reveal the time-varying pattern of meltwater during a discharge event from the Fourcade Glacier into Potter Cove, northern Antarctic Peninsula. The discharge is strongly dependent on local air temperature, and accumulates into an extremely thin, buoyant layer at the surface. This layer showed evidence of elevated turbidity, and responded rapidly to changes in atmospherically driven circulation to generate a strongly pulsed outflow from the cove to the broader ocean. These characteristics contrast with those further south along the Peninsula, where strong glacial frontal ablation is driven oceanographically by intrusions of warm deep waters from offshore. The Fourcade Glacier switched very recently to being land-terminating; if retreat rates elsewhere along the Peninsula remain high and glacier termini progress strongly landward, the structure and impact of the freshwater discharges are likely to increasingly resemble the patterns elucidated here.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Choosing the future of Antarctica

We present two narratives on the future of Antarctica and the Southern Ocean, from the perspective of an observer looking back from 2070. In the first scenario, greenhouse gas emissions remained unchecked, the climate continued to warm, and the policy response was ineffective; this had large ramifications in Antarctica and the Southern Ocean, with worldwide impacts. In the second scenario, ambitious action was taken to limit greenhouse gas emissions and to establish policies that reduced anthropogenic pressure on the environment, slowing the rate of change in Antarctica. Choices made in the next decade will determine what trajectory is realized.

HSP70 from the Antarctic sea urchin Sterechinus neumayeri: molecular characterization and expression in response to heat stress

Background

Heat stress proteins are implicated in stabilizing and refolding denatured proteins in vertebrates and invertebrates. Members of the Hsp70 gene family comprise the cognate heat shock protein (Hsc70) and inducible heat shock protein (Hsp70). However, the cDNA sequence and the expression of Hsp70 in the Antarctic sea urchin are unknown.

Methods

We amplified and cloned a transcript sequence of 1991 bp from the Antarctic sea urchin Sterechinus neumayeri, experimentally exposed to heat stress (5  and 10 °C for 1, 24 and 48 h). RACE-PCR and qPCR were employed to determine Hsp70 gene expression, while western blot and ELISA methods were used to determine protein expression.

Results

The sequence obtained from S. neumayeri showed high identity with Hsp70 members. Several Hsp70 family features were identified in the deduced amino acid sequence and they indicate that the isolated Hsp70 is related to the cognate heat shock protein type. The corresponding 70 kDa protein, called Sn-Hsp70, was immune detected in the coelomocytes and the digestive tract of S. neumayeri using a monospecific polyclonal antibody. We showed that S. neumayeri do not respond to acute heat stress by up-regulation of Sn-Hsp70 at transcript and protein level. Furthermore, the Sn-Hsp70 protein expression was not induced in the digestive tract.

Conclusions

Our results provide the first molecular evidence that Sn-Hsp70 is expressed constitutively and is non-induced by heat stress in S. neumayeri.

Uptake and Depuration Kinetics Influence Microplastic Bioaccumulation and Toxicity in Antarctic Krill ( Euphausia superba)

The discarding of plastic products has led to the ubiquitous occurrence of microplastic particles in the marine environment. The uptake and depuration kinetics of ingested microplastics for many marine species still remain unknown despite its importance for understanding bioaccumulation potential to higher trophic level consumers. In this study, Antarctic krill ( Euphausia superba) were exposed to polyethylene microplastics to quantify acute toxicity and ingestion kinetics, providing insight into the bioaccumulation potential of microplastics at the first-order consumer level. In the 10 day acute toxicity assay, no mortality or dose-dependent weight loss occurred in exposed krill, at any of the exposure concentrations (0, 10, 20, 40, or 80% plastic diet). Krill exposed to a 20% plastic diet for 24 h displayed fast uptake (22 ng mg^-1 h^-1) and depuration (0.22 h^-1) rates, but plastic uptake did not reach steady state. Efficient elimination also resulted in no bioaccumulation over an extended 25 day assay, with most individuals completely eliminating their microplastic burden in less than 5 days post exposure. Our results support recent findings of limited acute toxicity of ingested microplastics at this trophic level, and suggest sublethal chronic end points should be the focus of further ecotoxicological investigation.

Benthic colonization in newly ice-free soft-bottom areas in an Antarctic fjord

Extended glacier retreat is among the main consequences of the rapid warming of the West Antarctic Peninsula. Particularly, in the inner part of Potter Cove (South Shetland Islands, Antarctica) large areas are now exposed to open sea conditions owing to the retreat of Fourcade glacier. During the 2010 austral summer, underwater photographic surveys were undertaken by SCUBA diving up to 30 m in these new ice-free areas 80 m from the glacier front. Our main aim was to investigate colonization and early succession of the benthic assemblages on soft-bottom areas. Here, we reported a total of 1,146 animals belonging to 13 taxa. Filter-feeders comprised the largest trophic group and sessile fauna showed much higher coverages and densities than mobile fauna at all depths. The most abundant groups were ascidians and bryozoans, which together comprised ~90% of all taxa documented. In a region where most of marine-terminating glaciers are in retreat, these results are an important contribution to improve our knowledge on colonization in the newly ice-free areas.

Pelagic and benthic communities of the Antarctic ecosystem of Potter Cove: Genomics and ecological implications

Molecular technologies are more frequently applied in Antarctic ecosystem research and the growing amount of sequence-based information available in databases adds a new dimension to understanding the response of Antarctic organisms and communities to environmental change. We apply molecular techniques, including fingerprinting, and amplicon and metagenome sequencing, to understand biodiversity and phylogeography to resolve adaptive processes in an Antarctic coastal ecosystem from microbial to macrobenthic organisms and communities. Interpretation of the molecular data is not only achieved by their combination with classical methods (pigment analyses or microscopy), but furthermore by combining molecular with environmental data (e.g., sediment characteristics, biogeochemistry or oceanography) in space and over time. The studies form part of a long-term ecosystem investigation in Potter Cove on King-George Island, Antarctica, in which we follow the effects of rapid retreat of the local glacier on the cove ecosystem. We formulate and encourage new approaches to integrate molecular tools into Antarctic ecosystem research, environmental conservation actions, and polar ocean observatories.

Temporal controls on silicic acid utilisation along the West Antarctic Peninsula

The impact of climatic change along the Antarctica Peninsula has been widely debated in light of atmospheric/oceanic warming and increases in glacial melt over the past half century. Particular concern exists over the impact of these changes on marine ecosystems, not only on primary producers but also on higher trophic levels. Here we present a record detailing of the historical controls on the biogeochemical cycling of silicic acid [Si(OH)4] on the west Antarctica Peninsula margin, a region in which the modern phytoplankton environment is constrained by seasonal sea ice. We demonstrate that Si(OH)4 cycling through the Holocene alternates between being primarily regulated by sea ice or glacial discharge from the surrounding grounded ice sheet. With further climate-driven change and melting forecast for the twenty-first century, our findings document the potential for biogeochemical cycling and multi-trophic interactions along the peninsula to be increasingly regulated by glacial discharge, altering food-web interactions.