Posts Tagged 'Antarctic'

Effects of ocean acidification on Antarctic microbial communities

Antarctic waters are amongst the most vulnerable in the world to ocean acidification due to their cold temperatures, naturally low levels of calcium carbonate and upwelling that brings deep CO2-rich waters to the surface. A meta-analysis demonstrated groups of Antarctic marine biota in waters south of 60!S have a range of tolerances to ocean acidification. Invertebrates and phytoplankton showed negative effects above 500 μatm and 1000 μatm CO2 respectively, while bacteria appear tolerant to elevated CO2. Phytoplankton studied as part of a natural microbial community were found to be more
sensitive than those studied as a single species in culture. This highlights the importance of community and ecosystem level studies, which incorporate the interaction and competition among species and trophic levels, to accurately assess the effects of ocean acidification on the Antarctic ecosystem.

Antarctic marine microbes (comprising phytoplankton, protozoa and bacteria) drive ocean productivity, nutrient cycling and mediate trophodynamics and the biological pump. While they appear vulnerable to changes in ocean chemistry, little is known about the nature and magnitude of their responses to ocean acidification, especially for natural communities. To address this lack of information, a six level, dose-response ocean acidification experiment was conducted in Prydz Bay, East Antarctica, using 650 L incubation tanks (minicosms). The minicosms were filled with Antarctic nearshore water and adjusted to a gradient of carbon dioxide (CO2) from 343 to 1641 μatm. Microscopy
and phylogenetic marker gene sequence analysis found the microbial community
composition altered at CO2 levels above approximately 1000 μatm. The CO2-
induced responses of microeukaryotes (>20 μm) and nanoeukaryotes (2 to 20 μm) were taxon-specific. For diatoms the response of taxa was related to cell size with micro-sized diatoms (>20 μm) increasing in abundance with moderate CO2 (506 to 634 μatm), while above this level their abundance declined. In contrast, nano-size diatoms (<20 μm) tolerated elevated CO2. Like large diatoms, Phaeocystis antarctica increased in abundance between 343 to 634 μatm CO2 but fell at higher levels. 18S and 16S rDNA sequencing showed that picoeukaryotic and prokaryotic composition was unaffected by CO2, despite having higher abundances at CO2 levels !634 μatm. This was likely due to the lower abundance of heterotrophic nanoflagellates at CO2 levels exceeding 953 μatm, which reduced the top-down control of their pico- and nanoplanktonic prey. As a result of the differences in the tolerance of individual taxa/size categories, CO2 caused a
significant change in the microbial community structure to one dominated by nano-sized diatoms, picoeukaryotes and prokaryotes.

Based on the CO2-induced changes in the microbial community, modelling was performed to investigate the future effects of different levels of elevated CO2 on the structure and function of microbial communities in Antarctic coastal systems. These models indicate CO2 levels predicted toward the end of the century under a “business as usual scenario” elicit changes in microbial composition, significantly altering trophodynamic pathways, reducing energy transfer to higher trophic levels and favouring respiration of carbon within the microbial loop. Such responses would alter elemental cycles, jeopardise the productivity that underpins the wealth and diversity of life for which Antarctica is renowned. In addition, it would reduce carbon sequestration in coastal Antarctic waters thereby having a positive feedback on global climate change.

Continue reading ‘Effects of ocean acidification on Antarctic microbial communities’

Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978-2018

Antarctic bottom waters (AABWs) are known as a long term sink for anthropogenic CO2 (Cant) but is hardly quantified because of the scarcity of the observations, specifically at an interannual scale. We present in this manuscript an original dataset combining 40 years of carbonate system observations in the Indian sector of the Southern Ocean (Enderby Basin) to evaluate and interpret the interannual variability of Cant in the AABW. This investigation is based on regular observations collected at the same location (63° E/56.5° S) in the frame of the French observatory OISO from 1998 to 2018 extended by GEOSECS and INDIGO observations (1978, 1985 and 1987).

At this location the main sources of AABW sampled is the fresh and younger Cape Darnley bottom water (CDBW) and the Weddell Sea deep water (WSDW). Our calculations reveal that Cant concentrations increased significantly in AABW, from about +7 µmol.kg-1 in 1978-1987 to +13 µmol.kg-1 18 in 2010-2018. This is comparable to previous estimates in other SO basins, with the exception of bottom waters close to their formation sites where Cant concentrations are about twice as large. Our analysis shows that the CT and Cant increasing rates in AABW are about the same over the period 1978-2018, and we conclude that the long-term change in CT is mainly due to the uptake of anthropogenic CO2 in the different formation regions. This is however modulated by significant interannual to pluriannual variability associated with variations in hydrological (ϴ, S) and biogeochemical (CT, AT, O2) properties. A surprising result is the apparent stability of Cant concentrations in recent years despite the increase in CT and the gradual acceleration of atmospheric CO2.

The Cant sequestration by AABWs is more variable than expected and depends on a complex combination of physical, chemical and biological processes at the formation sites and during the transit of the different AABWs. The interannual variability at play in AABW needs to be carefully considered on the extrapolated estimation of Cant sequestration based on sparse observations over several years.

Continue reading ‘Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978-2018’

Effects of ocean acidification on Antarctic marine organisms: a meta‐analysis

Southern Ocean waters are among the most vulnerable to ocean acidification. The projected increase in the CO2 level will cause changes in carbonate chemistry that are likely to be damaging to organisms inhabiting these waters. A meta‐analysis was undertaken to examine the vulnerability of Antarctic marine biota occupying waters south of 60°S to ocean acidification. This meta‐analysis showed that ocean acidification negatively affects autotrophic organisms, mainly phytoplankton, at CO2 levels above 1,000 μatm and invertebrates above 1,500 μatm, but positively affects bacterial abundance. The sensitivity of phytoplankton to ocean acidification was influenced by the experimental procedure used. Natural, mixed communities were more sensitive than single species in culture and showed a decline in chlorophyll a concentration, productivity, and photosynthetic health, as well as a shift in community composition at CO2 levels above 1,000 μatm. Invertebrates showed reduced fertilization rates and increased occurrence of larval abnormalities, as well as decreased calcification rates and increased shell dissolution with any increase in CO2 level above 1,500 μatm. Assessment of the vulnerability of fish and macroalgae to ocean acidification was limited by the number of studies available. Overall, this analysis indicates that many marine organisms in the Southern Ocean are likely to be susceptible to ocean acidification and thereby likely to change their contribution to ecosystem services in the future. Further studies are required to address the poor spatial coverage, lack of community or ecosystem‐level studies, and the largely unknown potential for organisms to acclimate and/or adapt to the changing conditions.

Continue reading ‘Effects of ocean acidification on Antarctic marine organisms: a meta‐analysis’

Combined stress of ocean acidification and warming influence survival and drives differential gene expression patterns in the Antarctic pteropod, Limacina helicina antarctica

The ecologically important thecosome pteropods in the Limacina spp. complex have recently been the focus of studies examining the impacts global change factors – e.g., ocean acidification (OA) and ocean warming (OW) – on their performance and physiology. This focus is driven by conservation concerns where the health of pteropod populations is threatened by the high susceptibility of their shells to dissolution in low aragonite saturation states associated with OA and how coupling of these stressors may push pteropods past the limits of physiological plasticity. In this manipulation experiment, we describe changes in the transcriptome of the Antarctic pteropod, Limacina helicina antarctica, to these combined stressors. The conditions used in the laboratory treatments met or exceeded those projected for the Southern Ocean by the year 2100. We made two general observations regarding the outcome of the data: (1) Temperature was more influential than pH in terms of changing patterns of gene expression, and (2) these Antarctic pteropods appeared to have a significant degree of transcriptomic plasticity to respond to acute abiotic stress in the laboratory. In general, differential gene expression was observed amongst the treatments; here, for example, transcripts associated with maintaining protein structure and cell proliferation were up-regulated. To disentangle the effects of OA and OW, we used a weighted gene co-expression network analysis to explore patterns of change in the transcriptome. This approach identified gene networks associated with OW that were enriched for transcripts proposed to be involved in increasing membrane fluidity at warmer temperatures. Together these data provide evidence that L.h.antarctica has a limited capacity to acclimate to the combined conditions of OA and OW used in this study. This reduced scope of acclimation argues for continued study of how adaptation to polar aquatic environments may limit the plasticity of present-day populations in responding to future environmental change.

Continue reading ‘Combined stress of ocean acidification and warming influence survival and drives differential gene expression patterns in the Antarctic pteropod, Limacina helicina antarctica’

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.

Continue reading ‘Antarctic environmental change and biological responses’

Changes in genome-wide methylation and gene expression in response to future pCO2 extremes in the Antarctic pteropod Limacina helicina antarctica

Epigenetic processes such as variation in DNA methylation may promote phenotypic plasticity and the rapid acclimatization of species to environmental change. The extent to which an organism can mount an epigenetic response to current and future climate extremes may influence its capacity to acclimatize or adapt to global change on ecological rather than evolutionary time scales. The thecosome pteropod Limacina helicina antarctica is an abundant macrozooplankton endemic to the Southern Ocean and is considered a bellwether of ocean acidification as it is highly sensitive to variation in carbonate chemistry. In this study, we quantified variation in DNA methylation and gene expression over time across different ocean acidification regimes. We exposed L. helicina antarctica to pCO2 levels mimicking present-day norms in the coastal Southern Ocean of 255 μatm pCO2, present-day extremes of 530 μatm pCO2, and projected extremes of 918 μatm pCO2 for up to 7 days before measuring global DNA methylation and sequencing transcriptomes in animals from each treatment across time. L. helicina antarctica significantly reduced DNA methylation by 29–56% after 1 day of exposure to 918 μatm pCO2 before DNA methylation returned to control levels after 6 days. In addition, L. helicina antarctica exposed to 918 μatm pCO2 exhibited drastically more differential expression compared to cultures replicating present-day pCO2 extremes. Differentially expressed transcripts were predominantly downregulated. Furthermore, downregulated genes were enriched with signatures of gene body methylation. These findings support the potential role of DNA methylation in regulating transcriptomic responses by L. helicina antarctica to future ocean acidification and in situ variation in pCO2 experienced seasonally or during vertical migration. More broadly, L. helicina antarctica was capable of mounting a substantial epigenetic response to ocean acidification despite little evidence of metabolic compensation or recovery of the cellular stress response in this species at future pCO2 levels.

Continue reading ‘Changes in genome-wide methylation and gene expression in response to future pCO2 extremes in the Antarctic pteropod Limacina helicina antarctica’

Polar opposites; bacterioplankton susceptibility and mycoplankton resistance to ocean acidification

Microorganisms form the basis of ocean ecosystems yet the effects of perturbations such as decreasing pH on microbial community structure, interactions and functionality remain compared to multicellular organisms. Using an experimental manipulation of Southern Ocean seawater, we subjected bacterioplankton and mycoplankton to artificial pH decreases, which are predicted to occur in the future. We show that acidification led to substantial increases of bacterioplankton diversity, while in contrast it had no effect on mycoplankton diversity. Our analyses revealed a loss of putative keystone taxa and a decrease in predicted community interactions as a response to lower pH levels. Bacterioplankton shifted from generalist to specialist community members, suggesting a specific stress response to unfavourable conditions. In addition, enzyme activities involved in nitrogen acquisition were lower at reduced pH levels, suggesting altered organic matter cycling in a more acidic ocean. Our findings suggest that bacterioplankton and mycoplankton may respond differentially to future ocean acidification, with potentially negative impacts on community structure and biogeochemical cycling in the Southern Ocean.

Continue reading ‘Polar opposites; bacterioplankton susceptibility and mycoplankton resistance to ocean acidification’

Towards an intensified summer CO2 sink behaviour in the Southern Ocean coastal regions

Highlights

• We analysed the FCO2 and CO2 system in an important region of the Southern Ocean.

• The Gerlache Strait acts as a stronger CO2 sink than nearby open ocean areas during the austral summer.

• We identified both strong and near-equilibrium sink scenarios for FCO2.

• The pattern of variability of FCO2 has changed since 2012 to a higher frequency of years with a strong CO2 sink.

Abstract

The Southern Ocean is a globally important carbon sink region. However, the austral coastal zones are usually not considered in global estimations due to their general undersampling and large regional dynamics. Thus, estimations of carbon uptake in the Southern Ocean may differ considerably from current values, i.e., without accounting for coastal regions. Here, we conducted a case study in the Gerlache Strait, an ecologically important Antarctic coastal zone. We show that the net sea-air CO2 flux (FCO2) in the strait may reach the same or greater magnitudes than those in large open sea regions around Antarctica during summer, despite having a much smaller area. A large mean FCO2 of –31 ± 19 mmol m–2 d–1 was observed in the strong CO2 sink years (i.e., FCO2 < –12 mmol m–2 d–1), in contrast to –1 ± 7 mmol m–2 d–1 in CO2 near-equilibrium conditions (i.e., CO2 sea–air difference ≈ 0). This variability is mainly modulated by phytoplankton activity and likely upwelling processes. We also identified two cycles of variability with 2-year and 4-year periodicities from 1999 to 2017. The 2-year periodicity becomes stronger after 2012, intensifying the strong CO2 sink scenario in the Gerlache Strait. Our findings reinforce the importance of polar coastal zones as CO2 sinks during the austral summer and the need to broaden our understanding of the role of these regions at other time scales.

Continue reading ‘Towards an intensified summer CO2 sink behaviour in the Southern Ocean coastal regions’

A meta-analysis of microcosm experiments shows that dimethyl sulfide (DMS) production in polar waters is insensitive to ocean acidification

Emissions of dimethylsulfide (DMS) from the polar oceans play a key role in atmospheric processes and climate. Therefore, it is important to increase our understanding of how DMS production in these regions may respond to climate change. The polar oceans are particularly vulnerable to ocean acidification (OA). However, our understanding of the polar DMS response is limited to two studies conducted in Arctic waters, where in both cases DMS concentrations decreased with increasing acidity. Here, we report on our findings from seven summertime shipboard microcosm experiments undertaken in a variety of locations in the Arctic Ocean and Southern Ocean. These experiments reveal no significant effects of short-term OA on the net production of DMS by planktonic communities. This is in contrast to similar experiments from temperate north-western European shelf waters where surface ocean communities responded to OA with significant increases in dissolved DMS concentrations. A meta-analysis of the findings from both temperate and polar waters (n=18 experiments) reveals clear regional differences in the DMS response to OA. Based on our findings, we hypothesize that the differences in DMS response between temperate and polar waters reflect the natural variability in carbonate chemistry to which the respective communities of each region may already be adapted. If so, future temperate oceans could be more sensitive to OA, resulting in an increase in DMS emissions to the atmosphere, whilst perhaps surprisingly DMS emissions from the polar oceans may remain relatively unchanged. By demonstrating that DMS emissions from geographically distinct regions may vary in their response to OA, our results may facilitate a better understanding of Earth’s future climate. Our study suggests that the way in which processes that generate DMS respond to OA may be regionally distinct, and this should be taken into account in predicting future DMS emissions and their influence on Earth’s climate.

Continue reading ‘A meta-analysis of microcosm experiments shows that dimethyl sulfide (DMS) production in polar waters is insensitive to ocean acidification’

Ocean freshening and acidification differentially influences mortality and behavior of the Antarctic amphipod Gondogeneia antarctica

Highlights

• Glacier retreat induced by global warming can decrease pH and salinity of the Antarctic ocean.

• The Antarctic amphipod Gondogeneia antarctica was exposed to low pH (7.6) and low salinity (27 psμ) conditions.

• Low pH increased mortality, impaired food detection, reduced shelter-use during daytime. .

• Low salinity increased cannibalism and induced abnormal swimming.

• Ocean acidification and freshening act as independent stressors influencing behavior and physiology of Antarctic amphipods.

Abstract

The Western Antarctic Peninsula (WAP) has experienced rapid atmospheric and ocean warming over the past few decades and many marine-terminating glaciers have considerably retreated. Glacial retreat is accompanied by fresh meltwater intrusion, which may result in the freshening and acidification of coastal waters. Marian Cove (MC), on King George Island in the WAP, undergoes one of the highest rates of glacial retreat. Intertidal and shallow subtidal waters are likely more susceptible to these processes, and sensitive biological responses are expected from the organisms inhabiting this area. The gammarid amphipod Gondogeneia antarctica is one of the most abundant species in the shallow, nearshore Antarctic waters, and it occupies an essential ecological niche in the coastal marine WAP ecosystem. In this study, we tested the sensitivity of G. antarctica to lowered salinity and pH by meltwater intrusion following glacial retreat. We exposed G. antarctica to four different treatments combining two salinities (34 and 27 psμ) and pH (8.0 and 7.6) levels for 26 days. Mortality, excluding cannibalized individuals, increased under low pH but decreased under low salinity conditions. Meanwhile, low salinity increased cannibalism, whereas low pH reduced food detection. Shelter use during the daytime decreased under each low salinity and pH condition, indicating that the two stressors act as disruptors of amphipod behavior. Under low salinity conditions, swimming increased during the daytime but decreased at night. Although interactions between low salinity and low pH were not observed during the experiment, the results suggest that each stressor, likely induced by glacial melting, causes altered behaviors in amphipods. These environmental factors may threaten population persistence in Marian Cove and possibly other similar glacial embayments.

Continue reading ‘Ocean freshening and acidification differentially influences mortality and behavior of the Antarctic amphipod Gondogeneia antarctica’


Subscribe to the RSS feed

Powered by FeedBurner

Follow AnneMarin on Twitter

Blog Stats

  • 1,356,928 hits

OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book