What makes polar marine species amongst the most threatened by future climate change.
The polar regions are becoming ‘hot-spots’ of global-warming, with sea-surface temperatures having risen 1°C around the Western-Antarctic-Peninsula. Polar marine-species represent a unique community that has evolved in harsh ocean-environments; the nature of conditions varies between the Northern-and Southern sea-beds, yet both are characterised by cold-extremes resulting in similar adaptations. In many ways, it is the species’ adaptation to their environments which make them vulnerable to climate-change; especially their adaptation to relative stability. The increasingly isolated nature of Antarctica means that these species are even less-adapted to change compared to Arctic-species. Climate-change poses two main challenges for polar-marine organisms; temperature changes and ocean-acidification.
The responses are limited to genetic-evolution and phenotypic-plasticity as migration of native-species is not viable due to the relative-isolation and geographical-position. In this essay I suggest that the aspects making polar-marine species some of the most threatened by climate-change are mostly biological; their evolutionary-trajectory has resulted in poor-adaptive-capacities due to long-periods in a stable-environment. There has been no selection for traits which confer adaptability, thus in the face of environmental-change they are particularly vulnerable.
Here, I consider particular-adaptations in turn, demonstrating how each results in maladaptive-outcomes in the context of climate-change, these include: reproduction-metabolism-seasonality and cellular-responses. I also consider non-analogue environmental-changes as factors enhancing the threat. This discussion focusses-mainly on the Southern-sea-bed examples as adaptations and associated-vulnerabilities are more pronounced; many of these are applicable for Northern-polar-marine-species. Whilst both polar-regions contain species highly threatened by climate change, the threats themselves vary; in the Arctic the dominating threat is invasion and trophic cascades, whereas in Antarctica, the lack of adaptive-capacity to higher-temperatures threatens species more-significantly.
Ocean-acidification
Studies on the impacts of ocean-acidification on Antarctic-species is limited, however, it is indicated that there is a better-than-predicted response to lower-pH-conditions. Where long-term and multi-generational studies have been conducted, conclusions of progressive-adaptation and positive impacts of parental-acclimation have been found. In this respect, polar-marine-species are not considered highly-threatened. Nevertheless, the impacts of warming on both-biology and environment suggests a significantly reduced adaptive-capacity.
Reproduction|Development
The reproductive strategies of Antarctic marine species make them vulnerable to climate change due slow development and relatively few offspring; thus, reducing the potential rate for genetic-evolution. In Antarctica, large egg-size and reduced egg-numbers have been observed in a number of marine-species, including amphipod crustaceans and shrimps. Increasing data on fish also shows a trend from smaller, pelagic eggs at low latitudes to bigger, demersal eggs at higher latitudes(Leis-et-al-(2013). These trends are associated with the gradient in marine-productivity, and the energetic costs of egg production, previously-related to pelagic-changes in Thorson’s-rule, disproved with the advent of taxon-data. Nevertheless, Antarctic marine species tend to have fewer offspring and this pattern limits the rate at which they are able to evolve genetically. This feature makes them increasingly vulnerable to climactic-change.
In addition to long generation-times, increased temperatures threaten the dispersal potential of larvae because higher temperatures cause the larval-development-stage to be passed through more rapidly. The consequences of reduced-dispersal are species-specific due to varied development-strategies. In the example of Sterechinus neumayeri an increase in temperature from −2°Cto+0.5°C reduces the time from fertilisation to settlement from-120-to<90-days. These changes are substantial however, full elucidation of the impacts requires further investigation. It is nevertheless clear that changes in marine-temperature will impact dispersal and perhaps overall-distribution of certain species.
Cellular/Molecular
A number of cellular-adaptations which evolved to benefit marine-species in the extreme-cold of Antarctic-environments have been shown to negatively-impact the potential of species to acclimate in warmer-conditions. A prominent cellular-adaptation to cold environments is the increased lipid content in cell-membranes, an adaptation conferring membrane fluidity to maintain the structural/functional-integrity of membrane-proteins/ionic-transport at low temperatures. When warmed, these adaptations have negative-implications; one study finds that in two species of notothenioids, over a period of warming there is no plastic-response in the cell-membrane. Resoonse to warming in this way leads to failures including un-controlled membrane-fluidity with wide-ranging impacts on cellular functions such as ion. impacted. In this way, cellular-adaptations to cold limit the ability of marine-species to adapt to warming.
Enzyme functions showing adaptation to cold are thermally-limited in a similar way to membranes. To combat the high-oxygenation-levels, antioxidant enzymes are needed; optimization of these enzymes requires a a trade-off with their thermal-stability. Recent studies show that responses vary between species. One example where the mechanism is better elucidated is the species Aequiyoldia eightsii, in which a cytotoxic chemical O-propionyl-carnitine is produced as part of the antioxidant-inducing pathway. Whilst this adaptation confers protection against lipid peroxidation increasing temperatures cause this trait to become mal-adaptive (Sayed-Ahmed-et-al-(2001)).
A further example of cellular-level changes contributing to maladaptive-traits in climate-change conditions is the presence of step-wise genetic changes. The presence of step-changes has been facilitated by the prolonged-periods of stability, where there has been no fitness-cost to lacking plasticity or adaptive-capacity. A trait-example of this is the loss of globin-genes; due to high oxygen-levels in the water and low-metabolic demands for oxygen, the selective-pressure for erythrocytes is relaxed. Contemporaneously, erythrocytes/oxygen-carriers pose a disadvantage as they increase blood-viscosity, increasing energetic-costs for blood-transport. In 7/8 families of Nototheniodei, globin-genes have been reduced significantly. This step-change poses a challenge to thermal-adaptation as re-gaining genes requires longer time-scales than is permitted by current rates of ocean-warming. The lack of heat-shock response due to loss of chaperone-protein-expression is an additional stepped-change which limits the ability of species to respond to stress-factors, including thermal-challenges.
An additional molecular-adaptation to the cold is antifreeze-compounds, evolved from a pancreatic-trypsinogen-serine-protease-progenitor (structurally similar antifreeze-glycoproteins also found in Arctic-gadoids, arising from a conergent evolutionary-route). Biosynthesis of this compound is energetically-expensive, hence at higher temperatures will become maladaptive. Antarctic-Notothenioids synthesise antifreeze continuously; Arctic-fish exhibit seasonal patterns of antifreeze-biosynthesis. This is an efficient energy-saving strategy, that avoids costly biosynthesis when freezing is not a danger. These findings suggests that even a small increase in environmental temperatures will not pass unnoticed in biosynthesis control which is not viable in warmer conditions.
Resources/Seasonality
Climate change is predicted to impact the metabolic energy-costs of the overwinter-period in marine-systems. If the stored resources are insufficient, winter survival may be compromised. The majority of Antarctic-primary-consumers feed during the summer phytoplankton-bloom, which occurs for a short period annually. In response to the low-food-availability in winter, many Antarctic-species stop feeding and reduce metabolic-rates, sometimes to a hypometabolic-state, observed in molluscs, fish and starfish. This seasonality of primary-production is driven by light/nutrient availability, which is unlikely to change due to global-warming. Increased temperature also increases the rate at which digestive enzymes function, thus the rate at which food is processed; this leads to a scenario where marine-species are vulnerable to starvation during the winter due to increased metabolic costs in warmer waters.
Metabolism
Marine-polar species have limited tolerance of changes in water-oxygen content, making species vulnerable to thermal increases due to the impacts on oxygen%. This progressive loss in capacity for aerobic activity with temperature aligns with predictions from the oxygen-and-capacity-limited-thermal-tolerance(OCLTT) hypothesis (Figure 1). Clarke-et-al-(2017) suggest that there is no-uniform mechanism on which the upper thermal-tolerance is dependant. The temperature limits vary with the stage of the organism, and the most vulnerable stage is also species dependent. Studies also suggest that the time over which temperature change occurs impacts the response; although there is evidence of adaptation over time, it has also been shown that the accumulation of negative impacts caused by increased temperatures can decrease the ability of a species to adapt over time. These species-specific and complex results require further long-term-studies to elucidate the processes, but it is evident that OCLTT contributes to polar-marine-species’ sensitivity to climate-change.
Non-analogue environmental-change
Additional to biological-components contributing to the vulnerability of polar-marine-species, the nature of environmental-change in response to climate-change. This includes the secondary impacts of marine ice loss such as primary productivity timings as well as increased scour of the seabed due to collisions of ice-bergs. These complex interactions are not fully understood, evidence suggests that the polar-ecosystems are likely to be impacted by tipping points (Planetary-boundaries-Steffan-et-al-2009)causing un-predictable, rapid-change, with wide-ranging consequences for marine-species. The creation of models combined with robust-monitoring-systems will facilitate improved understanding of impacts in the long-term.
Having reviewed environmental and biological aspects which make polar-marine-species vulnerable to climate-change, it is clear that prolonged-stability has made species poorly suited to environmental-change. The capacity to adapt to warming-conditions is similar to that of tropical-species, where conditions are similarly-stable, however, tropical-species have the ability to migrate, and this is not a viable option for polar-marine-species. Furthermore, the period required to re-set their physiology following changes in temperature requires much-longer than tropical or temperate-species. This indicates that their ability to cope using phenotypic-plasticity is limited, compounded by long generation-times and fewer-eggs, reducing the ability to adapt-genetically. Anthropogenic influences, including the potential-dissolution of the Antarctic-Treaty, will contribute to threats in these regions, and should considered-parallel to biological/environmental-threats. Considering the uncertainty about future changes in the marine-environment, robust predictions of species responses are not yet possible; in this regard, there is a need for more long-term studies which incorporate transgenerational-effects in the context of ecosystems instead of isolated species. This knowledge is vital to inform future conservation-decisions.
