Soil characteristics and Collembola and Acari abundance in control and warming plots at Abisco Research Station

Extreme weather events can have negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (2-10 °C for 2-14 days), but returning to cold winter climate exposes the ecosystem to lower temperatures by the loss of insulating snow. Soil animals, which play an integral part in soil processes, may be very susceptible to such events depending on the intensity of soil warming and low temperatures following these events. We simulated week-long extreme winter warming events - using infrared heating lamps, alone or with soil warming cables - for two consecutive years in a sub-Arctic dwarf shrub heathland. Minimum temperatures were lower and freeze-thaw cycles were 2-11 times more frequent in treatment plots compared with control plots. Following the second event, Acari populations decreased by 39%; primarily driven by declines of Prostigmata (69%) and the Mesostigmatic nymphs (74%). A community-weighted vertical stratification shift occurred from smaller soil dwelling (eu-edaphic) Collembola species dominance to larger litter dwelling (hemi-edaphic) species dominance in the canopy-with-soil warming plots compared with controls. The most susceptible groups to these winter warming events were the smallest individuals (Prostigmata and eu-edaphic Collembola). This was not apparent from abundance data at the Collembola taxon level, indicating that life forms and species traits play a major role in community assembly following extreme events. The observed shift in soil community can cascade down to the micro-flora affecting plant productivity and mineralization rates. Short-term extreme weather events have the potential to shift community composition through trait composition with potentially large consequences for ecosystem development.

Seawater carbonate chemistry and buffer capacity of the coelomic fluid in echinoderms in a laboratory experiment

The increase in atmospheric CO2 due to anthropogenic activity results in an acidification of the surface waters of the oceans. The impact of these chemical changes depends on the considered organisms. In particular, it depends on the ability of the organism to control the pH of its inner fluids. Among echinoderms, this ability seems to differ significantly according to species or taxa. In the present paper, we investigated the buffer capacity of the coelomic fluid in different echinoderm taxa as well as factors modifying this capacity. Euechinoidea (sea urchins except Cidaroidea) present a very high buffer capacity of the coelomic fluid (from 0.8 to 1.8 mmol/kg SW above that of seawater), while Cidaroidea (other sea urchins), starfish and holothurians have a significantly lower one (from -0.1 to 0.4 mmol/kg SW compared to seawater). We hypothesize that this is linked to the more efficient gas exchange structures present in the three last taxa, whereas Euechinoidea evolved specific buffer systems to compensate lower gas exchange abilities. The constituents of the buffer capacity and the factors influencing it were investigated in the sea urchin Paracentrotus lividus and the starfish Asterias rubens. Buffer capacity is primarily due to the bicarbonate buffer system of seawater (representing about 63% for sea urchins and 92% for starfish). It is also partly due to coelomocytes present in the coelomic fluid (around 8% for both) and, in P. lividus only, a compound of an apparent size larger than 3 kDa is involved (about 15%). Feeding increased the buffer capacity in P. lividus (to a difference with seawater of about 2.3 mmol/kg SW compared to unfed ones who showed a difference of about 0.5 mmol/kg SW) but not in A. rubens (difference with seawater of about 0.2 for both conditions). In P. lividus, decreased seawater pH induced an increase of the buffer capacity of individuals maintained at pH 7.7 to about twice that of the control individuals and, for those at pH 7.4, about three times. This allowed a partial compensation of the coelomic fluid pH for individuals maintained at pH 7.7 but not for those at pH 7.4.

Effect of increased pCO2 level on early shell development in great scallop (Pecten maximus Lamarck) larvae

As a result of high anthropogenic CO2 emissions, the concentration of CO2 in the oceans has increased, causing a decrease in pH, known as ocean acidification (OA). Numerous studies have shown negative effects on marine invertebrates, and also that the early life stages are the most sensitive to OA. We studied the effects of OA on embryos and unfed larvae of the great scallop (Pecten maximus Lamarck), at pCO(2) levels of 469 (ambient), 807, 1164, and 1599 µatm until seven days after fertilization. To our knowledge, this is the first study on OA effects on larvae of this species. A drop in pCO(2) level the first 12 h was observed in the elevated pCO(2) groups due to a discontinuation in water flow to avoid escape of embryos. When the flow was restarted, pCO(2) level stabilized and was significantly different between all groups. OA affected both survival and shell growth negatively after seven days. Survival was reduced from 45% in the ambient group to 12% in the highest pCO(2) group. Shell length and height were reduced by 8 and 15 %, respectively, when pCO(2) increased from ambient to 1599 µatm. Development of normal hinges was negatively affected by elevated pCO(2) levels in both trochophore larvae after two days and veliger larvae after seven days. After seven days, deformities in the shell hinge were more connected to elevated pCO(2) levels than deformities in the shell edge. Embryos stained with calcein showed fluorescence in the newly formed shell area, indicating calcification of the shell at the early trochophore stage between one and two days after fertilization. Our results show that P. maximus embryos and early larvae may be negatively affected by elevated pCO(2) levels within the range of what is projected towards year 2250, although the initial drop in pCO(2) level may have overestimated the effect of the highest pCO(2) levels. Future work should focus on long-term effects on this species from hatching, throughout the larval stages, and further into the juvenile and adult stages.

Experimental ocean acidification alters the allocation of metabolic energy

Energy is required to maintain physiological homeostasis in response to environmental change. Although responses to environmental stressors frequently are assumed to involve high metabolic costs, the biochemical bases of actual energy demands are rarely quantified. We studied the impact of a near-future scenario of ocean acidification [800 µatm partial pressure of CO2 (pCO2)] during the development and growth of an important model organism in developmental and environmental biology, the sea urchin Strongylocentrotus purpuratus. Size, metabolic rate, biochemical content, and gene expression were not different in larvae growing under control and seawater acidification treatments. Measurements limited to those levels of biological analysis did not reveal the biochemical mechanisms of response to ocean acidification that occurred at the cellular level. In vivo rates of protein synthesis and ion transport increased 50% under acidification. Importantly, the in vivo physiological increases in ion transport were not predicted from total enzyme activity or gene expression. Under acidification, the increased rates of protein synthesis and ion transport that were sustained in growing larvae collectively accounted for the majority of available ATP (84%). In contrast, embryos and prefeeding and unfed larvae in control treatments allocated on average only 40% of ATP to these same two processes. Understanding the biochemical strategies for accommodating increases in metabolic energy demand and their biological limitations can serve as a quantitative basis for assessing sublethal effects of global change. Variation in the ability to allocate ATP differentially among essential functions may be a key basis of resilience to ocean acidification and other compounding environmental stressors.

Threatened Caribbean coral is able to mitigate the adverse effects of ocean acidification on calcification by increasing feeding rate

Global climate change threatens coral growth and reef ecosystem health via ocean warming and ocean acidification (OA). Whereas the negative impacts of these stressors are increasingly well-documented, studies identifying pathways to resilience are still poorly understood. Heterotrophy has been shown to help corals experiencing decreases in growth due to either thermal or OA stress; however, the mechanism by which it mitigates these decreases remains unclear. This study tested the ability of coral heterotrophy to mitigate reductions in growth due to climate change stress in the critically endangered Caribbean coral Acropora cervicornis via changes in feeding rate and lipid content. Corals were either fed or unfed and exposed to elevated temperature (30°C), enriched pCO2 (800 ppm), or both (30°C/800 ppm) as compared to a control (26°C/390 ppm) for 8 weeks. Feeding rate and lipid content both increased in corals experiencing OA vs. present-day conditions, and were significantly correlated. Fed corals were able to maintain ambient growth rates at both elevated temperature and elevated CO2, while unfed corals experienced significant decreases in growth with respect to fed conspecifics. Our results show for the first time that a threatened coral species can buffer OA-reduced calcification by increasing feeding rates and lipid content.

Seawater carbonate chemistry and skeletal development, size, weight, total lipid and symbiont density of coral Favia fragum in a laboratory experiment

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO3) production of framework-building corals thus reducing the amount of CaCO3 the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 µatm) or significantly elevated (1,311 µatm) CO2 conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO2 condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO2 level, fed corals accreted more CaCO3 than unfed corals, and fed corals reared under 1,311 µatm CO2 accreted as much CaCO3 as unfed corals reared under ambient CO2. However, feeding did not alter the sensitivity of calcification to increased CO2; Delta calcification/Delta Omega was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO3 production under OA than those in nutritionally limited environments.