Transcriptomic response to shell damage in the Antarctic clam, Laternula elliptica: time scales and spatial localisation

Mollusc shell is built up by secretion from the mantle and is the result of a controlled biological process termed biomineralisation. In general mollusc shells are well characterised however, the molecular mechanisms used by molluscs to produce shell remain largely unknown. One tractable method to study molecular biomineralisation mechanisms are shell damage-repair experiments, which stimulate calcification pathways. The present study used the Antarctic clam (Laternula elliptica) as a model to better understand when and where molecular biomineralisation events occur in the mantle. Two approaches were used: one experiment used high-throughput RNA-sequencing to study molecular damage-repair responses over a 2 month time series, and a second experiment used targeted semi-quantitative PCR to investigate the spatial location of molecular mechanisms in response to damage. Shell repair in L. elliptica was slow, lasting at least 2 months, and expression results revealed different biological processes were important at varying time scales during repair. A spatial pattern in relation to a single drilled hole was revealed for some, but not all, candidate genes suggesting the mantle may be functionally zoned and can respond to damage both locally and ubiquitously across the mantle. Valuable data on the temporal and spatial response of shell damage-repair provide a baseline not only for future studies in L. elliptica, but also other mollusc

Pteropod eggs released at high pCO2 lack resilience to ocean acidification

The effects of ocean acidification (OA) on the early recruitment of pteropods in the Scotia Sea, was investigated considering the process of spawning, quality of the spawned eggs and their capacity to develop. Maternal OA stress was induced on female pteropods (Limacina helicina antarctica) through exposure to present day pCO2 conditions and two potential future OA states (750 μatm and 1200 μatm). The eggs spawned from these females, both before and during their exposure to OA, were incubated themselves in this same range of conditions (embryonic OA stress). Maternal OA stress resulted in eggs with lower carbon content, while embryonic OA stress retarded development. The combination of maternal and embryonic OA stress reduced the percentage of eggs successfully reaching organogenesis by 80%. We propose that OA stress not only affects the somatic tissue of pteropods but also the functioning of their gonads. Corresponding in-situ sampling found that post-larval L. helicina antarctica concentrated around 600 m depth, which is deeper than previously assumed. A deeper distribution makes their exposure to waters undersaturated for aragonite more likely in the near future given that these waters are predicted to shoal from depth over the coming decades

Shifting ocean carbonate chemistry during the Eocene-Oligocene climate transition: implications for deep ocean Mg/Ca paleothermometry

To date, no conclusive evidence has been identified for intermediate or deep water cooling associated with the > 1 parts per thousand benthic delta O-18 increase at the Eocene-Oligocene transition (EOT) when large permanent ice sheets first appeared on Antarctica. Interpretation of this isotopic shift as purely ice volume change necessitates bipolar glaciation in the early Oligocene approaching that of the Last Glacial Maximum. To test this hypothesis, it is necessary to have knowledge about deep water temperature, which previous studies have attempted to reconstruct using benthic foraminiferal Mg/Ca ratios. However, it appears likely that contemporaneous changes in ocean carbonate chemistry compromised the Mg/Ca temperature sensitivity of benthic foraminifera at deep sites. New geochemical proxy records from a relatively shallow core, ODP Site 1263 (estimated paleodepth of 2100 m on the Walvis Ridge), reveal that carbonate chemistry change across the EOT was not limited to deep sites but extended well above the lysocline, critically limiting our ability to obtain reliable estimates of deep-ocean cooling during that time. Benthic Li/Ca measurements, used as a proxy for [CO32-], suggest that [CO32-] increased by similar to 29 mu mol/kg at Site 1263 across the EOT and likely impacted benthic foraminiferal Mg/Ca. A [CO32-]-benthic Mg/Ca relationship is most apparent during the early EOT when the overall increase in [CO32-] is interrupted by an apparent dissolution event. Planktonic d18O and Mg/Ca records suggest no change in thermocline temperature and a delta O-18(seawater) increase of up to 0.6 parts per thousand at this site across the EOT, consistent with previous estimates and supporting the absence of extensive bipolar glaciation in the early Oligocene

Millennial-scale ice-ocean-climate variability during the last glacial: high-resolution records from the NE Atlantic

Marine sediment core MDO1-2461 recovered from the European Margin, SW of Ireland (51 45' N, 12 55' W) at a water depth of 1153m provides material for multi-decadal to centennial scale investigation into ice-ocean-climate variability during the period 60 to 8 kyrs BP. Particular focus is placed on the oceanic and climatic conditions under which periodic collapse of the North American Laurentide ice sheet (LIS) occurred, so called Heinrich (H) events, and the involvement of the NW European ice sheets (NWEIS) within episodes of abrupt climate change. Presented here are records of circum-North Atlantic ice sheet growth, dynamics and decline from lithological and geochemical analysis of ice-rafted debris. Paired Mg/Ca and 8180 data from the surface dwelling Globigerina bulloides and subsurface dwelling Neogloboquadrina pachyderma sinistral are used to determine late-glacial variability of temperature, salinity and stratification of the upper water column, and benthic 813C records from C. wuellerstorfi documents the interchange of glacial northern and southern-sourced intermediate water signatures along the European Margin when compared to similar records from the Portuguese Margin and high-latitude North Atlantic basin. A picture emerges that the BIS was in a continuing state of readjustment and never fully reached steady state. Persistent 2 kyr instability of the BIS fluxes throughout the late glacial (26.5-10 kyr BP) suggests that 'precursor events' were not directly implicated in the collapse of the LIS. However, negative salinity excursions of up to 2.6 indicate significant incursions of meltwater associated with peaks in NWEIS instability, stratifying the upper water column. Such events are associated with reduced ventilation of intermediate waters, recorded in depleted epibenthic 813C suggesting that instability and meltwater forcing of the NWEIS temporarily weakened Glacial North Atlantic Intermediate Water formation, allowing transient advance of southern-sourced waters to the site of MDO1-2461. NWEIS-instability and associated perturbation to the North Atlantic thermohaline circulation may have set the stage for H event initiation regional cooling associated with a preliminary reduction in MOC may have promoted the growth of a LIS-fringing ice-shelf and subsequent subsurface warming and sea level rise may have destabilised the ice margin. Additionally, high-resolution summer sea surface temperature records based on the Mg/Ca G. bulloides suggest that mild, even warm summer months may be a have been a feature of H events

Sea Butterflies Defend Their Homes Against an Acidic Ocean

We all know that carbon dioxide (CO2) is produced from burning fossil fuels, and that it contributes to global warming. But have you heard about the “evil twin” of global warming, also caused by CO2–ocean acidification? The ocean absorbs about 30% of the CO2 that humans emit each year. As CO2 dissolves into the ocean, it forms carbonic acid, effectively making the ocean acidic. Animals that use a substance called calcium carbonate to build their shells and skeletons are vulnerable to acidic conditions, as their hard parts may dissolve. Microscopic swimming snails called pteropods or sea butterflies are common in the Southern Ocean. With incredibly delicate shells thinner than a human hair, pteropods are often considered to be the organisms most vulnerable to ocean acidification. Using microscopes and X-rays, we took extremely detailed images of these tiny shells and found that pteropods have a couple of clever tactics to defend their “homes” against ocean acidification

Evolutionary conservation and divergence of the transcriptional regulation of bivalve shell secretion across life history stages

Acknowledgements and Funding Information We are grateful to Andrew Gillis for use for paraffin histology and microscopy equipment and enthusiastic support of this work and the Rothera marine team for collecting the adult Laternula elliptica broodstock. This work was supported by UKRI Natural Environment Research Council (NERC) Core Funding to the British Antarctic Survey, a DTG Studentship (Project Reference: NE/J500173/1) and a Junior Research Fellowship to V.A.S. from Wolfson College, University of Cambridge. Review History: https://www.webofscience.com/api/gateway/wos/peer-review/10.1098/rsos.221022/Peer reviewedPublisher PD

Outer organic layer and internal repair mechanism protects pteropod Limacina helicina from ocean acidification

Scarred shells of polar pteropod Limacina helicina collected from the Greenland Sea in June 2012 reveal a history of damage, most likely failed predation, in earlier life stages. Evidence of shell fracture and subsequent re-growth is commonly observed in specimens recovered from the sub-Arctic and further afield. However, at one site within sea–ice on the Greenland shelf, shells that had been subject to mechanical damage were also found to exhibit considerable dissolution. It was evident that shell dissolution was localised to areas where the organic, periostracal sheet that covers the outer shell had been damaged at some earlier stage during the animal’s life. Where the periostracum remained intact, the shell appeared pristine with no sign of dissolution. Specimens which appeared to be pristine following collection were incubated for four days. Scarring of shells that received periostracal damage during collection only became evident in specimens that were incubated in waters undersaturated with respect to aragonite, ΩAr≤1. While the waters from which the damaged specimens were collected at the Greenland Sea sea–ice margin were not ΩAr≤1, the water column did exhibit the lowest ΩAr values observed in the Greenland and Barents Seas, and was likely to have approached ΩAr≤1 during the winter months. We demonstrate that L. helicina shells are only susceptible to dissolution where both the periostracum has been breached and the aragonite beneath the breach is exposed to waters of ΩAr≤1. Exposure of multiple layers of aragonite in areas of deep dissolution indicate that, as with many molluscs, L. helicina is able to patch up dissolution damage to the shell by secreting additional aragonite internally and maintain their shell. We conclude that, unless breached, the periostracum provides an effective shield for pteropod shells against dissolution in waters ΩAr≤1, and when dissolution does occur the animal has an effective means of self-repair. We suggest that future studies of pteropod shell condition are undertaken on specimens from which the periostracum has not been removed in preparation

"Live” (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: implications for calcification depths

It is widely held that benthic foraminifera exhibit species-specific calcification depth preferences, with their tests recording sediment pore water chemistry at that depth (i.e. stable isotope and trace metal compositions). This assumed depth habitat-specific pore water chemistry relationship has been used to reconstruct various palaeoenvironmental parameters, such as bottom water oxygenation. However, many deep-water foraminiferal studies show wide intra-species variation in sediment living depth but relatively narrow intra-species variation in stable isotope composition. To investigate this depth habitat- stable isotope relationship on the shelf we analysed depth distribution and stable isotopes of “living” (Rose Bengal stained) benthic foraminifera from two box cores collected on the South Georgia shelf (ranging from 250–300 m water depth). We provide a comprehensive taxonomic analysis of the benthic fauna, comprising 79 taxonomic groupings. The fauna shows close affinities with shelf assemblages from around Antarctica. We find “live” specimens of a number calcareous species from a range of depths in the sediment column. Stable isotope ratios (δ13C and δ18O) were measured on stained specimens of three species, Astrononion echolsi, Cassidulinoides porrectus and Buccella sp. 1, at 1 cm depth intervals within the down-core sediment sequences. In agreement with studies in deep water settings, we find no significant intraspecies variability in either δ13C foram or δ18O foram with sediment living depth on the South Georgia shelf. Our findings add to the growing evidence that infaunal benthic foraminiferal species calcify at a fixed depth. Given the wide range of depths that we find “living” ‘infaunal’ species, we speculate that they may actually calcify predominantly at the sediment-seawater interface, where carbonate ion concentration and organic carbon availability is at a maximum