Julie B. Schram

The Mg-Calcite Composition of Antarctic Echinoderms: Important Implications for Predicting the Impacts of Ocean Acidification

The Southern Ocean is considered to be the canary in the coal mine with respect to the first effects of ocean acidification (OA). This vulnerability is due to naturally low carbonate ion concentrations that result from the effect of low temperature on acid-base dissociation coefficients, from the high solubility of CO2 at low temperature, and from ocean mixing. Consequently, the two calcium carbonate polymorphs, aragonite and calcite, are expected to become undersaturated in the Southern Ocean within 50 and 100 years, respectively. Marine invertebrates such as echinoderms, whose skeletons are classified as high-magnesium carbonate (>4% mol MgCO3), are even more vulnerable to OA than organisms whose skeletons consist primarily of aragonite or calcite, with respect to both increased susceptibility to skeletal dissolution and further challenge to their production of skeletal elements. Currently, despite their critical importance to predicting the effects of OA, there is almost no information on the Mg-calcite composition of Antarctic echinoderms, a group known to be a major contributor to the global marine carbon cycle. Here we report the Mg-calcite compositions of 26 species of Antarctic echinoderms, representing four classes. As seen in tropical and temperate echinoderms, Mg-calcite levels varied with taxonomic class, with sea stars generally having the highest levels. When combined with published data for echinoderms from primarily temperate and tropical latitudes, our findings support the hypothesis that Mg-calcite level varies inversely with latitude. Sea stars and brittle stars, key players in Antarctic benthic communities, are likely to be the first echinoderms to be challenged by near-term OA.

Multi-frequency observations of seawater carbonate chemistry on the central coast of the western Antarctic Peninsula

Assessments of benthic coastal seawater carbonate chemistry in Antarctica are sparse. The studies have generally been short in duration, during the austral spring/summer, under sea ice, or offshore in ice-free water. Herein we present multi-frequency measurements for seawater collected from the shallow coastal benthos on a weekly schedule over one year (May 2012–May 2013), daily schedule over three months (March–May 2013) and semidiurnal schedule over five weeks (March–April 2013). A notable pH increase (max pH = 8.62) occurred in the late austral spring/summer (November–December 2012), coinciding with sea-ice break-out and subsequent increase in primary productivity. We detected semidiurnal variation in seawater pH with a maximum variation of 0.13 pH units during the day and 0.11 pH units during the night. Daily variation in pH is likely related to biological activity, consistent with previous research. We calculated the variation in dissolved inorganic carbon (DIC) over each seawater measurement frequency, focusing on the primary DIC drivers in the Palmer Station region. From this, we estimated net biological activity and found it accounts for the greatest variations in DIC. Our seasonal data suggest that this coastal region tends to act as a carbon dioxide source during austral winter months and as a strong sink during the summer. These data characterize present-day seawater carbonate chemistry and the extent to which these measures vary over multiple time scales. This information will inform future experiments designed to evaluate the vulnerability of coastal benthic Antarctic marine organisms to ocean acidification.

Trophic Transfer of Macroalgal Fatty Acids in Two Urchin Species: Digestion, Egestion, and Tissue Building

Sea urchins are ecosystem engineers of nearshore benthic communities because of their influence on the abundance and distribution of macroalgal species. Urchins are notoriously inefficient in assimilation of their macroalgal diets, so their fecal production can provide a nutritional subsidy to benthic consumers that cannot capture and handle large macroalgae. We studied the assimilation of macroalgal diets by urchins by analyzing the profiles of trophic biomarkers such as fatty acids (FAs). We tracked macroalgal diet assimilation in both Strongylocentrotus droebachiensis and S. purpuratus. Juvenile S. droebachiensis and adult S. purpuratus were maintained for 180 and 70 days, respectively, on one of three monoculture diets from three algal phyla: Nereocystis luetkeana, Pyropia sp., or Ulva sp. We then analyzed FA profiles of the macroalgal tissue fed to urchins as well as urchin gonad, gut, digesta, and egesta (feces) to directly evaluate trophic modification and compare nutritional quality of urchin food sources, urchin tissues, and fecal subsidies. In the S. purpuratus assay, there were significantly more total lipids in the digesta and egesta than in the algae consumed. The FA profiles of urchin tissues differed among urchin species, all diets, and tissue types. Despite these differences, we observed similar patterns in the relationships between the urchin and macroalgal tissues for both species. Egesta produced by urchins fed each of the three diets were depleted with respect to the concentration of important long chain polyunsaturated fatty acids (LCPUFAs), but did not differ significantly from the source alga consumed. Both urchin species were shown to synthesize and selectively retain both the precursor and resulting LCPUFAs involved in the synthesis of the LCPUFAs 20:4⍵6 and 20:5⍵3. S. droebachiensis and S. purpuratus exhibited consistent patterns in the respective depletion and retention of precursor FAs and resulting LCPUFAs of Pyropia and Ulva tissues, suggesting species level control of macroalgal digestion or differential tissue processing by gut microbiota. For both S. droebachiensis and S. purpuratus, macroalgal diet was a surprisingly strong driver of urchin tissue fatty acids; this indicates the potential of fatty acids for future quantitative trophic estimates of urchin assimilation of algal phyla in natural settings.