Microbialites, which are organosedimentary structures formed by microbial communities through binding and trapping and/or in situ precipitation, have a wide array of distinctive morphologies and long geologic record. The origin of morphological variability is hotly debated; elucidating the cause or causes of microfabric differences could provide insights into ecosystem functioning and biogeochemistry during much of Earth's history. Although rare today, morphologically distinct, co-occurring extant microbialites provide the opportunity to examine and compare microbial communities that may be responsible for establishing and modifying microbialite microfabrics. Highborne Cay, Bahamas, has extant laminated (i.e., stromatolites) and clotted (i.e., thrombolites) marine microbialites in close proximity, allowing focused questions about how community composition relates to physical attributes. Considerable knowledge exists about prokaryotic composition of microbialite mats (i.e., stromatolitic and thrombolitic mats), but little is known about their eukaryotic communities, especially regarding heterotrophic taxa. Thus, the heterotrophic eukaryotic communities of Highborne stromatolites and thrombolites were studied. Here, we show that diverse foraminiferal communities inhabit microbialite mat surfaces and subsurfaces; thecate foraminifera are relatively abundant in all microbialite types, especially thrombolitic mats; foraminifera stabilize grains in mats; and thecate reticulopod activities can impact stromatolitic mat lamination. Accordingly, and in light of foraminiferal impacts on modern microbialites, our results indicate that the microbialite fossil record may reflect the impact of the radiation of these protists.Neoproterozoic | geobiology | sedimentology | ooid

The responses of marine taxa to ocean acidification are varied, with, for example, some exhibiting decreased and some increased calcification rates. Experiments were conducted to assess the effect of elevated atmospheric carbon dioxide concentrations on the survival, fitness, shell microfabric and growth of Amphistegina gibbosa, a symbiont-bearing, coral-reef dwelling, benthic foraminiferal species that precipitates low-Mg calcite tests, using CO 2 partial pressure (pCO 2) levels similar to those likely to occur in shallow marine pore waters in the decades ahead. Specimens were cultured at constant temperature and controlled p CO 2 (ambient, 1000 parts per million by volume [ppmv], and 2000 ppmv) for 6 wk, and total alkalinity and dissolved inorganic carbon were measured every 2 wk to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using adenosine triphosphate analyses, and test microstructure and growth were evaluated using high resolution scanning electron microscopy and image analysis. Fitness and survival were not directly affected by elevated p CO 2 and the concomitant decrease in pH and calcite saturation states (Ω c). Test growth was not affected by elevated p CO 2. However, areas of dissolution were observed after 6 wk, even though Ω c was >1 in all treatments; the fraction of test area dissolved increased with decreasing Ω c. Test dissolution occurred only in small, well defined patches that appeared to be distributed randomly over the whole test surface. Similar dissolution was observed in offspring produced in the 2000 ppmv p CO 2 treatments. The long-term ecological consequences of the effects observed are not yet known.

We conducted experiments to assess the effect of elevated atmospheric carbon dioxide concentrations on survival, fitness, shell microfabric and growth of two species of symbiont-bearing coral-reef benthic foraminifera, using <i>p</i>CO₂ Ievels similar to those likely to occur in shallow marine pore waters in the decades ahead. Foraminifera were cultured at constant temperature and controlled <i>p</i>CO₂ (385 ppmv, 1000 ppmv, and 2000 ppmv) for six weeks, and total alkalinity and dissolved inorganic carbon were measured to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using Adenosine Triphosphate (ATP) analyses, and test microstructure and growth were evaluated using high resolution SEM and image analysis. Fitness and survival of <i>Amphistegina (A.) gibbosa</i> and <i>Archaias (A.) angulatus</i> were not directly affected by elevated <i>p</i>CO₂ and the concomitant decrease in pH and calcite saturation states (&Omega;_c values) of the seawater (pH and &Omega;_c values of 8.12, 7.86, and 7.50, and 5.4, 3.4, and 1.5, for control, 1000 ppmv, and 2000 ppmv, respectively). In <i>A. gibbosa</i>, a species precipitating low-Mg calcite, test growth was not affected by elevated <i>p</i>CO₂, but areas of dissolved calcium carbonate were observed even though &Omega;_c was >1 in all treatments; the fraction of test area dissolved increased with decreasing &Omega;_c. Similar dissolution was observed in offspring produced in the 2000 ppmv <i>p</i>CO₂ treatments. In <i>A. angulatus</i>, whose tests are more-solubile high-Mg calcite, growth was greatly diminished in the 2000 ppmv <i>p</i>CO₂ treatment compared to the control. These non-lethal effects of ocean acidification – reduced growth in <i>A. angulatus</i>, and enhanced dissolution in <i>A. gibbosa</i> – may reflect differences in test mineralogy for the two species; the long-term ecological consequences of these effects are not yet known

Specimens of Bolivina argentea and Bulimina marginata, two widely distributed temperate benthic foraminiferal species, were cultured at constant temperature and controlled pCO 2 (ambient, 1000 ppmv, and 2000 ppmv) for six weeks to assess the effect of elevated atmospheric CO 2 concentrations on survival and fitness using Adenosine Triphosphate (ATP) analyses and on shell microfabric using high-resolution SEM and image analysis. To characterize the carbonate chemistry of the incubation seawater, total alkalinity and dissolved inorganic carbon were measured approximately every two weeks. Survival and fitness were not directly affected by elevated pCO 2 and the concomitant decrease in seawater pH and calcite saturation states (Ω c ), even when seawater was undersaturated with respect to calcite. These

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Abstract. Insights into oceanographic environmental conditions such as paleoproductivity, deep-water temperatures, salinity, ice volumes, and nutrient cycling have all been obtained from geochemical analyses of biomineralized carbonate of marine organisms. However, we cannot fully understand geochemical proxy incorporation and the fidelity of such in species until we better understand fundamental aspects of their ecology such as where and when these (micro)organisms calcify. Here, we present an innovative method using osmotic pumps and the fluorescent marker calcein to help identify where and when calcareous meiofauna calcify in situ. Method development initially involved juvenile quahogs (Mercenaria mercenaria); subsequent method refinement involved a neritic benthic foraminiferal community. Future applications of this method will allow determining the in situ growth rate in calcareous organisms and provide insights about microhabitats where paleoceanographically relevant benthic foraminifera actually calcify.

Abstract. Insights into oceanographic environmental conditions such as paleoproductivity, sea-surface temperatures, deep-water temperatures, salinity, ice volumes, circulation patterns, and nutrient cycling have all been obtained from geochemical analyses of biomineralized carbonate of marine organisms. However, we cannot fully understand geochemical proxy incorporation and the fidelity of such in species until we better understand fundamental aspects of their ecology such as where and when these (micro)organisms calcify. Here, we present an innovative method using osmotic pumps and the fluorescent marker calcein to help identify where and when calcareous meiofauna calcify in situ. Method development initially involved juvenile quahogs (Mercenaria mercenaria); subsequent method refinement involved a neritic benthic foraminiferal community. Future applications of this method will allow determinations of in situ growth rate in calcareous organisms and provide insights about microhabitats where paleoceanographically relevant benthic foraminifera actually calcify.

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