Holcomb Michael

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  • Article
    Why corals care about ocean acidification : uncovering the mechanism
    (Oceanography Society, 2009-12) Cohen, Anne L. ; Holcomb, Michael
    Stony corals build hard skeletons of calcium carbonate (CaCO3) by combining calcium with carbonate ions derived, ultimately, from seawater. The concentration of carbonate ions relative to other carbonate species in seawater is rather low, so corals expend energy to raise the pH of seawater sequestered in an isolated, extracellular compartment where crystal growth occurs. This action converts plentiful bicarbonate ions to the carbonate ions required for calcification, allowing corals to produce CaCO3 about 100 times faster than it could otherwise form. It is this rapid and efficient production of CaCO3 crystals that enables corals to build coral reefs. Ocean acidification reduces the pH and thus the abundance of carbonate ions in seawater. Corals living in acidified seawater continue to produce CaCO3 and expend as much energy as their counterparts in normal seawater to raise the pH of the calcifying fluid. However, in acidified seawater, corals are unable to elevate the concentration of carbonate ions to the level required for normal skeletal growth. In several experiments, we found that boosting the energetic status of corals by enhanced heterotrophic feeding or moderate increases in inorganic nutrients helped to offset the negative impact of ocean acidification. However, this built-in defense is unlikely to benefit corals as levels of CO2 in the atmosphere continue to rise. Most climate models predict that the availability of inorganic nutrients and plankton in the surface waters where corals live will decrease as a consequence of global warming. Thus, corals and coral reefs may be significantly more vulnerable to ocean acidification than previously thought.
  • Preprint
    Factors affecting B/Ca ratios in synthetic aragonite
    ( 2015-09) Holcomb, Michael ; DeCarlo, Thomas M. ; Gaetani, Glenn A. ; McCulloch, Malcolm
    Measurements of B/Ca ratios in marine carbonates have been suggested to record seawater carbonate chemistry, however experimental calibration of such proxies based on inorganic partitioning remains limited. Here we conducted a series of synthetic aragonite precipitation experiments to evaluate the factors influencing the partitioning of B/Ca between aragonite and seawater. Our results indicate that the B/Ca ratio of synthetic aragonites depends primarily on the relative concentrations of borate and carbonate ions in the solution from which the aragonite precipitates; not on bicarbonate concentration as has been previously suggested. The influence of temperature was not significant over the range investigated (20 – 40°C), however, partitioning may be influenced by saturation state (and/or growth rate). Based on our experimental results, we suggest that aragonite B/Ca ratios can be utilized as a proxy of [CO32-]. Boron isotopic composition (δ11B) is an established pH proxy, thus B/Ca and δ11B together allow the full carbonate chemistry of the solution from which the aragonite precipitated to be calculated. To the extent that aragonite precipitation by marine organisms is affected by seawater chemistry, B/Ca may also prove useful in reconstructing seawater chemistry. A simplified boron purification protocol based on amberlite resin and the organic buffer TRIS is also described.
  • Preprint
    Cleaning and pre-treatment procedures for biogenic and synthetic calcium carbonate powders for determination of elemental and boron isotopic compositions
    ( 2015-01) Holcomb, Michael ; DeCarlo, Thomas M. ; Schoepf, Verena ; Dissard, Delphine ; Tanaka, Kiyoshi ; McCulloch, Malcolm
    In preparing calcium carbonate samples for the measurement of various geochemical proxies, it is often necessary to remove contaminating phases while leaving the phase of interest altered as little as possible. Here we evaluate the effects of some common cleaning protocols (rinsing (H2O), bleach (~3% NaOCl), hydrogen peroxide (30%), sodium hydroxide (0.006 – 0.1 M NaOH), and acid leaching (0.05 N HNO3)) on the elemental (Li, B, Na, Mg, Sr, Ba, Pb, and U) and boron isotope composition of both biogenic and synthetic calcium carbonates formed in marine environments. In untreated samples, the presence of elevated concentrations of Na and Mg, the most abundant cations in seawater, can be reduced with minimal cleaning (e.g. rinsing). Cleaning protocols that cause partial dissolution are problematic, especially for samples that are compositionally heterogeneous because the remaining sample may be biased towards particular phases with distinctive elemental or isotopic compositions. We show that the use of either acid or unbuffered hydrogen peroxide can lead to partial dissolution which was associated with an increase in the U/Ca ratio of the remaining sample. Bleaching or rinsing with water did not result in significant sample dissolution, suggesting these cleaning techniques may be safely used on heterogeneous samples. Cleaning treatments, other than those resulting in significant dissolution of heterogeneous samples, had no significant effect on δ11B, suggesting that boron isotopes are generally robust to the effects of sample pre-treatment.
  • Article
    Acidification alters the composition of ammonia‑oxidizing microbial assemblages in marine mesocosms
    (Inter-Research, 2013-10-31) Bowen, Jennifer L. ; Kearns, Patrick J. ; Holcomb, Michael ; Ward, Bess B.
    Increasing atmospheric CO2 concentrations are causing decreased pH over vast expanses of the ocean. This decreasing pH may alter biogeochemical cycling of carbon and nitrogen via the microbial process of nitrification, a key process that couples these cycles in the ocean, but which is often sensitive to acidic conditions. Recent reports have indicated a decrease in oceanic nitrification rates under experimentally lowered pH. How the composition and abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) assemblages respond to decreasing oceanic pH is unknown. We sampled microbes from 2 different acidification experiments and used a combination of qPCR and functional gene microarrays for the ammonia monooxygenase gene (amoA) to assess how acidification alters the structure of ammonia oxidizer assemblages. We show that despite widely different experimental conditions, acidification consistently altered the community composition of AOB by increasing the relative abundance of taxa related to the Nitrosomonas ureae clade. In one experiment, this increase was sufficient to cause an increase in the overall abundance of AOB. There were no systematic shifts in the community structure or abundance of AOA in either experiment. These different responses to acidification underscore the important role of microbial community structure in the resiliency of marine ecosystems.
  • Preprint
    An evaluation of staining techniques for marking daily growth in scleractinian corals
    ( 2012-12) Holcomb, Michael ; Cohen, Anne L. ; McCorkle, Daniel C.
    In situ skeletal markers have been widely used to quantify skeletal growth rates of scleractinian corals on sub-annual time-scales. Nevertheless, an evaluation of different techniques, both in terms of their efficacy and potential impacts on the growth process itself, has not been undertaken. Here the effects of exposure to four different dyes (alizarin, alizarin complexone, calcein, oxytetracycline) and isotope spikes (Ba and Sr) on the growth rates of scleractinian corals are compared. Oxytetracycline increased coral growth. Alizarin, alizarin complexone, calcein, and Sr and Ba isotope spikes had no significant effect on coral growth, but polyp extension appeared reduced during exposure to alizarin and alizarin complexone. Calcein provided a more intense fluorescent mark than either alizarin or alizarin complexone. Isotope spikes were challenging to locate using isotope ratio analysis techniques. Thus, calcein appears best suited for marking short-term calcification increments in corals, while a combination of alizarin or alizarin complexone and calcein may be useful for dual labeling experiments as there is little overlap in their fluorescence spectra.
  • Preprint
    Long-term effects of nutrient and CO2 enrichment on the temperate coral Astrangia poculata (Ellis and Solander, 1786)
    ( 2010-01-15) Holcomb, Michael ; McCorkle, Daniel C. ; Cohen, Anne L.
    Zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown under combinations of ambient and elevated nutrients (5 μM NO3 -, 0.3 μM PO4 -3, and 2 nM Fe+2) and CO2 (~780 ppmv) treatments for a period of 6 months. Coral calcification rates, estimated from buoyant weights, were not significantly affected by moderately elevated nutrients at ambient CO2 and were negatively affected by elevated CO2 at ambient nutrient levels. However, calcification by corals reared under elevated nutrients combined with elevated CO2 was not significantly different from that of corals reared under ambient conditions, suggesting that CO2 enrichment can lead to nutrient limitation in zooxanthellate corals. A conceptual model is proposed to explain how nutrients and CO2 interact to control zooxanthellate coral calcification. Nutrient limited corals are unable to utilize an increase in dissolved inorganic carbon (DIC) as nutrients are already limiting growth, thus the effect of elevated CO2 on saturation state drives the calcification response. Under nutrient replete conditions, corals may have the ability to utilize more DIC, thus the calcification response to CO2 becomes the product of a negative effect on saturation state and a positive effect on gross carbon fixation, depending upon which dominates, the calcification response can be either positive or negative. This may help explain how the range of coral responses found in different studies of ocean acidification can be obtained.
  • Article
    An investigation of the calcification response of the scleractinian coral Astrangia poculata to elevated pCO2 and the effects of nutrients, zooxanthellae and gender
    (Copernicus Publications on behalf of the European Geosciences Union, 2012-01-04) Holcomb, Michael ; Cohen, Anne L. ; McCorkle, Daniel C.
    The effects of nutrients and pCO2 on zooxanthellate and azooxanthellate colonies of the temperate scleractinian coral Astrangia poculata (Ellis and Solander, 1786) were investigated at two different temperatures (16 °C and 24 °C). Corals exposed to elevated pCO2 tended to have lower relative calcification rates, as estimated from changes in buoyant weights. Experimental nutrient enrichments had no significant effect nor did there appear to be any interaction between pCO2 and nutrients. Elevated pCO2 appeared to have a similar effect on coral calcification whether zooxanthellae were present or absent at 16 °C. However, at 24 °C, the interpretation of the results is complicated by a significant interaction between gender and pCO2 for spawning corals. At 16 °C, gamete release was not observed, and no gender differences in calcification rates were observed – female and male corals showed similar reductions in calcification rates in response to elevated CO2 (15% and 19% respectively). Corals grown at 24 °C spawned repeatedly and male and female corals exhibited two different growth rate patterns – female corals grown at 24 °C and exposed to CO2 had calcification rates 39% lower than females grown at ambient CO2, while males showed a non-significant decline of 5% under elevated CO2. The increased sensitivity of females to elevated pCO2 may reflect a greater investment of energy in reproduction (egg production) relative to males (sperm production). These results suggest that both gender and spawning are important factors in determining the sensitivity of corals to ocean acidification, and considering these factors in future research may be critical to predicting how the population structures of marine calcifiers will change in response to ocean acidification.
  • Thesis
    Coral calcification : insights from inorganic experiments and coral responses to environmental variables
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2010-02) Holcomb, Michael
    Coral calcification is examined using a laboratory model and living corals. In the laboratory model, abiogenic aragonite formed at high saturation state (Ω>~20) had a granular appearance and was enriched in trace elements, similar to centers of calcification and dark bands in corals. Abiogenic aragonite formed fibrous crystals at lower saturation states, similar to crystals which radiate out from centers of calcification. These similarities suggest the calcifying environment of the coral experiences a range of saturation states. To estimate when high or low saturation states occur within the coral, living corals were stained, staining patterns suggest dark bands form between dusk and dawn. A model is proposed in which daytime saturation state is limited by the availability of CO2. To test the potential for photosynthesis to limit CO2 availability to calcification, corals were grown under altered CO2 and nutrient levels. Elevated CO2 levels decreased calcification in zooxanthellate corals, however addition of nutrients reduced the negative impact of CO2. This suggests nutrient availability may limit photosynthesis under elevated pCO2 conditions. The effects of nutrients, CO2, and temperature were further tested on both zooxanthellate and azooxanthellate coral colonies. Unexpectedly, a gender difference was found in the effect of CO2 on calcification.