Developing and applying new proxies based on organic material for paleoceanographical and paleoclimatological reconstructions.
Microbial ecology; linking “genotypes” with “phenotypes” in situ through the use of organism specific organic compounds and their stable carbon, hydrogen and/or nitrogen isotope ratios.
• Isolating and cultivating (relevant) organisms and studying their lipid content and isotope fractionation patterns in vitro.
• Studying organic compounds and stable isotopic fractionation in natural ecosystems and relating this to culture studies and molecular information.
• Labeling experiments using stable isotopes to study organism and ecosystem functioning.
July 2014- present: Scientist NIOZ
May 2010-June 2014: Tenure Track Scientist NIOZ May 2009-May 2010: Post-doc NIOZ February 2006-February 2009: Post-doc NIOZ January 2005-April 2005: Post-doc NIOZ August 2001-August 2004: Post-doc Montana State University April 1997-April 2001: PhD student NIOZ
How salty was the sea? A crucial question to predict climate change.
Testing climate models for future climate change critically depend on our ability to quantitatively reconstruct past climate. Paleosalinity is the single most important oceanographic parameter which currently can still not be accurately quantified from sedimentary records. To date, the most promising tool to estimate paleosalinity variations combines reconstructions of paleotemperature and foraminiferal δ18O. Foraminiferal δ18O varies as a function of temperature and ambient seawater δ18O which is directly coupled to seawater salinity. The close relation between the stable hydrogen isotope 2H (deuterium, D) and δ18O in precipitation and seawater (so-called meteoric water line) enables an alternative approach to deconvolve palaeosalinity. Deuterium is incorporated into marine organic matter during photosynthesis and can be extracted from seafloor sediments. Thus, δD analyses on marine organic matter could provide an alternative proxy for seawater palaeosalinity.
Paul (2002) and Englebrecht and Sachs (2005) reported constant fractionations of ~225-232‰ for alkenones from batch cultures of Emiliania huxleyi. In contrast, for this species and another common oceanic haptophyte alga, Gephyrocapsa oceanica, cultured at different salinities and temperatures, hydrogen isotope fractionation was found to depend on salinity and, to some degree, on growth rate (Schouten et al., 2006) and growth phase (Wolhowe et al., 2009). Thus, the δD of C37 alkenones of E. huxleyi largely depends on salinity and the δD of water, which in itself is correlated to salinity in the natural environment (see above).
We recently cultivated a coastal alkenone-producing species, Isochrysis galbana, and again found a strong positive linear correlation between the fractionation factor α, between the C37 alkenones and the culture water, and salinity with approximately 0.002 change in fractionation per salinity unit (Fig. 1; M’Boule et al., 2014). However, although the different haptophyte species show a similar response to salinity change, i.e. the slopes of the α-salinity relationship are similar (0.0019 and 0.0022), the absolute fractionation is different as indicated by the different intercepts (0.835 and 0.739 for I. galbana and E. huxleyi, respectively).
Initial applications of the δD of C37 alkenones to sedimentary material revealed their potential to reconstruct salinity. At the onset of the deposition of sapropel S5 in the Eastern Mediterranean a large and abrupt shift towards more negative values in δDalkenones suggests a freshening of approximately 6, from a salinity of 39 to 33 (van der Meer et al., 2007). In the Black Sea the δD record of C37 alkenones suggests a substantial freshening for the last the last 3000 years (van der Meer et al., 2008; Giosan et al., 2012). Finally, we recently obtained evidence that the hydrogen isotopic composition of alkenones can be used to estimate paleo sea surface salinity shifts in open ocean systems (Kasper et al., 2014). We observed a shift to more negative δDalkenone values of approximately 14‰ during glacial Termination I and approximately 13‰ during Termination II. Approximately half of these shifts can be attributed to the change in global ice volume, while the residual isotope shift is attributed to changes in salinity, suggesting relatively high salinities at the core site during glacials with subsequent freshening during glacial terminations. Rough estimates suggests that δDalkenone changes represent a freshening of ca. 1.7-2 during Termination I and ca. 1.5-1.7 during Termination II.
Thus far, our results indicate that the δD of alkenones of different haptophytes are all sensitive to salinity changes and that they apparently record salinity shifts even in open ocean settings. Nevertheless, there are a number of challenges in improving the reliability of this novel proxy. Although the slopes of the different α-salinity relationships for the different species are similar, absolute fractionation may be species-specific (e.g. Fig. 1). Furthermore, growth rate and growth phase are also known to impact the δD of long chain alkenones (Schouten et al., 2006; Wolhowe et al., 2009), potentially offsetting the salinity signal in the δD of alkenones. Schwab and Sachs (2011) recently showed that the hydrogen isotope ratios for the C37:2 and C37:3 alkenones in suspended particulate matter from the Chesapeake Bay estuary seem to depend only on the hydrogen isotopic composition of the water without the additional salinity-depended biological fractionation, potentially due to a shift in alkenone producing haptophyte species along this salinity gradient (M’Boule et al., 2014).
People involved: Gabriella Weiss (PhD)
Funding: Netherlands Earth System Science Centre (NESSC), GW) NWO Innovational Research Incentives Scheme VIDI and NWO Medium Sized Investments (GC-TC-irMS).
Developing and validating a new culture independent method to assess the core metabolisms of microorganisms in situ based on the hydrogen isotopic composition of their membrane lipids.
Microorganisms are involved in all elemental cycles and therefore it is important to study their metabolism in the natural environment. A recent technique to investigate this is the hydrogen isotopic composition of microbial fatty acids, i.e. heterotrophic microorganisms produce fatty acids enriched in deuterium (D) while photoautotrophic and chemoautotrophic microorganisms produce fatty acids depleted in D compared to the water in the culture medium (growth water). However, the impact of factors other than metabolism have not been investigated. Here, we evaluate the impact of growth phase compared to metabolism on the hydrogen isotopic composition of fatty acids of different environmentally relevant microorganisms with heterotrophic, photoautotrophic and chemoautotrophic metabolisms. Fatty acids produced by heterotrophs are enriched in D compared to growth water with εlipid/water between 82 ‰ and 359 ‰ when grown on glucose or acetate, respectively. Photoautrophs (εlipid/water between -149 ‰ and -264 ‰) and chemoautotrophs (εlipid/water between -217 ‰ and -275 ‰) produce fatty acids depleted in D. Fatty acids become, in general, enriched by between 4 and 46 ‰ with growth phase which is minor compared to the influence of different metabolisms. Therefore, the D/H ratio of fatty acids is a promising tool to investigate community metabolism in nature.
Figure 2. Box plots of D/H fractionations between the C16:0 fatty acid and culture medium observed in different culture experiments. Cultures included from this study are Thiocapsa roseopersicina, Halochromatium glycolicum, Isochrysis galbana, Thiobacillus denitrificans and Pseudomonas sp.. Additionally, published data for Isochrysis galbana, Ascophyllum sp., Alexandrium fundyense, Methylococcus capsulatus, Saragassum filicinum, Undararia pinnatifida, Binghamia californica, Gelidium japonica, Sporomusa sp., Botrycoccus braunii, Eudorina unicocca, Volvox aureus, Desulfobacterium autotrophicum, Cupriavidus oxalaticus, Cupriavidus necator, Escherichia coli, Rhodopseudomonas palustris, Tetrahymena thermophile and Moritella japonica DSK 1 have been included (Sessions et al., 1999; Sessions et al., 2002; Chikaraishi et al., 2004; Valentine et al., 2004; Zhang and Sachs, 2007; Campbell et al., 2009; Zhang et al., 2009a; Dirghangi and Pagani, 2013; Fang et al., 2014). From Heinzelmann et al. (2015) Frontiers in Microbiology http://dx.doi.org/10.3389/fmicb.2015.00408.
Funding: NIOZ internal competition (SH) and NWO Medium Sized Investments (GC-TC-irMS).