Alkenones

Long chain alkenones are a class of C35–C40 unsaturated ketones produced by members of the division Haptophyceae. The producer of alkenones in the marine environment is dominated by calcifying haptophytes Emiliania huxleyi and Gephyrocapsa oceanica in the open ocean (de Leeuw et al., 1980, Volkman et al., 1995) and by non-calcifying species Isochrysis galbana and Chrysotila lamellosa in the coastal marine setting (Marlowe et al., 1984). However, in lacustrine environments, the diversity of haptophytes producing alkenones varies according to their ecology and environment.

Alkenones are of interest because analysis of these compounds provides a method for quantitative reconstruction of past water temperature based on the degree of unsaturation in the ketone molecule (e.g. Prahl and Wakeham, 1987, Brassell et al., 1986, Herbert et al., 2003). The most abundant alkenones produced, are those with chain lengths of 37 and 38 carbon atoms with two, three or four double bonds (Table 1.1). The carbonyl functional group is either located at the second or third carbon in the chain. The relative abundances of C37:2, C37:3 and C37:4 alkenones within the synthesising haptophyte species changes with growth temperature (Brassell et al., 1986, Prahl and Wakeham, 1987). When the temperature is warmer, the relative abundance of more unsaturated alkenones decreases resulting in high values. This relationship has led to the formulation of the alkenone unsaturation index ( ; Equation 4a) by Brassel et al. (1986). Subsequent research showed that in the marine environments there was no empirical benefit to including the tetra-unsaturated ketone in the paleo-temperature equation because this alkenone is not abundant outside polar and sub-polar regions (Prahl and Wakeham, 1987). The Index

9

was suggested (Prahl and Wakeham, 1987) and has been widely adopted (Equation 4b). Calibration equations are used to convert and values into water temperatures resulting in quantitative reconstructed past water temperature.

Table 1-1: alkenones and their shorthand notation

Shorthand calibration makes it possible to reconstruct SST using marine sediments from any location in the world. Whereas the temperature calibration used in lacustrine settings depend on the alkenone-producer(s) present within the lake.

Although the alkenone producers in the marine environment are well constrained, the alkenone producer(s) in a given lake depends on their ecology and environment. In addition, there can be several alkenone-producers in one given lake (Theroux et al., 2010). Alkenone producers in lacustrine environments are similar to the haptophyte species Isochrysis galbana (Coolen et al., 2004a)

10

and Chrysotila lamellosa (Sun et al., 2007). In addition, several novel alkenone producers, which have not been fully classified taxonomically, have also been discovered (D'Andrea et al., 2006). Recently, there have been several key studies using environmental genomics to identify the alkenone-produce(s) in lakes (e.g., D'Andrea et al., 2006, Randlett et al., 2014) using the 18S molecular marker. The 18S molecular marker is present in all eukaryotes (Olsen et al., 1986, Woese, 1987) and therefore provides species level taxonomic resolution (Sogin et al., 1986, Edvardsen et al., 2000).

The identification of the alkenone-producer(s) in lakes prior to reconstructing temperature is important in order to chose the correct calibration as different alkenone-producers have the potential to produce different temperature calibrations, individual alkenone-containing lakes require unique calibration datasets dependent on species of haptophyte algae present (Chu et al., 2005, D'Andrea et al., 2006). Therefore prior to alkenone-based temperature reconstructions in lacustrine settings, it is important to identify the alkenone-prodcer(s) within the lake and determine the response of haptophyte species to temperature. There has been several studies which have successfully produced temperature calibrations for lakes based on a number of different calibration methods. The main site-specific temperature calibration methods are; examining surface sediments (Zink et al., 2001; Chu et al., 2005), isolation and culturing of alkenone producer(s) to create an experimental calibration (Sun et al., 2007) and also filtering water column samples over an extended time period (Toney et al., 2010; D’Andrea et al., 2011) to produce an in-situ calibration. Zink et al (2001) established a temperature calibration by examining surface sediments from a transect of lakes in Germany. During this study the best correlation existed between the U^K’37 Index and the summer average lake surface temperature (r²

=0.90). This calibration approach makes large assumptions that all lakes sampled contain the same haptophyte species or that different species have the same relationship to temperature. Chu et al (2005) also adopted this calibration method and examined 37 surface sediments of lakes in China. This study found a good

11

relationship between temperature and alkenone signature. They found a significant relationship between the U^K37 Index and water temperature (r²= 0.75) but in contrast to other studies, they found no relationship between U^K’37 and water temperature (r² = 0.14). D’Andrea et al (2011) developed a temperature calibration for Lakes in Kangerlussuaq, West Greenland based on U^K37 of filtered alkenones and in situ temperature. The combined the results with Zink et al (2001) to extend the narrow temperature range. On the other hand, Sun et al (2007) collected and isolated Chrysotila lamellosa from an inland saline Lake in China. They created a culture-based calibration using the U^K’37 Index that correlated with growth temperature varying from 10°C to 22°C.

Although it is evident that alkenones produced by haptophytes have a unique relationship with temperature, it is still not known why haptophytes produce alkenones and hence why the variations in alkenone ratios are related to temperature. Some early studies have suggested that alkenones are membrane lipids, which are produced for fluidity and rigidity (Prahl et al., 1988). This would also explain why the unsaturation ratio of alkenones are related to temperature; e.g. increased unsaturation at lower temperatures would decrease the melting point of the lipids and hence keep membranes fluid. However, other studies have suggested that alkenones are not membrane lipids and instead are produced for buoyancy (Fernández et al., 1994, Epstein et al., 2001) or energy storage (Pond and Harris, 1996, Epstein et al., 2001, Eltgroth et al., 2005). On the other hand, the function of alkenones may not be related to the relationship between temperature and unsaturation. Instead, the temperature dependence of alkenone unsaturation may reflect different biochemical pathways (Epstein et al., 2001). Epstein et al. (2001) suggested that the synthesis and degradation of alkenones may require different enzymes with different temperature optima.