Temperature calibration and phylogenetically distinct distributions for freshwater alkenones: Evidence from northern Alaskan lakes

Abstract

Alkenones are a class of unsaturated long-chain ketone biomarkers that have been used to reconstruct sea surface temperature and, more recently, continental temperature, by way of alkenone unsaturation indices (e.g. and). Alkenones are frequently found in brackish and saline lakes, however species effects confound temperature reconstructions when multiple alkenone-producing species with different temperature responses are present. Interestingly, available genetic data indicate that numerous freshwater lakes host a distinct phylotype of alkenone-producing haptophyte algae (the Group I or Greenland phylotype), providing evidence that species effects may be diminished in freshwater lakes. These findings encourage further investigation of alkenone paleotemperature proxies in freshwater systems. Here, we investigated lakes from northern Alaska (n=35) and show that alkenones commonly occurred in freshwater lakes, where they featured distinct distributions, characterized by dominant C 37:4 alkenones and a series of triunsaturated alkenone isomers. The distributions were characteristic of Group I-type alkenone distributions previously identified in Greenland and North America. Our analysis of suspended particulate matter from Toolik Lake (68° 38'N, 149° 36'W) yielded the first in situ freshwater calibration (=0.021*T-0.68; r 2 =0.85; n=52; RMSE=±1.37 °C). We explored the environmental significance of the tri-unsaturated isomers using our northern Alaskan lakes dataset in conjunction with new data from haptophyte cultures and Canadian surface sediments. Our results show that these temperature-sensitive isomers are biomarkers for the Group I phylotype and indicators of multiple-species effects. Together, these findings highlight freshwater lakes as valuable targets for continental alkenone-based paleotemperature reconstructions and demonstrate the significance of the recently discovered tri-unsaturated isomers.

Next‐Generation Sequencing to Identify Lacustrine Haptophytes in the Canadian Prairies: Significance for Temperature Proxy Applications

“…Interestingly, Richmond and Deadmoose lakes each contained two main LCA‐producing haptophyte species, while the other studied lakes only contained a single dominant LCA‐haptophyte species. These results are consistent with previous studies that have shown that Group I haptophytes only occur in freshwater and oligohaline lakes, whereas Group II haptophytes occur in oligohaline to hyperhaline lakes (Longo et al, , ; Plancq, McColl, et al, ; Simon et al, ; Theroux et al, ; Toney et al, ). When looking at the LCA profiles (Figure ), OTU_21 is associated with a LCA profile with tri‐unsaturated isomers and the presence of C 38 Me LCAs, which has been shown to be characteristic of Group I haptophytes (Longo et al, , ).…”

“…Richmond Lake displayed a predominance of C 38 alkenones over C 37 alkenones, with particularly elevated concentrations of the C 38:3 alkenone. Finally, Lenore Lake showed the presence of tri‐unsaturated alkenone isomers, including the C 37:3b isomer, which has been shown to be characteristic of the Group I of LCA‐producing haptophytes (Dillon et al, ; Longo et al, , , ). Total concentrations of LCAs ranged from 12.9 to 2,184 μg/g dry sediment, with the highest concentrations (>1,000 μg/g dry sediment) recorded in Manitou and Middle lakes (Table ).…”

“…In principle, the presence of multiple species in a single lake complicates the use of the proxy, as the relationship between temperature and the LCA unsaturation index could differ between taxa (Randlett et al, ; Theroux et al, ; Toney et al, ). Lakes containing a single LCA‐producing species would thus be ideal for paleotemperature reconstructions since they would be immune to “species mixing effects” (Longo et al, ). Among our study lakes, each seems susceptible to such “species mixing effects,” even though Middle, Manitou, Dewey, and Lenore lakes each contain one predominant LCA‐producer, but several other OTUs.…”

Successional blooms of alkenone‐producing haptophytes in Lake George, North Dakota: Implications for continental paleoclimate reconstructions

“…Alkenone production is restricted to haptophyte algae in the order Isochrysidales and alkenone‐producing species have been categorized into three main groups based on phylogenetic relationships and commonly associated habitats (Theroux et al ; Longo et al ). Group I alkenone‐producing haptophytes (hereafter, Group I) have been found in fresh and oligohaline lakes (Theroux et al ; Simon et al ; Longo et al ), and are believed to only occupy environments with salinities lower than ~ 5 psu (Longo et al ). Group II alkenone‐producing haptophytes (Group II) are common in brackish and saline lakes and coastal environments, and Group III alkenone‐producing haptophytes (Group III) are common in open ocean marine environments (Theroux et al ; Longo et al ).…”

“…A narrow phylogenetic range in marine alkenone‐producers allows for the application of a universal calibration for sea surface temperature reconstructions, a calibration that was first developed empirically through culture work (Brassell et al ; Prahl and Wakeham ) and later validated with a global core‐top survey (Müller et al ; Conte et al ). For freshwater environments where Group I haptophytes have been identified, namely a series of arctic lakes in Greenland (D'Andrea et al ) and Alaska (Longo et al ), studies have found a uniform species occurrence that allows for a consistent temperature calibration to be applied in individual lakes. In Lake BrayaSø, Greenland, for example, an in situ calibration derived from a single haptophyte species matched a downcore alkenone profile spanning 6000 yr (Theroux et al , ; D'Andrea et al ).…”