Culturing-based Temperature Calibration of a Genetically Distinct, Alkenone-producing Haptophyte Species isolated from Lake George, ND

Abstract:

Lacustrine alkenones are rapidly becoming an important tool for continental paleoclimate reconstructions. However, DNA sequencing of 18S ribosomal RNA marker genes has uncovered multiple species of haptophytes in different lakes. To date, there are only two isolated lacustrine species Chrysotila lamellosa and Isochrysis galbana available for culture studies. In our study, we report the isolation of a new haptophyte species from Lake George (LG) that, based on analyses of partial large subunit rRNA gene sequences, is genetically distinct from both Chrysotila lamellosa and Isochrysis galbana. We examined alkenone unsaturation index U^K₃₇ values for the LG species at 4°C, 10°C, 15°C, 20°C and 25°C as a function of temperature in a culture experiment. The temperature sensitivity of the new species was significantly higher than previously cultured Isochrysis galbana and Chrysotila lamellosa strains, with a slope that was 25 to 100 % higher. We found that the best linear relationship was obtained when two double-bond alkenones were excluded from the calibration (we developed an index termed U^K''₃₇ = [C_37:4] / [C_37:3+C_37:4]). In particular, U^K''₃₇ is more linear to the growth temperature than U^K₃₇ at low (4-10°C) and high (20-25°C) temperature ranges. Our experiments show that both U^K₃₇ and U^K''₃₇ of this new alkenone-produced species is strongly controlled by culture temperature and can be used for paleoclimate reconstruction. However, we recommend the use of U^K''₃₇ index to reconstruct temperature if the haptophyte's growing environment falls within temperature extremes (4-10°C and 20-25°C). This newly cultivated species broadens our ability of applying lacustrine haptophyte calibrations to continental paleothermometry.