Main
The cryosphere, including sea ice, lake ice, and glaciers, is a crucial element of the Earth’s climate system. It regulates the Earth’s heat budget with its high albedo and energy transfer between the Earth’s surface and atmosphere. Pervasive changes in the cryosphere have already occurred under a warming climate 1. Current Arctic sea ice cover has decreased to its lowest level since at least 1850 CE, with a continuous loss of ~ 12.3 % per decade since 1978 when satellite observation began1. Loss of lake ice has also been observed over the recent decades, and the decrease in ice duration and thickness is projected to intensify at an unprecedented pace2. As sea ice continues to melt, the Arctic Ocean becomes increasingly accessible for resource exploration, shipping routes, and military activities, all of which are rapidly and drastically reshaping the global economical and geopolitical frameworks. We urgently need to accurately and quantitatively project the courses of Arctic sea ice change in the coming decades under various scenarios of anthropogenic greenhouse gas emissions. Quantitative reconstruction of past cryosphere changes beyond instrumental records, especially during warmer periods in the geological past, is essential for calibrating future projections using climate models.
A new quantitative sea-ice proxy based on characteristic alkenone distributions produced by Group 2i Isochrysidales (an order of haptophytes) was recently proposed by Wang et al. 3. Phylogenetically based on 18S rRNA gene, Isochrysidales has been classified into 3 groups, with Group 1 inhabiting freshwater and oligohaline environments, Group 2 species in saline lakes and estuaries, and Group 3 in open ocean settings4,5. Group 2 can be further separated into ice-associated Group 2i and warm-season blooming Group 2w (e.g., Isochrysis galbana , Ruttnera lamellosa )6,4,7,8,3,9. Alkenones produced by Group 3 Isochrysidales have been widely used for paleo sea surface temperature (SST) reconstructions since the 1980s 10,11. However, alkenones from sea-ice-laden oceans often lead to abnormal SST reconstructions and display high value of %C37:4(C37:4/( C37:2+ C37:3+ C37:4)) compared to mid-to-low latitude oceans, which was previously attributed to meltwater input and decreased salinity12,13. Recent culture experiments show that %C37:4 of alkenones produced by Group 2i and 3 Isochrysidales is not affected by salinity14. The increased %C37:4 observed in high-latitude ocean is rather an effect of input from Group 2i during increased sea-ice cover which coincide with low surface salinity3. Cells and DNA sequences of Group 2i has been widely observed within sea ice3, and the correlation between occurrence of Group 2i and high %C37:4 has been used to reconstruct past changes in sea ice in regions such as the Gulf of Alaska and the Fram Strait15,16,3. However, we have a limited understanding of the ecology of Group 2i Isochrysidales in the natural environments. For instance, it is unclear whether ice is a prerequisite for the presence of Group 2i in the global ocean and lacustrine environments. Further, there are currently no constraints on the growth conditions (e.g., temperature or salinity) or seasonality of Group 2i phytoplankton blooms in natural waters.
Here, we made the first global map of known occurrences of Group 2i and its habitable temperature ranges based on environmental DNA data sequenced from sediment samples collected in Baltic Sea, Chesapeake Bay, and Greenland fjords and re-analyzed environmental DNA data from published studies in global marine and lacustrine environments (Supplementary Data). We also examined the seasonal distribution of Group 2i during annual cycles to understand their role in bloom successions. We propose potential growth strategies adopted by Group 2i that enable its success in cold settings and discuss the implications for paleoclimate reconstructions.