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.