Spatial distribution of Group 2i Isochrysidales in marine and lacustrine environments
We compiled global environmental DNA data from ice, water, and sediment samples in lacustrine and marine environments that targeted haptophytes, and mapped the global distribution of Group 2 Isochrysidales detected in different ocean environments (Fig.1). Based on previous study, Group 2i is widely identified in seawater and ice samples in the Arctic Ocean, where other Isochrysidales are rare 3. Here, we investigated DNA recovered from seawater samples collected in the Antarctic Region and report the first identification of Group 2i in the Southern Ocean, from seawater samples collected during the austral summer in the northern Antarctic Peninsula region 17(Fig. S1). Group 2i DNA is found in water column samples from ocean adjacent to the South Orkney Islands. The water temperature at the time of sampling (January, 2018) range from -0.5 to 3 ˚C and salinity range from 33.1 to 34.4 psu. No other Group 2 Isochrysidales has been found in the Southern Ocean water samples studies we examined (Supplementary Data).
We analyzed Isochrysidales DNA and alkenone profiles from sediment samples in estuaries that encompass three temperature regimes: Greenland fjords, the Baltic Sea, and Chesapeake Bay (Fig. S2). The Upernavik Fjord and Kangerlussuaq Fjord are located on the west and east coast of Greenland, respectively (Fig. S3). The modern annual SST of the two sample sites ranges between -2 ~ 4˚C (Fig. S2). Both fjords are major outlets of the retreating Greenland Ice Sheet 18,19. Isochrysidales 28S rRNA gene sequences were recovered from downcore sediment samples from two core sites along Upernavik Fjord. Phylogenetic analyses based on 28S rRNA gene sequences show the Upernavik Fjord Isochrysidales sequences belong to Group 2i, as it clustered with Group 2i sequences recovered from surface sediment from Lake Fryxell3,5 (Fig. S4). Group 2i Isochrysidales was also detected through the 18S rRNA gene from surface sediment at the Kangerlussuaq Fjord. The DNA of other Isochrysidales is not detected in Greenland fjords during this study. However, the alkenone profile from Kangerlussuaq Fjord suggests E. huxleyi also exists at the site, and the alkenones are produced by both Group 2i and E. huxleyi , as %C37:4 (14% in Kangerlussuaq sample) is not as high as in typical Group 2i cultures (~80% for strain RCC5486 under 3˚C 14) and C38Mealkenones are relatively high as well (Fig. S5).
The Baltic Sea is an intra-continental shelf sea characterized by strong salinity and temperature gradients (Fig. 2, Fig. S2). The south of the Baltic Sea is connected with the North Sea, where salinity is ~35 psu, in contrast to the northern end of Bothnian Bay and the Gulf of Finland where salinity is ~2 psu. During winter, Bothnian Bay is covered by consolidated ice. Moving towards the North Sea along the transect, winter ice cover transitions to more fractured ice and open water. All three groups of Isochrysidales were detected in the Baltic Sea region 20–22. Skagerrak is dominated by E. huxleyi with small alkenone contributions from other Groups. From Skagerrak toward the Gulf of Bothnian estuary, the alkenone-producing community changed progressively (Fig. S5). The inner part of the Baltic Sea, where sea ice is more prominent, is dominated by Group 2i, in addition to Group 1 Isochrysidales in low salinity (< 6 psu) waters21,22.
Chesapeake Bay’s annual SST ranges from 4 to 27 ˚C, and the seawater remains unfrozen during winter (Fig. S2). Isochrysidales 18S rRNA gene sequences were recovered from seven surface sediment samples in the lower part of Chesapeake Bay through next-generation sequencing (NGS).Emiliania huxleyi was detected in one sample near the mouth of the bay. The dominant amplicon sequence variants (ASVs) in all samples were Isochrysis galbana (a Group 2w Isochrysidales), which agrees with the microscopic identification conducted in a previous study23. The unidentified ASVs recovered all fall into Group 2w and have 99.2-99.7% similarity to Isochrysis galbana(Fig. S6). Importantly, no Group 2i was identified in these samples despite the recovery of abundant Group 2w and Group 3 Isochrysidales reads. The alkenone profile from surface sediment is similar to theIsochrysis galbana culture with a small input fromEmiliania huxleyi (Fig. S5).
Our results show that although Group 2i can occur across a wide salinity range from the typical open ocean (~ 35 psu) to oligohaline waters, as shown in the Baltic Sea and off shore Antarctic Peninsula, they are strongly associated with low temperatures. The absence of Group 2i in Chesapeake Bay demonstrates that they are limited to regions with seasonal or perennial ice cover.
Group 2i is also widespread in icy lacustrine environments (Fig. 1). However, it is generally absent in freshwater lakes where Group 1 Isochrysidales flourish 24,25,5,26, regardless of the temperature. A combination of Group 2i and 2w is commonly found in surface sediments from saline lakes with seasonal ice cover, throughout a range of minimal monthly temperatures from -23 ˚C to 3 ˚C. In modern samples from lakes with perennial ice cover (e.g., Lakes Fryxell and Bonney in the Antarctic Dry Valley), Group 2i was the only Isochrysidales found 5,27,28. In contrast, only Group 2w was found in temperate saline lakes in Spain, France, Tanzania, and Brazil, where lake water remains unfrozen throughout the year29–31.