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.