A new constraint on the antiquity of ancient haloalkaliphilic green algae that flourished in a ca. 300 Ma Paleozoic lake

Abstract

It is established that green algae and land plants progressively colonized freshwater and terrestrial habitats throughout the Paleozoic Era, but little is known about the ecology of Paleozoic saline lakes. Here, we report lipid biomarker and petrographic evidence for the occurrence of a green alga as a major primary producer in a late Paleozoic alkaline lake (Fengcheng Formation; 309–292 Ma). A persistently saline and alkaline lacustrine setting is supported by mineralogical and lipid biomarker evidence alongside extremely enriched δ15Nbulk values (+16 to +24‰) for the lake depocenter. The prominence of C28 and C29 steroids, co‐occurring with abundant carotene‐derived accessory pigment markers in these ancient rocks, is suggestive of prolific primary production and elevated source inputs from haloalkaliphilic green algae. The high C28/C29‐sterane ratios (0.78–1.29) are significantly higher than the typical marine value reported for late Paleozoic rocks (<0.5) and thus are associated with certain groups of chlorophytes. Adaptation to such extreme lacustrine environments, aided by enhanced biosynthesis of certain cell membrane lipids, likely played an important role in the evolution and physiological development of ancient green algae.

Organic petrology, geochemistry, and hydrocarbon generation capacity of Permo–Carboniferous source rocks in the Mahu Sag, northwestern Junggar Basin, China

“…Dunaliella) can accumulate b-carotane to >14% of their dry cell weight, and adapt to a high-salinity environment (Ding et al, 2020;Francavilla et al, 2010;Tao et al, 2019). Unlike the Fengcheng Formation, in which the b-carotane index is >1.0 (Figure 10(a); Xia et al, 2020a), the b-carotane index values of most Carboniferous, Jiamuhe Formation, and Lower-Wuerhe Formation samples are <1.0. This indicates that the water salinity was lower than for the Fengcheng Formation (Figure 10(a)).…”

“…We found that the CPI and OEP values of most samples vary from 0.5 to 1.5, although the CPI values of a small number of (Bray and Evans, 1961); OEP ¼ [(C i þ6C iþ2 þC iþ4 )/(4 C iþ1 þ4C iþ4 )] (-1)iþ1 , where i þ 1 is the peak of n-alkanes (Scalan and Smith, 1970). Source: The Fengcheng Formation data were taken from Xia et al (2020a). samples are >1.5.…”

“…For example, Cao et al (2015) showed that the Fengcheng Formation source rocks are highquality and were deposited in a high-salinity alkaline lake. Evidence for this includes N isotope data that indicate a water pH of >9.25, the presence of wegscheiderite, abundant spherical bacteria fossils, a low clay mineral content, and geochemical features indicative of a high-salinity, strongly reducing environment that was affected by hydrothermal fluids (Cao et al, 2020;Qin et al, 2016;Xia et al, 2020aXia et al, , 2020b. Zhi et al (2016) and Wang et al (2018) showed that the Fengcheng Formation source rocks are voluminous, contain good kerogen types, and have a high hydrocarbon generation potential.…”

“…(a) d 13 C kerogen vs. TOC contents; (b) T max vs. HI; (c) OI vs. HI. Source: The classification of organic matter types is from Espitali e et al(1977), and the Fengcheng Formation data were taken fromXia et al (2020a).…”

“…CPI ¼ [(C 25 þC 27 þC 29 þC 31 þC 33 )/(C 24 þC 26 þC 28 þC 30 þC 32 )þ(C 25 þC 27 þC 29 þC 31 þC 33 )/ (C 26 þC 28 þC 30 þC 32 þC 34 )]/2(Bray and Evans, 1961); OEP ¼ [(C i þ6C iþ2 þC iþ4 )/(4 C iþ1 þ4C iþ4 )] (-1)iþ1 , where i þ 1 is the peak of n-alkanes(Scalan and Smith, 1970). Source: The Fengcheng Formation data were taken fromXia et al (2020a).…”