School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York, USA.
Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, USA.
Geobiology. 2024 Mar-Apr;22(2):e12593. doi: 10.1111/gbi.12593.
Biological processes in the Proterozoic Ocean are often inferred from modern oxygen-deficient environments (MODEs) or from stable isotopes in preserved sediment. To date, few MODE studies have simultaneously quantified carbon fixation genes and attendant stable isotopic signatures. Consequently, how carbon isotope patterns reflect these pathways has not been thoroughly vetted. Addressing this, we profiled planktonic productivity and quantified carbon fixation pathway genes and associated organic carbon isotope values (δ C ) of size-fractionated (0.2-2.7 and >2.7 μm) particulate matter from meromictic Fayetteville Green Lake, NY, USA. The high-O Calvin-Benson-Bassham (CBB) gene (cbbL) was most abundant in the <2.7 μm size fraction in shallow oxic and deep hypoxic waters, corresponding with cyanobacterial and eukaryote algal populations. The low-O CBB gene (cbbM) was most abundant near the lower oxycline boundary in the larger size fraction, coincident with purple sulfur bacteria populations. The reverse citric acid cycle gene (aclB) was equally abundant in both size fractions in the deepest photic zone, coinciding with green sulfur bacteria populations. Methane coenzyme reductase A (mcrA), of anaerobic methane cyclers, was most abundant at the lower oxycline boundary in both size fractions, coinciding with Methanoregula populations. δ C values overlapped with the high-O CBB fixation range except for two negative excursions near the lower oxycline boundary, likely reflecting assimilation of isotopically-depleted groundwater-derived carbon by autotrophs and sulfate-reducers. Throughout aphotic waters, δ C values of the large size fraction became C-enriched, likely reflecting abundant purple sulfur bacterial aggregates. Eukaryote algae- or cyanobacteria-like isotopic signatures corresponded with increases in cbbL, cbbM, and aclB, and enrichment of exopolymer-rich prokaryotic photoautotrophs aggregates. Results suggest that δ C values of preserved sediments from areas of the Proterozoic Ocean with sulfidic photic zones may reflect a mixture of alternate carbon-fixing populations exported from the deep photic zone, challenging the paradigm that sedimentary stable carbon isotope values predominantly reflect oxygenic photosynthesis from surface waters.
从现代贫氧环境 (MODEs) 或保存下来的沉积物中的稳定同位素中，人们常常推断出元古宙海洋中的生物过程。迄今为止，很少有 MODE 研究同时定量了碳固定基因和相关的稳定同位素特征。因此，碳同位素模式如何反映这些途径还没有经过彻底的审查。为了解决这个问题，我们对浮游生物生产力进行了分析，并定量了大小分级（0.2-2.7μm 和>2.7μm）颗粒物质中的碳固定途径基因和相关的有机碳同位素值（δC）来自美国纽约州费耶特维尔绿湖的分层湖，该湖为分层湖。高-O 卡尔文-本森-巴斯汉姆（CBB）基因（cbbL）在浅层含氧和深层缺氧水中的<2.7μm 大小部分最为丰富，与蓝细菌和真核藻类种群相对应。低-O CBB 基因（cbbM）在较大尺寸部分的下氧跃变边界附近最为丰富，与紫色硫细菌种群相对应。反向柠檬酸循环基因（aclB）在最深光区的两个尺寸部分中含量相等，与绿硫细菌种群相对应。厌氧甲烷循环物的甲烷辅酶还原酶 A（mcrA）在两个尺寸部分的下氧跃变边界处最为丰富，与 Methanoregula 种群相对应。δC 值与高-O CBB 固定范围重叠，除了在下氧跃变边界附近的两个负偏移外，可能反映了自养生物和硫酸盐还原菌对同位素贫化地下水碳的同化。在整个无光区，大尺寸部分的δC 值变得更加丰富，可能反映了大量的紫色硫细菌聚集体。真核藻类或蓝细菌样的同位素特征与 cbbL、cbbM 和 aclB 的增加相对应，并富集了富含外聚合物的原核光自养生物聚集体。结果表明，元古宙海洋中具有硫化光区的地区保存的沉积物中的δC 值可能反映了从深光区输出的替代碳固定种群的混合物，这挑战了沉积物稳定碳同位素值主要反映地表水有氧光合作用的范式。
