Resolving sources of marine DOM via novel sulfur isotope analyses

利用硫同位素解析海洋有机质的来源海洋中含有硫的有机分子知之甚少。有机硫丰富，可能有助于稳定溶解的有机物，结合重要的痕量金属，并为浮游植物提供微量营养。然而，目前尚不清楚有机硫分子是来自生物过程，还是来自涉及硫化氢(H2S)的非生物“硫化”反应。现有的知识预测，这两条路径应该会产生非常不同的--而且容易测量的--硫同位素分布(即原子质量不同的元素硫)。这项研究将使用一种新的、高灵敏度的方法来测量溶解有机分子的硫同位素值，以区分这两种形成途径。如果成功，这种方法将被用来计算每一条途径产生多少溶解的有机硫。该项目将与当地一所女子高中合作，在为期一年的实习期间为当地高中生提供从事实验室研究的机会。此外，该项目支持的研究生将在女性从事科学的Instagram社交媒体平台上主持一次“生活中的一天”信息会议。她将在为该项目采集样本的研究邮轮上回答这个Instagram账号的关注者提出的关于海洋学和海洋科学职业的问题。最近，几种证据都集中在对海洋溶解有机硫的兴趣上，包括认识到许多异养细菌需要外部来源的“固定”有机硫；有机硫分子--特别是硫醇--在结合和隔离痕量金属方面发挥了主要作用；在6700Tg S处，溶解有机硫是海洋中第二大硫磺池(仅次于溶解的硫酸盐)。也许最重要的是，DOS有数千年的历史，这意味着它不会迅速回收利用，并挑战了DOS主要来自不稳定的生物分子的预期，如半胱氨酸、蛋氨酸和二甲基磺酸丙酸酯(DMSP)。一种合理的替代方案是，非生物硫化反应与来自缺氧孔隙水的硫化氢反应，而水柱是顽固的海洋DOS的来源。区分这些形成途径对于理解DOS动力学至关重要，但到目前为止还很难。根据已有的数据，认为海洋浮游植物形成的DOS的硫同位素值(δ34S)应在20‰(VCDT)附近。另外，非生物硫化形成的DOS应具有更大的可变性，平均值为-20‰。因为这些同位素值是不同的，他们应该能够区分这两条路径。分析的主要障碍是DOS的低浓度(微米)，以及在海水中浓缩的困难。这项研究将使用加州理工学院开发的新分析技术来测量几个地点的DOS的δ34S值，以及百慕大和夏威夷(热)时间序列地点的半胱氨酸和蛋氨酸的化合物特定δ34S值。这项工作将提供有关海洋硫磺循环的新信息，并提高对DOS在稳定深海碳中所起作用的理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Resolving sources of marine organic matter using sulfur isotopes Organic molecules that contain sulfur in the ocean are poorly understood. Organic sulfur is abundant, and likely helps stabilize dissolved organic matter, binds important trace metals, and provides trace nutrients to phytoplankton. However, it is not known whether organic sulfur molecules derive from biological processes, or from non-biological “sulfurization” reactions involving hydrogen sulfide (H2S). Existing knowledge predicts that the two pathways should generate very different -- and easily measurable -- distributions of sulfur isotopes (i.e., elemental sulfur with varied atomic mass). This study will use a new, highly sensitive method to measure the sulfur isotope values of dissolved organic molecules to distinguish between the two formation pathways. If successful, this method will be used to calculate how much dissolved organic sulfur results from each pathway. This project will provide an opportunity for a local high school student to engage in laboratory research during a one year internship, in partnership with a local all-girls high school. Additionally, the graduate student supported by this project will host a “Day in the Life” information session on the Women Doing Science Instagram social media platform. She will answer questions about oceanography and marine science careers from followers of this Instagram account while on a research cruise collecting samples for the project. Several lines of evidence have recently converged to focus interest on marine dissolved organic sulfur (DOS), including the recognition that many heterotrophic bacteria require exogenous sources of ‘fixed’ organic sulfur; that organosulfur molecules -- particularly thiols -- play a major role in binding and sequestering trace metals; and that at 6700 Tg S, DOS is the second largest (after dissolved sulfate) sulfur pool in the oceans. Perhaps most importantly, DOS is thousands of years old, implying that it is not rapidly recycled and challenging the expectation that DOS derives mainly from labile biomolecules such as cysteine, methionine and dimethylsulfoniopropionate (DMSP). A plausible alternative is that abiotic sulfurization reactions with H2S from anoxic porewaters and the water column is a source of recalcitrant marine DOS. Distinguishing between these formation pathways is critical for understanding DOS dynamics, but thus far has been difficult. Based on existing data, it is thought that DOS formed from marine phytoplankton should have a sulfur isotope value (δ34S) near +20‰ (VCDT). Alternatively, DOS formed from abiotic sulfurization should be more variable, with average values of-20‰. Because these isotope values are distinct, they should be able to distinguish between the two pathways. The primary obstacle to analysis has been the low (µM) concentration of DOS, and difficulties associated with concentrating it in seawater. This study will use new analytical techniques developed at Caltech to measure the δ34S values of DOS at several locations, and compound-specific δ34S values for cysteine and methionine from the Bermuda (BATS) and Hawaii (HOT) time series locations. This work will contribute new information about the marine sulfur cycle, and improve understanding of the role that DOS plays in stabilizing carbon in the deep oceans.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.