Resolving sources of marine DOM via novel sulfur isotope analyses

人们对海洋中含有硫的有机分子知之甚少。有机硫含量丰富，可能有助于稳定溶解的有机物，结合重要的微量金属，并为浮游植物提供微量营养物质。然而，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，这两种途径应该产生非常不同的硫同位素分布（即具有不同原子质量的单质硫），并且很容易测量。本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，以区分两种形成途径。如果成功，该方法将用于计算每个途径产生的溶解有机硫的数量。该项目将为当地一名高中生提供一个机会，与当地一所女子高中合作，在为期一年的实习期间从事实验室研究。此外，该项目支持的研究生将在女性做科学Instagram社交媒体平台上主持“生活中的一天”信息会议。她将在为该项目收集样本的研究巡航中回答这个Instagram账户的粉丝关于海洋学和海洋科学职业的问题。最近，一些证据集中在海洋溶解有机硫（DOS）上，包括认识到许多异养细菌需要外源的“固定”有机硫；有机硫分子——尤其是硫醇——在结合和隔离微量金属方面发挥着重要作用；DOS的硫含量为6700 Tg S，是海洋中第二大硫库（仅次于溶解的硫酸盐）。也许最重要的是，DOS有数千年的历史，这意味着它不能迅速回收，这挑战了DOS主要来自半胱氨酸、蛋氨酸和二甲基磺酰丙酸（DMSP）等不稳定生物分子的预期。一种合理的替代方法是，来自缺氧孔隙水和水柱的H2S与非生物硫化反应是顽固的海洋DOS的来源。区分这些形成路径对于理解DOS动力学至关重要，但迄今为止一直很困难。根据现有资料，认为海洋浮游植物形成的DOS的硫同位素值（δ34S）应接近+20‰（VCDT）。非生物硫化形成的DOS变化较大，其平均值为-20‰。因为这些同位素值是不同的，他们应该能够区分这两种途径。分析的主要障碍是DOS浓度低（µM），以及在海水中浓缩的困难。本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 δ34S值，以及百慕大（BATS）和夏威夷（HOT）时间序列地点的半胱氨酸和蛋氨酸的化合物特异性δ34S值。这项工作将提供关于海洋硫循环的新信息，并提高对DOS在稳定深海碳中所起作用的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.