While the history of taxonomic diversification in open ocean lineages of ray-finned fish and elasmobranchs is increasingly known, the evolution of their roles within the open ocean ecosystem remains poorly understood. To assess the relative importance of these groups through time, we measured the accumulation rate of microfossil fish teeth and elasmobranch dermal denticles (ichthyoliths) in deep-sea sediment cores from the North and South Pacific gyres over the past 85 million years (Myr). We find three distinct and stable open ocean ecosystem structures, each defined by the relative and absolute abundance of elasmobranch and ray-finned fish remains. The Cretaceous Ocean (pre-66 Ma) was characterized by abundant elasmobranch denticles, but low abundances of fish teeth. The Palaeogene Ocean (66-20 Ma), initiated by the Cretaceous/Palaeogene mass extinction, had nearly four times the abundance of fish teeth compared with elasmobranch denticles. This Palaeogene Ocean structure remained stable during the Eocene greenhouse (50 Ma) and the Eocene-Oligocene glaciation (34 Ma), despite large changes in the overall accumulation of both groups during those intervals, suggesting that climate change is not a primary driver of ecosystem structure. Dermal denticles virtually disappeared from open ocean ichthyolith assemblages approximately 20 Ma, while fish tooth accumulation increased dramatically in variability, marking the beginning of the Modern Ocean. Together, these results suggest that open ocean fish community structure is stable on long timescales, independent of total production and climate change. The timing of the abrupt transitions between these states suggests that the transitions may be due to interactions with other, non-preserved pelagic consumer groups.

Non-targeted liquid chromatography−tandem mass spectrometry (LC−MS/MS) is a widely used tool for metabolomics analysis, enabling the detection and annotation of small molecules in complex environmental samples. Data-dependent acquisition (DDA) of product ion spectra is thereby currently one of the most frequently applied data acquisition strategies. The optimization of DDA parameters is central to ensuring high spectral quality, coverage, and number of compound annotations. Here, we evaluated the influence of 10 central DDA settings of the Q Exactive mass spectrometer on natural organic matter samples from ocean, river, and soil environments. After data analysis with classical and feature-based molecular networking using MZmine and GNPS, we compared the total number of network nodes, multivariate clustering, and spectrum quality-related metrics such as annotation and singleton rates, MS/MS placement, and coverage. Our results show that automatic gain control, microscans, mass resolving power, and dynamic exclusion are the most critical parameters, whereas collision energy, TopN, and isolation width had moderate and apex trigger, monoisotopic selection, and isotopic exclusion minor effects. The insights into the data acquisition ergonomics of the Q Exactive platform presented here can guide new users and provide them with initial method parameters, some of which may also be transferable to other sample types and MS platforms.

Non-targeted liquid chromatography tandem mass spectrometry (LC-MS/MS) is a widely used tool for the detection and annotation of small molecules in complex environmental samples. Data Dependent Acquisition (DDA) of product ion spectra is thereby currently one of the most frequently applied data acquisition strategies. The optimization of DDA parameters is central to ensuring high spectral quality, coverage and number of compound annotations. Here, we evaluated the influence of 10 central DDA settings of the Q Exactive mass spectrometer on natural organic matter samples from ocean, river, and soil environments. After data analysis with Classical and Feature-based Molecular Networking using MZmine and GNPS, we compared the total number of network nodes, multivariate clustering, and spectrum quality related metrics such as annotation and singleton rates, MS/MS placement and coverage. Our results show that Automatic Gain Control, Microscans, Mass Resolution, and Dynamic Exclusion are the most critical parameters, whereas Collision Energy, TopN, and Isolation Width had moderate and Apex Trigger, Monoisotopic Selection, and Isotopic Exclusion minor effects. The insights into the data acquisition ergonomics of the Orbitrap platform presented here can guide new users and provide them with initial method parameters, some of which may also be transferable to other sample types and MS platforms.