Glossary

Abiotic: non-biological (e.g., physical or geochemical).

Absorbent: a material that picks up and retains a liquid distributed throughout its molecular structure causing the solid to swell (50% or more). The absorbent is at least 70% insoluble in excess fluids.

Acute-to-chronic ratio: the ratio of the acute and chronic toxicity values for a given compound, usually the average of the ratios for a variety of species; used to estimate the chronic toxicity of a compound or a mixture of compounds, from the measured or modeled acute toxicity when no chronic toxicity data are available.

Adsorbent: an insoluble material that is coated by a liquid on its surface including pores and capillaries without the solid swelling more than 50% in excess fluid.

Adverse outcome pathways (AOPs): direct associations between specific molecular networks and deleterious effects to biological systems that can be used as signatures of exposure.

Allision: the striking of a vessel with a fixed or stationary object.

American Petroleum Institute (API) gravity: industry scale expressing the gravity or density of liquid petroleum products. The measuring scale is calibrated in terms of degrees API, it may be calculated using the following formula: Degrees API = (141.5/specific gravity at 15.6°C) – 131.5. The higher the API gravity, the lighter the compound. Light crudes generally exceed 38° API and heavy crudes are commonly labeled as all crudes with an API gravity of 22° or below. Intermediate crudes fall in the range of 22–38° API gravity. Light crudes yield more gasoline.

Anaerobic: metabolisms that occur independent of oxygen. Annual exceedance probability: the chance or probability of a natural hazard event (usually a rainfall or flooding event) occurring annually, usually expressed as a percentage. Bigger rainfall events occur (are exceeded) less often and will therefore have a lesser annual probability.

Anoxic: depleted of oxygen.

Archaea: one of two prokaryote domains (microscopic single-celled organisms that have neither a distinct nucleus with a membrane nor other specialized organelles) that includes microorganisms that live in extreme environments.

Arctic: generally defined as the region above the Arctic Circle (i.e., above approximately 66° 34’ N latitude, although other definitions are based on the tree line or on temperature data).

Aromatics: class of hydrocarbons having one or more five or six membered “fused” (clustered together) carbon atom planar rings. The single six carbon membered ring is named benzene. The multiple rings are made up of multiple six, and less often five, carbon atom planar rings, with single carbon to carbon (sigma) bonds between carbon atoms and a shared Pi electrons resonating between the carbon atoms in a “cloud” of electrons above and below the planar ring(s). Some aromatics are heteroatom aromatic compounds for which a carbon atom is replaced by a nitrogen, sulfur or oxygen atom. Multiple fused ring aromatic hydrocarbons are named polycyclic aromatic hydrocarbons and those with a nitrogen, sulfur, or oxygen atom are named polycyclic aromatic compounds (PACs). The abbreviation PAC is often used to mean all of the aromatic ring compounds. The benzene rings are often referred to as the “phenyl ring” when bonded to other aromatic ring structures or to other atoms in non-aromatic ring structures.

Aryl hydrocarbon receptor: a cellular protein that binds planar or plate-like polycyclic aromatic compounds having a molecular shape and dimensions that resemble 2,3,7,8 tetrachorodibenzo(p)dioxin.

Asphalt: a dark-brown-to-black cement-like material containing bitumens as the predominant constituent obtained by petroleum processing that is used primarily for road construction. It includes crude asphalt as well as finished products such as cements, fluxes, the asphalt content of emulsions (exclusive of water), and petroleum distillates blended with asphalt to make cutback asphalts.

Asphaltenes: class of petroleum compounds of high molecular weight and complexity; defined as the oil fraction that precipitates in low molecular weight n-alkanes (e.g., C5–C7) but is soluble in toluene.

Authigenic: a geological deposit that has formed, via geochemical processes, in the same site where it is currently located.

Bacteria: microscopic, single-celled organisms that lack a nucleus. One of the two kingdoms, along with the Archaea, that comprise the prokaryotes.

Ballast tank: tank that carries water for weight needed for maintaining ship stability or draft.

Barrel: term used as the standard measurement of volume for crude oil and large quantities of refined products in the petroleum industry. A unit of volume equal to 42 U.S. gallons (159 L), often abbreviated as bbl.

Bioaccumulation (or bioconcentration): the tendency of substances to accumulate in the body of organisms; the net uptake from their diet, respiration or transfer across skin and loss due to excretion or metabolism. The bioaccumulation factor or bioconcentration factor is the ratio of concentrations in tissue to concentrations in a source (i.e., water or diet).

Bioavailability or biological availability: the extent to which a compound can be assimilated by a living organism; also, the proportion of a chemical in an environmental compartment (e.g., water) that can be taken up by an organism.

Biodegradation: a natural process in which microbes enzymatically transform organic chemicals, such as oil, under aerobic or anaerobic conditions and typically resulting in cell growth; oil biodegradation usually requires nutrients, such as nitrogen and phosphorus; transformation may be complete, producing water, carbon dioxide and/or methane, or incomplete, producing partially oxidized chemicals.

Biofilm: any consortium of microbes that stick to each other (e.g., marine snow) and/or are attached to living or inanimate surfaces by biological macromolecules such as extracellular polymeric substances or transparent extracellular polymers.

Biofuel: a fuel derived from biomass, such as plant or animal material.

Bioinformatician: scientist who conducts research using an interdisciplinary approach to develop methods and software for analyzing and interpreting biological data, particularly using large, complex datasets.

Bioinformatics: a field of study at the intersection of molecular biology and computer science that enables acquisition, analysis, comparison and archiving of biological data, such as DNA and amino acid sequences.

Biomagnification: a food chain or food web phenomenon whereby a substance or element increases in concentration at successive trophic levels; occurs when a substance is persistent and is accumulated from the diet faster than it is lost due to excretion or metabolism.

Biomarkers: a term used in two different ways, depending on discipline. In petroleum chemistry, a biomarker is a relic chemical relating its presence to the original biological source (microbial, plant, or animal); biomarkers are usually poorly or non-biodegradable and so persist in the oil, enabling their use as internal standards in petroleum analysis. In environmental toxicology, a biomarker is a biochemical process, product, or cellular response that indicates the organism’s exposure to a pollutant and/or the toxic effects of the pollutant.

Bioremediation: an environmental intervention strategy to enhance biodegradation of spilled oil (or other contaminants) ranging from no remedial action other than monitoring (monitored natural attenuation) to nutrient addition (biostimulation) to inoculation with competent microbial communities (bioaugmentation).

Bitumen: the heaviest class of petroleum, having high viscosity and density; widely considered to represent the residue from lighter oils that have undergone biodegradation over geological time.

BTEX: abbreviation for the mixture of benzene, toluene, ethylbenzene, and xylenes.

Bunker fuel: fuel oil used by ships for main propulsion or auxiliary power.

Chemically enhanced water-accommodated fraction of oil (CEWAF): a solution of hydrocarbons and a suspension of oil droplets created when a chemical dispersant is added to oil and water with stirring.

Chemotaxis: the movement of a motile cell or organism toward an attractant chemical or away from a repellent chemical, along the increasing or decreasing concentration gradient of that chemical.

Co-metabolism: simultaneous biodegradation of two compounds, in which the degradation of the second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate).

Commingling: mixing of two petroleum products with similar specifications. Most branded gasoline firms require that their product not be commingled to preserve the integrity of the brand.

Conventional crude oil: commonly defined as liquid petroleum that flows in the reservoir and in pipelines and is recovered from traditional oil wells using established methods, including primary recovery and water flooding (e.g., condensates, light and medium crude oils), versus unconventional crude oils.

Crude oil: synonymous with petroleum; a naturally occurring and typically liquid complex mixture of thousands of different hydrocarbon and non-hydrocarbon molecules.

Cx: a molecule, such as a hydrocarbon, having x carbon atoms.

Cyanobacteria: a phylum of bacteria capable of photosynthesis; colloquially (but incorrectly) also called “blue-green algae.”

CYP1A: a member of the cytochrome P450 family of proteins; an enzyme of vertebrates, including fish, birds, and mammals, that catalyzes the addition of oxygen to double bonds as a first step in the metabolism and excretion of polycyclic aromatic hydrocarbons.

cyp1a: the gene that codes for CYP1A proteins.

Dead oil: used to describe an oil at standard conditions in equilibrium with a gaseous headspace in a closed container. Dead oil has released most of the gaseous compounds dissolved in the oil but may retain all of its volatile constituents. See also live oil.

Dead weight tonnes: a measure of ships’ carrying capacity (cargo, fuels, ballast water, provisions, passengers, and crew).

Demersal: can be pelagic but living on or associated with the bottom sediments of marine and freshwater aquatic systems. Typical organisms are fish, such as flounder, and invertebrates, such as shrimp.

Dilbit: bitumen diluted with a lighter petroleum class, such as condensate or naphtha, typically 70% bitumen and 30% diluent. Dispersant: a chemical or mixture of chemicals applied, for example, to an oil spill to disperse the oil phase into small droplets in the water phase.

Dispersion: suspension of oil droplets in water accomplished by natural wind and wave action; production of biological materials (biosurfactants) and/or chemical dispersant formulations.

Distillate: No. 1 and No. 2 fuel oils, and No. 1 and No. 2 diesel fuels as well as some blends of gasoil. These are light fuel oils for transportation and heating oil, and include railroad engine fuel and diesel for agricultural machinery. Lower sulfur distillates also represent the preferable feedstock for power generation when utilities see natural gas curtailments or high natural gas prices.

Ebullition: the act or state of bubbling or boiling.

EC₅₀: the concentration of a substance that causes sublethal effects on the median or 50th percentile organism tested within a specified exposure time (e.g., 14-day EC₅₀).

Ecological risk assessment: process for analyzing and evaluating the possibility of adverse ecological effects caused by environmental pollutants.

Ecosystem: the interrelationships between all of the living things in an area and their non-living environment.

Effects-driven chemical fractionation (EDCF): the stepwise fractionation, toxicity testing, and chemical analysis of complex mixtures of compounds to isolate and identify the constituents responsible for the toxic effects of the whole mixture.

Emulsification: formation of water droplets in an oil matrix (water-in-oil) or conversely oil droplets in a water matrix (oil-in-water) achieved by the action of agitation, such as wind and wave activity; can be unstable, separating into oil and water phases again soon after formation, or stable for months or years (e.g., “chocolate mousse,” a water-in-oil emulsion).

Environmental impact assessment: the process of measuring or estimating the environmental effects of pollutants, such as oil spills, relative to conditions at a reference site or to a time prior to a spill.

Epigenomics: the study of reversible heritable changes in gene function that occur without a change in the sequence of nuclear DNA (e.g., DNA methylation, histone modifications).

Eukaryote: a single-celled or multicellular organism (in the domain Eukarya) that has a nucleus and other intracellular organelles; includes plants, animals, fungi, and protists.

Evaporation: the physical loss of low molecular weight components of an oil to the atmosphere by volatilization.

Flame ionization detector (FID): used with analytical instruments like gas chromatographs to detect components of petroleum by combustion ionization, hence GC-FID.

Foraminifera: single-celled organisms (protists) with tests (shells). They may be pelagic or benthic and are abundant as fossils for the past 540 million years. The shells of benthic species are commonly divided into chambers that are added during growth and may be made of organic compounds, sand grains or other particles cemented together, or crystalline calcium carbonate.

Fossil fuel: a fuel source (such as oil, condensate, natural gas, natural gas liquids, or coal) formed in the earth from plant or animal remains.

Fractions: the different cuts of petroleum products that come off a distillation column contingent on their volatility or boiling range. Fractions are essentially crude cut points at different boiling ranges that produce the various finished products.

Fugitive emissions: emissions of gases or vapors from pressurized equipment, including pipelines, due to leakage or unintended or irregular releases of gases.

Gas chromatography (GC): an analytical method used to characterize petroleum components; GC is combined with different detection methods, hence GC-FID, GC-MS, etc.

Genomics: the study of the genomes (the full DNA sequence) of organisms, including taxonomic information, biochemical potential, evolutionary history, etc.

Gross tonnes: a measure of volume of enclosed spaces on ships used to categorize ships in regulations and commercial transactions.

Harpacticoids: small crustaceans (usually <0.3 mm) of the order Copepoda. They are generally benthic grazers and common members of the meiofauna.

Heteroatom: in petroleum, an atom such as nitrogen, sulfur, and/or oxygen that is part of a hydrocarbon skeleton, such as found in the resins fraction of crude oils.

High-energy water-accommodated fraction of oil (HEWAF): a solution of hydrocarbons and a suspension of oil droplets created when oil and water are mixed by high-energy agitation.

High molecular weight: relative term referring to the molecular mass of chemicals; in oil, asphaltenes would be typical of high molecular weight compounds.

High-performance liquid chromatography (HPLC): an analytical method for separating chemicals in solution.

Hydrocarbon: a chemical that is composed of only carbon and hydrogen; chemicals containing heteroatoms, such as nitrogen, sulfur, and/or oxygen, are not hydrocarbons, even though they may be petroleum constituents.

Hydrocarbonoclastic: term defining microorganisms that can metabolize or transform hydrocarbons (e.g., in crude oil or refined petroleum products).

Hydrogen sulfide (H₂S): a toxic, flammable, and corrosive gas sometimes associated with petroleum.

Hydrophilic-lipophilic balance (HLB): a measure of the properties of dispersants and surface-active agents at the interface of polar and non-polar liquids, such as oil and water. The higher the surfactant HLB value, the more hydrophilic (water-loving) it is.

Hydrophobic: having a tendency to repel water.

Hyporheic flow: the percolating flow of water through the sand, gravel, sediment, and other permeable soils under and beside the streambed.

Immiscible: a liquid that cannot be mixed with another liquid without separating from it.

Intermediate fuel oil (IFO): the heaviest commercial class of refined petroleum diluted with a lighter refined product, commonly burned in furnaces, boilers or ship engines (e.g., IFO 180).

Isomers: chemicals that have the same molecular formula (i.e., elemental composition) but different structures; may also have different properties, including water solubility, biodegradability, and toxicity.

Kinorhynch: a segmented, limbless animals <1 mm, with a body consisting of a head, neck, and a trunk of 11 segments. They are externally armored with a number of spines along the body, plus up to seven circles of spines around the head.

Kolmogorov scale: length scale of turbulent flow below which the effects of molecular viscosity are non-negligible. In three-dimensional turbulence, the Kolmogorov scale is (ν2/ε)^1/4, in which ν is the kinematic viscosity and ε is the energy dissipation rate per unit mass.

K_ow: the partition coefficient describing the equilibrium concentration ratio of a dissolved chemical in octanol versus in water, in a two-phase system at a specific temperature; used in the prediction of partitioning between dissolved and particulate or organic film or particle coating phase, prediction of bioaccumulation in organisms and in the related prediction of toxicity.

Labile: the property of being easily broken down (e.g., organic chemicals degraded by microbes; foodstuff metabolized by higher organisms).

Lacustrine: relating to lakes.

Lagrangian Coherent Structure (LCS): repelling, attracting, and shearing material surfaces that form the skeletons of Lagrangian particle dynamics. The theory of LCS seeks to reveal special surfaces of fluid trajectories that organize the rest of the flow into ordered patterns. Adapted from Haller (2015).

Langmuir cells: an alternating series of counter-rotating, helical vortex cells set up by the interaction of winds with waves. These cells resulting in alternating convergence and divergence lines on the sea surface that give the appearance of so-called wind rows.

LC₅₀: the concentration of a substance toxic to the median or 50th percentile organism tested within a specified exposure time (e.g., 96-hour LC₅₀).

Lipidomics: the study of the complete profile (lipidome) of lipids produced by an organism, such as those comprising cell membranes (and organelles in eukaryotes), and including identity, structure, enzymatic modification, and function.

Liquefied natural gas (LNG): a natural gas that has been cooled to a liquid form for transportation and/or storage purposes.

Liquefied petroleum gas (LPG): any of a group of hydrocarbon-based gases derived from crude oil refining or natural gas stream fractionation that are often liquified, through pressurization, for ease of transport. They include ethane, propane, normal butane, and isobutane. Uses of these fuels include home heating, industrial, automotive fuel, petrochemical feedstocks, and for drying purposes in farming.

Live oil: used to describe an oil whose composition would change if brought to equilibrium with standard conditions. Normally, live oil exists at pressures higher than standard pressure and contains various dissolved gases. See also Dead oil.

Long-chain alkanes: commonly considered to be alkanes having more than 16 carbons (>C16).

Low molecular weight: relative term referring to the molecular mass of chemicals; in oil, monoaromatics, and aliphatics up to C10 would be typical of these compounds.

Marine oil snow (MOS): an oil-particle aggregate (OPA) that forms when dispersed oil droplets, dispersed, or dissolved oil chemicals attach to, or interact (adsorb/absorb) with marine snow. Marine oil snow also forms when oil droplets, dispersed, or dissolved oil chemicals interact with organisms (mainly microbes and phytoplankton), causing exudation of polymeric organic material (exopolymers) that interact with the oil droplets and/or dispersed or dissolved oil chemicals. It is differentiated in one manner from other types of OPAs by higher proportions of microbes and organic material.

Marine snow: a descriptive term referring to aggregates with a general range in size from >0.5 mm to 10 cm that form mainly in upper layers of the ocean. Their composition can include organic and inorganic material from various sources (e.g., phytoplankton, microbes, plankton, fecal pellets, sand, soot, dust, sources) incorporated into a hydrated matrix of biopolymers. After initial formation, marine snow can have a range of densities that cause it to sink, rise, or be neutrally buoyant during its existence.

Mass spectrometry (MS): an analytical method used for detailed characterization of petroleum components, often in combination with GC, hence GC-MS.

MCF and MMCF: units of volume measurement for natural gas. An abbreviation that combines the Roman numeral M as a stand-in for 1,000, or MM for 1,000,000, with the term “cubic feet” (CF).

Mechanism of action (MOA): describes a functional or anatomical change at the molecular level.

Medium molecular weight (MMW): relative terms referring to the molecular mass of chemicals; in oil, 3- to 6-ringed polycyclic aromatic hydrocarbons and aliphatics up to C20 would be typical of MMW compounds.

Mesocosm: an experimental system designed to study the behavior of natural ecosystems under controlled conditions. Mesocosm experiments can help to link laboratory studies with observational field studies.

Metabolism: the biochemical processes occurring in a living entity (a single cell, an organ, or an entire organism) that maintain life.

Metabolomics: comprehensive chemical analysis of small molecules produced as intermediates or end products of biochemical reactions (i.e., metabolites) by an organism or community of organisms (i.e., an environmental sample) under optimal or stressed conditions.

Metagenomics: DNA sequence-based analysis of the collective genomes of (micro)biological assemblages from an environmental sample that provides information about community composition and/or function.

Metatranscriptomics: analysis of the function and activity of the collective suite of RNA transcripts extracted from a community of (micro)organisms to assess and profile which genes are being expressed in that environment at the sampling time.

Microbiota: the complete community of microorganisms present in a particular location or environmental habitat, including on or in higher organisms.

Microcosm: a small-scale, controlled experimental system intended to replicate all or parts of the conditions of an ecosystem in order to simulate and predict the behavior of natural ecosystems under specific conditions.

Mid-chain alkanes: commonly considered to be alkanes of six to 16 carbons in length (C6–C16).

Mineralization: complete oxidation of a compound (e.g., hydrocarbon) to carbon dioxide or methane, water, and biomass; may be accomplished by a single species of organism or by a community of microbes.

Mode of action (MoA): describes a functional or anatomical change at the cellular level, resulting from the exposure of a living organism to a substance.

Monitored natural attenuation: an environmental remediation strategy in which there is no intervention, but the site is monitored using various parameters.

Monoaromatics: aromatic hydrocarbons having only a single benzene ring; may also have one or more alkyl side chains.

MOS sedimentation and flocculent accumulation (MOSSFA): a process describing the sinking of oil chemicals through the water column and deposition to surface sediments through sedimentation and flocculent accumulation. First observed at some sampling and observational locations during and after the Deepwater Horizon oil spill, this process is thought to be a result of increases in density through the combined processes of aggregation and microbial degradation.

Naphthenic acids: a class of polar petroleum compounds that contributes to aquatic toxicity and to petroleum infrastructure corrosion.

Natural attenuation: remediation of a contaminated site by natural processes alone, without human intervention.

Natural dispersion: ability of oil to naturally disperse under ambient environmental conditions.

Natural gas: naturally occurring hydrocarbon gases found in porous rock formations. Its principal component is usually methane. Non-hydrocarbon gases such as carbon dioxide and hydrogen sulfide can sometimes be present in natural gas.

Neat oil: an oil that has not been diluted or altered by the addition of other components such as diluents or processing chemicals.

Nekton: Living organisms that are able to swim and move independently of currents, as opposed to most phytoplankton. Nekton are heterotrophic, have a large size range, and include marsh fish, squid, and sharks.

Obligate hydrocarbonoclastic bacteria: bacteria that have adapted to use hydrocarbons as their sole source of carbon and energy for growth and metabolism.

Oil-mineral aggregate (OMA): floc-containing oil adhering to mineral particle(s), which may float, sink, or resuspend in a water column; historically referred to as clay–oil flocculation.

Oil-particle aggregate (OPA): more general term than OMA, describing oil adhering to particles that may be inorganic (minerals) and/or organic, including microbial cells.

Olefin: a hydrocarbon containing one or more double bonds, such as an alkene or cycloalkene.

Oleophilic: having a strong affinity for oils.

Oligochaeta: a subclass of animals in the phylum Annelida, which is made up of many aquatic and terrestrial worms. Specifically, oligochaetes usually have few setae (chaetae) or “bristles” on their outer body surfaces and lack parapodia, unlike Polychaeta.

Omics: a collective term for (meta)genomics, (meta)transcriptomics, proteomics, lipidomics, and/or metabolomics.

Ostracod: a small crustacean, typically 1 mm in size, with a flattened body from side to side and protected by a bivalve-like, chitinous, or calcareous valve or “shell.”

Oxidation: the loss of electrons by a molecule during a chemical reaction.

Partitioning: the diffusion of compounds between two immiscible liquid phases, including water and oil droplets and water and lipid membranes; may be described by the term K_ow, when at equilibrium.

Petroleum: synonymous with crude oil; a naturally occurring complex mixture of thousands of different hydrocarbon and non-hydrocarbon molecules.

Photochemical reaction: a chemical reaction initiated when a molecule absorbs light energy; a more general term than photo-oxidation.

Photo-enhanced toxicity: increased toxicity due to photooxidation in vivo.

Photo-oxidation: oxidation of chemicals due to the influence of photic energy, usually from UV light.

Phytoplankton: microscopic photosynthetic organisms that live in the upper regions of marine and freshwaters where light penetrates; includes Bacteria, eukaryotic protists, and microalgae.

Piezotolerant: capable of withstanding hydrostatic pressure (e.g., >10 MPa).

Pneumatophores: roots commonly found in mangrove species that grow in saline mud flats. The roots grow upward out of the mud and water to function as the site of oxygen intake for the submerged primary root system.

Polychaete: also known as bristle worms; a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called setae.

Polycyclic aromatic hydrocarbons (PAHs): a subclass of aromatic hydrocarbons having two or more fused benzene rings; may also have one or more alkyl side chains, generating large suites of isomers; some are considered “priority pollutants” because of their toxicity and/or potential carcinogenicity.

Produced water: water produced in connection with oil and natural gas exploration and development activities.

Prokaryote: a division of life comprising the two prokaryotic domains of Bacteria and Archaea. All prokaryotes lack a nucleus, in contrast to eukaryotes.

Proteomics: study of the total complement of proteins (the proteome) produced by an organism under specific conditions, including identification, quantification, enzymatic modification, and inference of function.

Pseudo-component: a component having averaged thermodynamic properties that is used to represent a group of compounds in a petroleum mixture. Any number or type of compounds can be included within an arbitrary pseudo-component. Pseudo-components are used to reduce the thousands of individual molecules present in a crude oil mixture to a tractable number of quantifiable groups.

Psychrophiles: organisms (typically prokaryotes) that are capable of growth only at low temperatures (typically -20°C to +10°C) and perish at higher temperatures (e.g., above +15°C). They inhabit environments that are constantly cold, such as the deep-sea and polar ecosystems.

Psychrotolerant: property of cold-tolerant organisms (often prokaryotes) that are able to grow at cold temperatures (e.g., 4°C) but have optimum growth at higher temperatures (e.g., 20°–40°C).

Psychrotroph: cold-loving organisms (often prokaryotes) with growth temperatures slightly higher than psychrophiles (e.g., maximum growth temperature of ~+20°C).

Q10 rule: a generalized relationship of temperature sensitivity of (bio)chemical reactions over a range of 10°C (e.g., a measure of the change in enzyme rate over 10°C, within a tolerated range). Traditionally, a factor of 2 has been applied to estimate that biological activity (e.g., microbial growth) an organism decreases by 2 for a 10°C drop in ambient temperature, or increases 2-fold for a 10°C increase in temperature, to a minimum or maximum tolerated temperature.

Rare biosphere: a term describing organisms that persist in an environment but are present at low abundance (e.g., <1% of the total community) under a given condition. The rare biosphere cohort can change if environmental conditions change, and rare taxa can increase many-fold, “blooming” transiently or permanently.

Reactive oxygen species (ROS): a group of chemicals that can cause cellular damage due to their high reactivity and are released during reactions that add oxygen to double bonds. In turn, ROS can react with double bonds in lipids, proteins, and nucleic acids to change their structure and function.

Recalcitrant: a term describing a compound or molecule that persists in nature for a long time and resists biodegradation or chemical breakdown.

Redox discontinuity layer: the sediment transition boundary between oxic and anoxic (no oxygen) conditions.

Resins: a solubility class of poorly characterized, polar petroleum compounds in which each molecule contains one or more atoms of nitrogen, sulfur, and/or oxygen in a hydrocarbon skeleton.

Riparian: related to being situated on or dwelling on the bank of a river or other body of water, such as a lake or tidewater.

Riverine: relating to rivers.

Saturated hydrocarbons: class of hydrocarbons that may be straight-chain, branched-chain or cyclic, in which all carbon atoms have single bonds to either carbon or hydrogen.

Shale: a very fine-grained sedimentary rock that is formed by the consolidation of clay, mud, or silt and that usually has a finely stratified or laminated structure. Certain shale formations contain large amounts of oil and natural gas.

Shale oil: also known as “tight oil” (but not to be confused with “oil shale”); liquid petroleum that is produced from shale oil reservoirs, typically by hydraulic fracturing methods.

Short-chain alkanes: commonly considered to be alkanes less than six carbons in length (<C6).

Slick: a film or layer of oil floating on the water surface, of varying thickness.

Sorbent: an insoluble material or mixture of materials used to recover liquids through the mechanisms of absorption or adsorption, or both.

Sour crude: petroleum that has a >1% total sulfur content that may be present as hydrogen sulfide and/or as organic forms of sulfur in a hydrocarbon backbone.

Sour gas: natural gas or any other gas containing significant amounts of hydrogen sulfide.

Species sensitivity distributions (SSD): these compare the cumulative proportion of species (percentile) affected by a chemical to a toxicity end point measured for each species (e.g., 96-hour LC₅₀); the SSD model assumes that species sensitivity is randomly distributed.

Submerged oil: oil that is in the water column below the water surface, such as dispersed oil droplets and neutrally buoyant tar balls.

Surfactant: a surface-active chemical agent that reduces the interfacial tension between two liquids such as oil and water; surfactants are the active component of commercial oil dispersants and household detergents.

Sweet crude: petroleum with low total sulfur content, variously defined as <0.5% or <1% sulfur.

Sweet gas: natural gas that contains little or no hydrogen sulfide.

Sympagic: organisms that complete either their entire life cycle within the sea ice or spend at least part of their life cycle attached to the ice.

Synbit: bitumen diluted with synthetic crude oil, typically at 1:1 ratio.

Synthetic crude oil: a partially refined fraction of bitumen; may be used as a diluent to make dilbit for transport.

Syntrophy: a condition of two bacterial strains that rely on each other for growth by sharing resources that neither can utilize alone; a form of long-term stable cross-feeding.

Target lipid model: estimates the aqueous concentration of organic compounds, or mixtures of organic compounds, that cause toxicity by narcosis.

Taxon (plural, taxa): a coherent group of organisms considered to be evolutionarily related. In practice, a taxon can represent any named taxonomic level (e.g., class, family, species, etc. that may or may not have a formally recognized name and for taxonomic purposes may exist only as a discrete DNA sequence, such as a region of the 16S rRNA gene).

Total petroleum hydrocarbons (TPHs): the total mass of all hydrocarbons in an oil or environmental sample, including the volatile and extractable (non-volatile) hydrocarbons; may be further defined by stating the analytical method used (e.g., GC-detectable TPH or TPH-F [TPH measured by fluorescence], which vary in their rigor).

Total polycyclic aromatic hydrocarbons (TPAHs): including alkyl-PAHs and parent (unsubstituted) PAHs; the sum of all concentrations of PAHs measured by GC-MS or similar analytical method.

Toxic unit (TU): ratio between the concentration of a compound in water and a toxic end point (e.g., 96-hour LC₅₀).

Transcriptomics: study of the RNA transcripts in an organism (i.e., the expression of the complement of genes that are active under specific conditions).

Unresolved complex mixture (UCM): petroleum constituents that are not resolved by conventional GC and appear as a “hump” in the gas chromatogram; comprised of many compounds eluting from the gas chromatogram in an overlapping fashion (i.e., not resolved by gas chromatography).

Unusual mortality event (UME): term coined by the National Oceanic and Atmospheric Administration to describe a greater-than-usual rate of mortality of marine mammals.

Viscosity: a fluid’s internal resistance to flow.

Volatile organic compounds (VOCs): chemicals having high vapor pressure at room temperature (and corresponding low boiling point) that therefore tend to evaporate or sublimate into the air (e.g., BTEX).

Water-accommodated fraction of oil (WAF): hydrocarbons that will partition from oil to water during gentle stirring or mixing; may contain droplets, in contrast to water-soluble fractions.

Water-soluble fraction of oil (WSF): aqueous solution of hydrocarbons that partition from oil; does not include droplet or particulate oil.

Weathering: a suite of processes that cause changes in spilled oil composition and properties. Weathering includes a variety of environmental processes such as spreading, evaporation, photo-oxidation, dissolution, emulsification, and biodegradation, among others.

Xenobiotic: a chemical that is foreign to an organism or ecological system.