Formation of Polycyclic Aromatic Hydrocarbons and Nitrogen Containing Polycyclic Aromatic Compounds in Titan's Atmosphere, the Interstellar Medium and Combustion

Several different mechanisms leading to the formation of (substituted) naphthalene and azanaphthalenes were examined using theoretical quantum chemical calculations. As a result, a series of novel synthetic routes to Polycyclic Aromatic Hydrocarbons (PAHs) and Nitrogen Containing Polycyclic Aromatic Compounds (N-PACs) have been proposed. On Earth, these aromatic compounds originate from incomplete combustion and are released into our environment, where they are known to be major pollutants, often with carcinogenic properties. In the atmosphere of a Saturn's moon Titan, these PAH and N-PACs are believed to play a critical role in organic haze formation, as well as acting as chemical precursors to biologically relevant molecules. The theoretical calculations were performed by employing the ab initio G3(MP2,CC)/B3LYP/6-311G** method to effectively probe the Potential Energy Surfaces (PES) relevant to the PAH and N-PAC formation. Following the construction of the PES, Rice-Ramsperger-Kassel-Markus (RRKM) theory was used to evaluate all unimolecular rate constants as a function of collision energy under single-collision conditions. Branching ratios were then evaluated by solving phenomenological rate expressions for the various product concentrations. The most viable pathways to PAH and N-PAC formation were found to be those where the initial attack by the ethynyl (C2H) or cyano (CN) radical toward a unsaturated hydrocarbon molecule led to the formation of an intermediate which could not effectively lose a hydrogen atom. It is not until ring cyclization has occurred, that hydrogen elimination leads to a closed shell product. By quenching the possibility of the initial hydrogen atom elimination, one of the most competitive processes preventing the PAH or N-PAC formation was avoided, and the PAH or N-PAC formation was allowed to proceed. It is concluded that these considerations should be taken into account when attempting to explore any other potential routes towards aromatic compounds in cold environments, such as on Titan or in the interstellar medium.

Effects of Land Use on Sources and Ages of Inorganic and Organic Carbon in Temperate Headwater Streams

The amounts, sources and relative ages of inorganic and organic carbon pools were assessed in eight headwater streams draining watersheds dominated by either forest, pasture, cropland or urban development in the lower Chesapeake Bay region (Virginia, USA). Streams were sampled at baseflow conditions six different times over 1 year. The sources and ages of the carbon pools were characterized by isotopic (δ13C and ∆14C) analyses and excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). The findings from this study showed that human land use may alter aquatic carbon cycling in three primary ways. First, human land use affects the sources and ages of DIC by controlling different rates of weathering and erosion. Relative to dissolved inorganic carbon (DIC) in forested streams which originated primarily from respiration of young, 14C-enriched organic matter (OM; δ13C = −22.2 ± 3 ‰; ∆14C = 69 ± 14 ‰), DIC in urbanized streams was influenced more by sedimentary carbonate weathering (δ13C = −12.4 ± 1 ‰; ∆14C = −270 ± 37 ‰) and one of pasture streams showed a greater influence from young soil carbonates (δ13C = −5.7 ± 2.5 ‰; ∆14C = 69 ‰). Second, human land use alters the proportions of terrestrial versus autochthonous/microbial sources of stream water OM. Fluorescence properties of dissolved OM (DOM) and the C:N of particulate OM (POM) suggested that streams draining human-altered watersheds contained greater relative contributions of DOM and POM from autochthonous/microbial sources than forested streams. Third, human land uses can mobilize geologically aged inorganic carbon and enable its participation in contemporary carbon cycling. Aged DOM (∆14C = −248 to −202 ‰, equivalent14C ages of 1,811–2,284 years BP) and POM (∆14C = −90 to −88 ‰, 14C ages of 669–887 years BP) were observed exclusively in urbanized streams, presumably a result of autotrophic fixation of aged DIC (−297 to −244 ‰, 14C age = 2,251–2,833 years BP) from sedimentary shell dissolution and perhaps also watershed export of fossil fuel carbon. This study demonstrates that human land use may have significant impacts on the amounts, sources, ages and cycling of carbon in headwater streams and their associated watersheds.

A Suite of Prey Traits Determine Predator and Nutrient Enrichment Effects in a Tri-Trophic Food Chain

Predation, predation risk, and resource quality affect suites of prey traits that collectively impact individual fitness, population dynamics, and community structure. However, studies of multi-trophic level effects generally focus on a single prey trait, failing to capture trade-offs among suites of covarying traits that govern population responses and emergent community patterns. We used structural equation models (SEM) to summarize the non-lethal and lethal effects of crayfish, Procambarus fallax, and phosphorus (P) addition, which affected prey food quality (periphyton), on the interactive effects of behavioral, morphological, developmental, and reproductive traits of snails, Planorbella duryi. Univariate and multivariate analyses suggested trade-offs between production (growth, reproduction) and defense (foraging behavior, shell shape) traits of snails in response to non-lethal crayfish and P addition, but few lethal effects. SEM revealed that non-lethal crayfish effects indirectly limited per capita offspring standing stock by increasing refuge use, slowing individual growth, and inducing snails to produce thicker, compressed shells. The negative effects of non-lethal crayfish on snails were strongest with P addition; snails increased allocation to shell defense rather than growth or reproduction. However, compared to ambient conditions, P addition with non-lethal crayfish still yielded greater per capita offspring standing stock by speeding individual snail growth enabling them to produce more offspring that also grew faster. Increased refuge use in response to non-lethal crayfish led to a non-lethal trophic cascade that altered the spatial distribution of periphyton. Independent of crayfish effects, snails stimulated periphyton growth through nutrient regeneration. These findings illustrate the importance of studying suites of traits that reveal costs associated with inducing different traits and how expressing those traits impacts population and community level processes.