Development of nascent autotrophic carbon fixation systems in various redox conditions of the fluid degassing in early Earth

<p><strong>Abstract.</strong> Strategies for the origin and development of primary metabolism on early Earth were determined by the two main regimes of degassing of Earth in the form of CO₂ or CH₄ fluid impulses. Among the existing theories of the autotrophic origin of the life, CO₂ is usually considered the carbon source for nascent autotrophic metabolism. However, the ancestral carbon used in metabolism may have been derived from CH₄ if the outflow of magma fluid to the surface of the Earth consisted mainly of methane. Primary biochemical systems are present in methane degassing regimes developed in an environment of high partial pressure of methane, which is a source of carbon for nascent metabolic systems. Due to the absence of molecular oxygen in the Archaean conditions, this metabolism would have been anaerobic, i.e., oxidation of methane must be realized by inorganic high-potential electron acceptors. In light of the primacy and predominance of CH₄-dependent metabolism in hydrothermal systems of the ancient Earth, we propose a model of carbon fixation, which is a sequence of reactions in a hypothetical methane-fumarate (MF) cycle. Thermodynamics calculations showed a high efficiency of oxidation of methane to acetate (methanotrophic acetogenesis) by oxidized nitrogen compounds in hydrothermal systems. Thermodynamically favorable were also reactions involving the introduction of carbon methane into the intermediates of the proposed MF cycle. The methane oxidation reactions with the use of oxygen of iron mineral buffers are closer to the equilibrium state, which apparently determines the possibilities of primordial cycle flow in the forward or reverse directions.</p>

Interspecific variation in tropical tree height and crown allometries in relation to life history traits

<p><strong>Abstract.</strong> Tree allometric relationships are widely employed to estimate forest biomass and production, and are basic building blocks of dynamic vegetation models. In tropical forests, allometric relationships are often modeled by fitting scale-invariant power functions to pooled data from multiple species, an approach that fails to reflect finite size effects at the smallest and largest sizes, and that ignores interspecific differences in allometry. Here, we analyzed allometric relationships of tree height (9884 individuals) and crown area (2425) with trunk diameter using species-specific morphological and life history data of 162 species from Barro Colorado Island, Panamá. We fit nonlinear, hierarchical models informed by species traits and assessed the performance of three alternative functional forms: the scale-invariant power function, and the saturating Weibull and generalized Michaelis-Menten (gMM) functions. The relationship of tree height with trunk diameter was best fit by a saturating gMM model in which variation in allometric parameters was related to interspecific differences in sapling growth rates, a measure of regeneration light demand. Light-demanding species attained taller heights at comparatively smaller diameters as juveniles and had shorter asymptotic heights at larger diameters as adults. The relationship of crown area with trunk diameter was best fit by a power function model incorporating a weak positive relationship between crown area and species-specific wood density. The use of saturating functional forms and the incorporation of functional traits in tree allometric models is a promising approach to improve estimates of forest biomass and productivity. Our results provide an improved basis for parameterizing tropical tree functional types in vegetation models.</p>

Sediment trap efficiency of paddy fields at the watershed scale in a mountainous catchment in Northwest Vietnam

Composite agricultural systems with permanent maize cultivation in the uplands and irrigated rice in the valleys are very common in mountainous Southeast Asia. The soil loss and fertility decline of the upland fields is well documented, but little is known about reallocation of these sediments within the landscape. In this study, a turbidity-based linear mixed model was used to quantify sediment inputs, from surface reservoir irrigation water and from direct overland flow, into a paddy area of 13 hectares. Simultaneously, the sediment load exported from the rice fields was determined. Mid-infrared spectroscopy was applied to analyze sediment particle size. Our results showed that per year, 64 Mg ha−1 of sediments were imported into paddy fields, of which around 75 % were delivered by irrigation water and the remainder by direct overland flow during rainfall events. Overland flow contributed one third of the received sandy fraction, while irrigated sediments were predominantly silty. Overall, rice fields were a net sink for sediments, trapping 28 Mg ha−1 a−1 or almost half of total sediment inputs. As paddy outflow consisted almost exclusively of silt- and clay-sized material, 24 Mg ha−1 a−1 of the trapped amount of sediment was estimated to be sandy. Under continued intensive upland maize cultivation, such a sustained input of coarse material could jeopardize paddy soil fertility, puddling capacity and ultimately also food security of the inhabitants of these mountainous areas. Preventing direct overland flow from entering the paddy fields, however, could reduce sand inputs by up to 34 %.

Evidence for methane production by marine algae (<i>Emiliana huxleyi</i>) and its implication for the methane paradox in oxic waters

Methane (CH4), an important greenhouse gas that affects radiation balance and consequently the earth's climate, still has uncertainties in its sinks and sources. The world's oceans are considered to be a source of CH4 to the atmosphere, although the biogeochemical processes involved in its formation are not fully understood. Several recent studies provided strong evidence of CH4 production in oxic marine and freshwaters but its source is still a topic of debate. Studies of CH4 dynamics in surface waters of oceans and large lakes have concluded that pelagic CH4 supersaturation cannot be sustained either by lateral inputs from littoral or benthic inputs alone. However, frequently regional and temporal oversaturation of surface waters occurs. This comprises the observation of a CH4 oversaturating state within the surface mixed layer, sometimes also termed the "oceanic methane paradox". In this study we considered marine algae as a possible direct source of CH4. Therefore, the coccolithophore Emiliania huxleyi was grown under controlled laboratory conditions and supplemented with two 13C-labelled carbon substrates, namely bicarbonate and a position-specific 13C-labelled methionine (R-S-13CH3). The CH4 production was 0.7 μg POC g−1 d−1, or 30 ng g−1 POC h−1. After supplementation of the cultures with the 13C labelled substrate, the isotope label was observed in headspace-CH4. Moreover, the absence of methanogenic archaea within the algal culture and the oxic conditions during CH4 formation suggest that marine algae such as Emiliania huxleyi contribute to the observed spatial and temporal restricted CH4 oversaturation in ocean surface waters.

Challenges associated with modeling low-oxygen waters in Chesapeake Bay: a multiple model comparison

As three-dimensional (3-D) aquatic ecosystem models are becoming used more frequently for operational water quality forecasts and ecological management decisions, it is important to understand the relative strengths and limitations of existing 3-D models of varying spatial resolution and biogeochemical complexity. To this end, two-year simulations of the Chesapeake Bay from eight hydrodynamic-oxygen models have been statistically compared to each other and to historical monitoring data. Results show that although models have difficulty resolving the variables typically thought to be the main drivers of dissolved oxygen variability (stratification, nutrients, and chlorophyll), all eight models have significant skill in reproducing the mean and seasonal variability of dissolved oxygen. In addition, models with constant net respiration rates independent of nutrient supply and temperature reproduced observed dissolved oxygen concentrations about as well as much more complex, nutrient-dependent biogeochemical models. This finding has significant ramifications for short-term hypoxia forecasts in the Chesapeake Bay, which may be possible with very simple oxygen parameterizations, in contrast to the more complex full biogeochemical models required for scenario-based forecasting. However, models have difficulty simulating correct density and oxygen mixed layer depths, which are important ecologically in terms of habitat compression. Observations indicate a much stronger correlation between the depths of the top of the pycnocline and oxycline than between their maximum vertical gradients, highlighting the importance of the mixing depth in defining the region of aerobic habitat in the Chesapeake Bay when low-oxygen bottom waters are present. Improvement in hypoxia simulations will thus depend more on the ability of models to reproduce the correct mean and variability of the depth of the physically driven surface mixed layer than the precise magnitude of the vertical density gradient.

Potential environmental impact of tidal energy extraction in the Pentland Firth at large spatial scales: results of a biogeochemical model

A model study was carried out of the potential large-scale (> 100 km) effects of marine renewable tidal energy generation in the Pentland Firth, using the 3-D hydrodynamics-biogeochemistry model GETM-ERSEM-BFM. A realistic 800 MW scenario and an exaggerated academic 8 GW scenario were considered. The realistic 800 MW scenario suggested minor effects on the tides, and undetectable effects on the biogeochemistry. The academic 8 GW scenario suggested effects would be observed over hundreds of kilometres away with changes of up to 10 % in tidal and ecosystem variables, in particular in a broad area in the vicinity of The Wash. There, waters became less turbid, and primary production increased with associated increases in faunal ecosystem variables. Moreover, a one-off increase in carbon storage in the sea bed was detected. Although these first results suggest positive environmental effects, further investigation is recommended of: (i) the residual circulation in the vicinity of the Pentland Firth and effects on larval dispersal using a higher resolution model, (ii) ecosystem effects with (future) state-of-the-art models if energy extraction substantially beyond 1 GW is planned.

Low Florida coral calcification rates in the Plio-Pleistocene

In geological outcrops and drill cores from reef frameworks, the skeletons of scleractinian corals are usually leached and more or less completely transformed into sparry calcite because the highly porous skeletons formed of metastable aragonite (CaCO3) undergo rapid diagenetic alteration. Upon alteration, ghost structures of the distinct annual growth bands may be retained allowing for reconstructions of annual extension (= growth) rates, but information on skeletal density needed for reconstructions of calcification rates is invariably lost. Here we report the first data of calcification rates of fossil reef corals which escaped diagenetic alteration. The corals derive from unlithified shallow water carbonates of the Florida platform (southeastern USA), which formed during four interglacial sea level highstands dated 3.2, 2.9, 1.8, and 1.2 Ma in the mid Pliocene to early Pleistocene. With regard to the preservation, the coral skeletons display smooth growth surfaces with minor volumes of marine aragonite cement within intra-skeletal porosity. Within the skeletal structures, dissolution is minor along centers of calcification. Mean extension rates were 0.44 ± 0.19 cm yr−1 (range 0.16 to 0.86 cm yr−1) and mean bulk density was 0.86 ± 0.36 g cm−3 (range 0.55 to 1.22 g cm−3). Correspondingly, calcification rates ranged from 0.18 to 0.82 g cm−2 yr−1 (mean 0.38 ± 0.16 g cm−2 yr−1), values which are 50 % of modern shallow-water reef corals. To understand the possible mechanisms behind these low calcification rates, we compared the fossil calcification with modern zooxanthellate-coral (z-coral) rates from the Western Atlantic (WA) and Indo-Pacific (IP) calibrated against sea surface temperature (SST). In the fossil data, we found an analogous relationship with SST in z-corals from the WA, i.e. density increases and extension rate decreases with increasing SST, but over a significantly larger temperature window during the Plio-Pleistocene. With regard to the environment of coral growth, stable isotope proxy data from the fossil corals and the overall structure of the ancient shallow marine communities are consistent with a well-mixed, open marine environment similar to the present-day Florida Reef Tract, but variably affected by intermittent upwelling. Upwelling along the platform may explain low rates of reef coral calcification and inorganic cementation, but is too localized to account for low extension rates of Pliocene z-corals recorded throughout the tropical Caribbean in the western Atlantic region. Low aragonite saturation on a more global scale in response to rapid glacial/interglacial CO2 cyclicity is also a potential factor, but Plio-Pleistocene atmospheric pCO2 is believed to have been broadly similar to the present-day. Heat stress related to globally high interglacial SST, only episodically moderated by intermittent upwelling affecting the Florida platform seems to be the most likely reason for low calcification rates. From these observations we suggest some present coral reef systems to be endangered from future ocean warming.

Larval development and settling of <i>Macoma balthica</i> in a large-scale mesocosm experiment at different <i>f</i>CO₂ levels

Anthropogenic carbon dioxide (CO2) emissions are causing severe changes in the global inorganic carbon balance of the oceans. Associated ocean acidification is expected to impose a major threat to marine ecosystems worldwide, and it is also expected to be amplified in the Baltic Sea where the system is already at present exposed to relatively large natural seasonal and diel pH fluctuations. The response of organisms to future ocean acidification has primarily been studied in single-species experiments, whereas the knowledge of community-wide responses is still limited. To study responses of the Baltic Sea pelagic community to a range of future CO2-scenarios, six ∼ 55 m3 pelagic mesocosms were deployed in the northern Baltic Sea in June 2012. In this specific study we focused on the tolerance, development and subsequent settlement process of the larvae of the benthic key-species Macoma balthica when exposed to different levels of future CO2. We found that the settling of M. balthica was delayed along the increasing CO2 gradient of the mesocosms. Also, when exposed to increasing CO2 levels larvae settled at a larger size, indicating a developmental delay. With on-going climate change, both the frequency and extent of regularly occurring high CO2 conditions is likely to increase, and a permanent pH decrease will likely occur. The strong impact of increasing CO2 levels on early-stage bivalves is alarming as these stages are crucial for sustaining viable populations, and a failure in their recruitment would ultimately lead to negative effects on the population.

A probabilistic assessment of calcium carbonate export and dissolution in the modern ocean

The marine cycle of calcium carbonate (CaCO3) is an important element of the carbon cycle and co-governs the distribution of carbon and alkalinity within the ocean. However, CaCO3 fluxes and mechanisms governing CaCO3 dissolution are highly uncertain. We present an observationally-constrained, probabilistic assessment of the global and regional CaCO3 budgets. Parameters governing pelagic CaCO3 export fluxes and dissolution rates are sampled using a Latin-Hypercube scheme to construct a 1000 member ensemble with the Bern3D ocean model. Ensemble results are constrained by comparing simulated and observation-based fields of excess dissolved calcium carbonate (TA*). The minerals calcite and aragonite are modelled explicitly and ocean–sediment fluxes are considered. For local dissolution rates either a strong, a weak or no dependency on CaCO3 saturation is assumed. Median (68 % confidence interval) global CaCO3 export is 0.82 (0.67–0.98) Gt PIC yr−1, within the lower half of previously published estimates (0.4–1.8 Gt PIC yr−1). The spatial pattern of CaCO3 export is broadly consistent with earlier assessments. Export is large in the Southern Ocean, the tropical Indo–Pacific, the northern Pacific and relatively small in the Atlantic. Dissolution within the 200 to 1500 m depth range (0.33; 0.26–0.40 Gt PIC yr−1) is substantially lower than inferred from the TA*-CFC age method (1 ± 0.5 Gt PIC yr−1). The latter estimate is likely biased high as the TA*-CFC method neglects transport. The constrained results are robust across a range of diapycnal mixing coefficients and, thus, ocean circulation strengths. Modelled ocean circulation and transport time scales for the different setups were further evaluated with CFC11 and radiocarbon observations. Parameters and mechanisms governing dissolution are hardly constrained by either the TA* data or the current compilation of CaCO3 flux measurements such that model realisations with and without saturation-dependent dissolution achieve skill. We suggest to apply saturation-independent dissolution rates in Earth System Models to minimise computational costs.

Carbon sequestration in managed temperate coniferous forests under climate change

Management of temperate forests has the potential to increase carbon sinks and mitigate climate change. However, those opportunities may be confounded by negative climate change impacts. We therefore need a better understanding of climate change alterations to temperate forest carbon dynamics before developing mitigation strategies. The purpose of this project was to investigate the interactions of species composition, fire, management and climate change on the Copper–Pine creek valley, a temperate coniferous forest with a wide range of growing conditions. To do so, we used the LANDIS-II modelling framework including the new Forest Carbon Succession extension to simulate forest ecosystems under four different productivity scenarios, with and without climate change effects, until 2050. Significantly, the new extension allowed us to calculate the Net Sector Productivity, a carbon accounting metric that integrates above and below-ground carbon dynamics, disturbances, and the eventual fate of forest products. The model output was validated against literature values. The results implied that the species optimum growing conditions relative to current and future conditions strongly influenced future carbon dynamics. Warmer growing conditions led to increased carbon sinks and storage in the colder and wetter ecoregions but not necessarily in the others. Climate change impacts varied among species and site conditions and this indicates that both of these components need to be taken into account in when considering climate change mitigation activities and adaptive management. The introduction of a new carbon indicator – Net Sector Productivity, promises to be useful in assessing management effectiveness and mitigation activities.