Climate pathways behind phytoplankton-induced atmospheric warming

We investigate the ways in which marine biologically mediated heating increases the surface atmospheric temperature. While the effects of phytoplankton light absorption on the ocean have gained attention over the past years, the impact of this biogeophysical mechanism on the atmosphere is still unclear. Phytoplankton light absorption warms the surface of the ocean, which in turn affects the air–sea heat and CO2 exchanges. However, the contribution of air–sea heat versus CO2 fluxes in the phytoplankton-induced atmospheric warming has not been yet determined. Different so-called climate pathways are involved. We distinguish heat exchange, CO2 exchange, dissolved CO2, solubility of CO2 and sea-ice-covered area. To shed more light on this subject, we employ the EcoGEnIE Earth system model that includes a new light penetration scheme and isolate the effects of individual fluxes. Our results indicate that phytoplankton-induced changes in air–sea CO2 exchange warm the atmosphere by 0.71 ∘C due to higher greenhouse gas concentrations. The phytoplankton-induced changes in air–sea heat exchange cool the atmosphere by 0.02 ∘C due to a larger amount of outgoing longwave radiation. Overall, the enhanced air–sea CO2 exchange due to phytoplankton light absorption is the main driver in the biologically induced atmospheric heating.

Bacteriohopanetetrol-<i>x</i>: constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system

Interpreting lipid biomarkers in the sediment archive requires a good understanding of their application and limitations in modern systems. Recently it was discovered that marine bacteria performing anaerobic ammonium oxidation (anammox), belonging to the genus Ca. Scalindua, uniquely synthesize a stereoisomer of bacteriohopanetetrol (“BHT-x”). The ratio of BHT-x over total bacteriohopanetetrol (BHT, ubiquitously synthesized by diverse bacteria) has been suggested as a proxy for water column anoxia. As BHT has been found in sediments over 50 Myr old, BHT-x has the potential to complement and extend the sedimentary biomarker record of marine anammox, conventionally constructed using ladderane lipids. Yet, little is known about the distribution of BHT-x in relation to the distribution of ladderanes and to the genetic evidence of Ca. Scalindua in modern marine systems. Here, we investigate the distribution of BHT-x and the application of the BHT-x ratio in relation to distributions of ladderane intact polar lipids (IPLs), ladderane fatty acids (FAs) and Ca. Scalindua 16S rRNA genes in suspended particulate matter (SPM) from the water column of the Benguela upwelling system (BUS), sampled across a large oxygen gradient. In BUS SPM, high BHT-x abundances were restricted to the oxygen-deficient zone on the continental shelf (at [O2] < 45 µmol L−1, in all but one case). High BHT-x abundances co-occurred with high abundances of the Ca. Scalindua 16S rRNA gene (relative to the total number of bacterial 16S rRNA genes) and ladderane IPLs. At shelf stations with [O2] > 50 µmol L−1, the BHT-x ratio was < 0.04 (in all but one case). In apparent contradiction, ladderane FAs and low abundances of BHT and BHT-x (resulting in BHT-x ratios > 0.04) were also detected in oxygenated offshore waters ([O2] up to 180 µmol L−1), whereas ladderane IPLs were undetected. The index of ladderane lipids with five cyclobutane rings (NL5) correlates with in situ temperature. NL5-derived temperatures suggested that ladderane FAs in the offshore waters were not synthesized in situ but were transported down-slope from warmer shelf waters. Thus, in sedimentary archives of systems with known lateral organic matter transport, such as the BUS, relative BHT and BHT-x abundances should be carefully considered. In such systems, a higher BHT-x ratio may act as a safer threshold for deoxygenation and/or Ca. Scalindua presence: our results and previous studies indicate that a BHT-x ratio of ≥ 0.2 is a robust threshold for oxygen-depleted waters ([O2] < 50 µmol kg−1). In our data, ratios of ≥ 0.2 coincided with Ca. Scalindua 16S rRNA genes in all samples (n=62), except one. Lastly, when investigating in situ anammox, we highlight the importance of using ladderane IPLs over BHT-x and/or ladderane FAs; these latter compounds are more recalcitrant and may derive from transported fossil anammox bacteria remnants.

Enhanced bioavailability of dissolved organic matter (DOM) in human-disturbed streams in Alpine fluvial networks

The influence of human activities on the role of inland waters in the global carbon (C) cycle is poorly constrained. In this study, we investigated the impact of human land use on the sources and biodegradation of dissolved organic matter (DOM) and its potential impact on bacterial respiration in 10 independent catchments of the Lake Geneva basin. Sites were selected along a gradient of human disturbance (agriculture and urbanization) and were visited twice during the winter high-flow period. Bacterial respiration and DOM bioavailability were measured in the laboratory through standardized dark bioassays, and the influence of human land uses on DOM sources, composition and reactivity was assessed from fluorescence spectroscopy. Bacterial respiration was higher in agro-urban streams but was related to a short-term bioreactive pool (0–6 d of incubation) of autochthonous origin, whose relative contribution to the total DOM pool increased with the degree of human disturbance. On the other hand, the degradation of a long-term (6–28 d) bioreactive pool related to terrestrial DOM was independent from the catchment land use and did not contribute substantially to aquatic bacterial respiration. From a greenhouse gas emission perspective, our results suggest that human activities may have a limited impact on the net C exchanges between inland waters and the atmosphere, as most CO2 fixed by aquatic producers in agro-urban streams is cycled back to the atmosphere after biomineralization. Although seasonal and longitudinal changes in DOM sources must be considered, the implications of our results likely apply more widely as a greater proportion of autochthonous-DOM signature is a common feature in human-impacted catchments. Yet, on a global scale, the influence of human activities remains to be determined given the large diversity of effects of agriculture and urbanization on freshwater DOM depending on the local environmental context.

Versatile soil gas concentration and isotope monitoring: optimization and integration of novel soil gas probes with online trace gas detection

Gas concentrations and isotopic signatures can unveil microbial metabolisms and their responses to environmental changes in soil. Currently, few methods measure in situ soil trace gases such as the products of nitrogen and carbon cycling or volatile organic compounds (VOCs) that constrain microbial biochemical processes like nitrification, methanogenesis, respiration, and microbial communication. Versatile trace gas sampling systems that integrate soil probes with sensitive trace gas analyzers could fill this gap with in situ soil gas measurements that resolve spatial (centimeters) and temporal (minutes) patterns. We developed a system that integrates new porous and hydrophobic sintered polytetrafluoroethylene (sPTFE) diffusive soil gas probes that non-disruptively collect soil gas samples with a transfer system to direct gas from multiple probes to one or more central gas analyzer(s) such as laser and mass spectrometers. Here, we demonstrate the feasibility and versatility of this automated multiprobe system for soil gas measurements of isotopic ratios of nitrous oxide (δ18O, δ15N, and the 15N site preference of N2O), methane, carbon dioxide (δ13C), and VOCs. First, we used an inert silica matrix to challenge probe measurements under controlled gas conditions. By changing and controlling system flow parameters, including the probe flow rate, we optimized recovery of representative soil gas samples while reducing sampling artifacts on subsurface concentrations. Second, we used this system to provide a real-time window into the impact of environmental manipulation of irrigation and soil redox conditions on in situ N2O and VOC concentrations. Moreover, to reveal the dynamics in the stable isotope ratios of N2O (i.e., 14N14N16O, 14N15N16O, 15N14N16O, and 14N14N18O), we developed a new high-precision laser spectrometer with a reduced sample volume demand. Our integrated system – a tunable infrared laser direct absorption spectrometry (TILDAS) in parallel with Vocus proton transfer reaction mass spectrometry (PTR-MS), in line with sPTFE soil gas probes – successfully quantified isotopic signatures for N2O, CO2, and VOCs in real time as responses to changes in the dry–wetting cycle and redox conditions. Broadening the collection of trace gases that can be monitored in the subsurface is critical for monitoring biogeochemical cycles, ecosystem health, and management practices at scales relevant to the soil system.

Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

Tropical rivers emit large amounts of carbon dioxide (CO2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km2) of the Nyong watershed (Cameroon 27 800 km2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO2 m−2 yr−1) compared to stream 1 (286 ± 228 gC-CO2 m−2 yr−1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr−1) and wetland (4.0 ± 1.5 MgC yr−1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr−1) and 73 % (33.0 ± 12.4 MgC yr−1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO2 degassing from the entire river network was 652 ± 161 GgC-CO2 yr−1 (23.4 ± 5.8 MgC CO2 km−2 yr−1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO2 m−2 yr−1, which implied that only ∼ 10 % of the CO2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr−1 (10.3 ± 5.8 MgC km−2 yr−1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

Derivation of seawater <i>p</i>CO₂ from net community production identifies the South Atlantic Ocean as a CO₂ source

A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite-derived NCP, net primary production (NPP) or Chl a to compare which biological proxy produces the most accurate fields of pCO2 (sw). Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. A perturbation analysis assessed the potential maximum reduction in pCO2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw) and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO2 sink.

Modeling polar marine ecosystem functions guided by bacterial physiological and taxonomic traits

Heterotrophic marine bacteria utilize organic carbon for growth and biomass synthesis. Thus, their physiological variability is key to the balance between the production and consumption of organic matter and ultimately particle export in the ocean. Here we investigate a potential link between bacterial traits and ecosystem functions in the rapidly warming West Antarctic Peninsula (WAP) region based on a bacteria-oriented ecosystem model. Using a data assimilation scheme, we utilize the observations of bacterial groups with different physiological traits to constrain the group-specific bacterial ecosystem functions in the model. We then examine the association of the modeled bacterial and other key ecosystem functions with eight recurrent modes representative of different bacterial taxonomic traits. Both taxonomic and physiological traits reflect the variability in bacterial carbon demand, net primary production, and particle sinking flux. Numerical experiments under perturbed climate conditions demonstrate a potential shift from low nucleic acid bacteria to high nucleic acid bacteria-dominated communities in the coastal WAP. Our study suggests that bacterial diversity via different taxonomic and physiological traits can guide the modeling of the polar marine ecosystem functions under climate change.

Bioaerosols and atmospheric ice nuclei in a Mediterranean dryland: community changes related to rainfall

Certain biological particles are highly efficient ice nuclei (IN), but the actual contribution of bioparticles to the pool of atmospheric IN and their relation to precipitation are not well characterized. We investigated the composition of bioaerosols, ice nucleation activity, and the effect of rainfall by metagenomic sequencing and freezing experiments of aerosol samples collected during the INUIT 2016 campaign in a rural dryland on the eastern Mediterranean island of Cyprus. Taxonomic analysis showed community changes related to rainfall. For the rain-affected samples, we found higher read proportions of fungi, particularly of Agaricomycetes, which are a class of fungi that actively discharge their spores into the atmosphere in response to humidity changes. In contrast, the read proportions of bacteria were reduced, indicating an effective removal of bacteria by precipitation. Freezing experiments showed that the IN population in the investigated samples was influenced by both rainfall and dust events. For example, filtration and heat treatment of the samples collected during and immediately after rainfall yielded enhanced fractions of heat-sensitive IN in the size ranges larger than 5 µm and smaller than 0.1 µm, which were likely of biological origin (entire bioparticles and soluble macromolecular bio-IN). In contrast, samples collected in periods with dust events were dominated by heat-resistant IN active at lower temperatures, most likely mineral dust. The DNA analysis revealed low numbers of reads related to microorganisms that are known to be IN-active. This may reflect unknown sources of atmospheric bio-IN as well as the presence of cell-free IN macromolecules that do not contain DNA, in particular for sizes < 0.1 µm. The observed effects of rainfall on the composition of atmospheric bioaerosols and IN may influence the hydrological cycle (bioprecipitation cycle) as well as the health effects of air particulate matter (pathogens, allergens).

Pelagic primary production in the coastal Mediterranean Sea: variability, trends, and contribution to basin-scale budgets

We estimated pelagic primary production (PP) in the coastal (<200 m depth) Mediterranean Sea from satellite-borne data, its contribution to basin-scale carbon fixation, its variability, and long-term trends during the period 2002–2016. Annual coastal PP was estimated at 0.041 Gt C, which approximately represents 12 % of total carbon fixation in the Mediterranean Sea. About 51 % of this production occurs in the eastern basin, whereas the western and Adriatic shelves contribute with ∼25 % each of total coastal production. Strong regional variability is revealed in coastal PP, from high-production areas (>300 g C m−2) associated with major river discharges to less productive provinces (<50 g C m−2) located in the southeastern Mediterranean. PP variability in the Mediterranean Sea is dominated by interannual variations, but a notable basin-scale decline (17 %) has been observed since 2012 concurring with a period of increasing sea surface temperatures in the Mediterranean Sea and positive North Atlantic Oscillation and Mediterranean Oscillation climate indices. Long-term trends in PP reveal slight declines in most coastal areas (−0.05 to −0.1 g C m−2 per decade) except in the Adriatic where PP increases at +0.1 g C m−2 per decade. Regionalization of coastal waters based on PP seasonal patterns reveals the importance of river effluents in determining PP in coastal waters that can regionally increase up to 5-fold. Our study provides insight into the contribution of coastal waters to basin-scale carbon balances in the Mediterranean Sea while highlighting the importance of the different temporal and spatial scales of variability.

An analysis of the variability in <i>δ</i>¹³C in macroalgae from the Gulf of California: indicative of carbon concentration mechanisms and isotope discrimination during carbon assimilation

The isotopic composition of carbon in macroalgae (δ13C) is highly variable, and its prediction is complex concerning terrestrial plants. The determinants of δ13C macroalgal variations were analyzed in a large stock of specimens that vary in taxa and morphology and were collected in shallow marine habitats in the Gulf of California (GC) with distinctive environmental conditions. A large δ13C variability (−34.6 ‰ to −2.2 ‰) was observed. Life-forms (taxonomy 57 %, morphology and structural organization 34 %) explain the variability related to carbon use physiology. Environmental conditions influenced the δ13C macroalgal values but did not change the physiology, which is most likely inherently species-specific. Values of δ13C were used as indicators of the presence or absence of carbon concentrating mechanisms (CCMs) and as integrative values of the isotope discrimination during carbon assimilation in the life cycle macroalgae. Based on δ13C signals, macroalgae were classified in three strategies relative to the capacity of CCM: (1) HCO3- uptake (δ13C > −10 ‰), (2) using a mix of CO2 and HCO3- uptake (-10<δ13C > −30 ‰), and (3) CO2 diffusive entry (δ13C < −30 ‰). Most species showed a δ13C that indicates a CCM using a mix of CO2 and HCO3- uptake. HCO3- uptake is also widespread among GC macroalgae, with many Ochrophyta species. Few species belonging to Rhodophyta relied on CO2 diffusive entry exclusively, while calcifying macroalgae species using HCO3- included only Amphiroa and Jania. The isotopic signature evidenced the activity of CCM, but it was inconclusive about the preferential uptake of HCO3- and CO2 in photosynthesis and the CCM type expressed in macroalgae. In the study of carbon use strategies, diverse, species-specific, and complementary techniques to the isotopic tools are required.