Autotrophic Abundance Research Articles

Long-term subsoiling and tillage rotation increase carbon storage in soil aggregates and the abundance of autotrophs

Autotrophic microorganisms in soil can increase soil carbon (C) sequestration by utilising chemical energy released from inorganic compounds to fix atmospheric CO2. Therefore, the effects of tillage systems on soil C stocks and autotrophic microbial community deserves in-depth study. The effects of five tillage systems, including no tillage (NoTill), subsoiling (SubS), rotary tillage (RotTill, local general tillage), no tillage-subsoiling-no tillage (NoTill-SubS), and rotary tillage-subsoiling-rotary tillage (RotTill-SubS) were investigated within a 17-year field experiment. Their effects on soil aggregates, C content and enzyme activities were studied, and the impacts on C-fixing microbial communities were analysed using the C-fixing gene cbbL. The macroaggregate portion and RubisCO and ATPase activities were the highest under SubS, causing the aggregate-associated organic C (AOC) to be 93 % higher than that under RotTill. Under RotTill-SubS, SOC, AOC and aggregate-associated microbial biomass C (AMBC) were larger than those under RotTill. RotTill-SubS reduced the microaggregate portion, increased enzyme activities and the relative abundance of C-fixing bacteria, including Sphingomonadales, Burkholderiales, and Nitrosomonadales. The macroaggregate portion under NoTill-SubS and the relative abundance of Sphingomonadales and Burkholderiales was the highest. Correspondingly, the SOC content was the largest (29 % higher than that in RotTill), and the AOC and AMBC contents were 42 % and 31 % larger, respectively, than those in RotTill. Structural equation modelling reveals that increasing tillage or lowering AOC content can directly increase differences in autotrophic bacterial communities. The reduced aggregate stability decreases C content and increases ATPase and RubisCO activities can indirectly increase differences in bacterial community composition.

Seasonal Snowpack Microbial Ecology and Biogeochemistry on a High Arctic Ice Cap Reveals Negligible Autotrophic Activity During Spring and Summer Melt

AbstractSnowpack ecosystem studies are primarily derived from research on snow‐on‐soil ecosystems. Greater research attention needs to be directed to the study of glacial snow covers as most snow cover lies on glaciers and ice sheets. With rising temperatures, snowpacks are getting wetter, which can potentially give rise to biologically productive snowpacks. The present study set out to determine the linkage between the thermal evolution of a snowpack and the seasonal microbial ecology of snow. We present the first comprehensive study of the seasonal microbial activity and biogeochemistry within a melting glacial snowpack on a High Arctic ice cap, Foxfonna, in Svalbard. Nutrients from winter atmospheric bulk deposition were supplemented by dust fertilization and weathering processes. NH4+ and PO43− resources in the snow therefore reached their highest values during late June and early July, at 22 and 13.9 mg m−2, respectively. However, primary production did not respond to this nutrient resource due to an absence of autotrophs in the snowpack. The average autotrophic abundance on the ice cap throughout the melt season was 0.5 ± 2.7 cells mL−1. Instead, the microbial cell abundance was dominated by bacterial cells that increased from an average of (39 ± 19 cells mL−1) in June to (363 ± 595 cells mL−1) in early July. Thus, the total seasonal biological production on Foxfonna was estimated at 153 mg C m−2, and the glacial snowpack microbial ecosystem was identified as net‐heterotrophic. This work presents a seasonal “album” documenting the bacterial ecology of glacial snowpacks.

Temperature sensitivity of dark CO2 fixation in temperate forest soils

Abstract. Globally, soil temperature to 1 m depth is predicted to be up to 4 ∘C warmer by the end of this century, with pronounced effects expected in temperate forest regions. Increased soil temperatures will potentially increase the release of carbon dioxide (CO2) from temperate forest soils, resulting in important positive feedback on climate change. Dark CO2 fixation by microbes can recycle some of the released soil CO2, and CO2 fixation rates are reported to increase under higher temperatures. However, research on the influence of temperature on dark CO2 fixation rates, particularly in comparison to the temperature sensitivity of respiration in soils of temperate forest regions, is missing. To determine the temperature sensitivity (Q10) of dark CO2 fixation and respiration rates, we investigated soil profiles to 1 m depth from beech (deciduous) and spruce (coniferous) forest plots of the Hummelshain forest, Germany. We used 13C-CO2 labelling and incubations of soils at 4 and 14 ∘C to determine CO2 fixation and net soil respiration rates and derived the Q10 values for both processes with depth. The average Q10 for dark CO2 fixation rates normalized to soil dry weight was 2.07 for beech and spruce profiles, and this was lower than the measured average Q10 of net soil respiration rates with ∼2.98. Assuming these Q10 values, we extrapolated that net soil respiration might increase 1.16 times more than CO2 fixation under a projected 4 ∘C warming. In the beech soil, a proportionally larger fraction of the label CO2 was fixed into soil organic carbon than into microbial biomass compared to the spruce soil. This suggests a primarily higher rate of microbial residue formation (i.e. turnover as necromass or release of extracellular products). Despite a similar abundance of the total bacterial community in the beech and spruce soils, the beech soil also had a lower abundance of autotrophs, implying a higher proportion of heterotrophs when compared to the spruce soil; hence this might partly explain the higher rate of microbial residue formation in the beech soil. Furthermore, higher temperatures in general lead to higher microbial residues formed in both soils. Our findings suggest that in temperate forest soils, CO2 fixation might be less responsive to future warming than net soil respiration and could likely recycle less CO2 respired from temperate forest soils in the future than it does now.

Denitrification performance of sulfur-based autotrophic denitrification and biomass‑sulfur-based mixotrophic denitrification in solid-phase denitrifying reactors using novel composite filters

Both the elemental sulfur-based autotrophic denitrification (ESAD) and the biomass‑sulfur-based mixotrophic (simultaneous autotrophic and heterotrophic) denitrification processes (BSMD) are efficient methods for removing nitrate from wastewater. However, a comparative analysis of the denitrification capacity of the BSMD and ESAD in the packed bed reactors is still lacking. In this paper, corncob powder was selected as the biomass source to prepare biomass‑sulfur-based composite filter (BSCF) for the BSMD process. The denitrification performances of the three identical lab-scale bioreactors packed with varying elemental sulfur-based composite filters (ESCFs) were compared under varying loading conditions, and the optimal ESCF of the ESAD system was 2:1 by weight ratio of sulfur powder to shell powder. In pilot-scale experiments, the results showed that BSCF could decrease the sulfate productivity and gave better denitrification performance than the ESCF with the optimal nitrate removal rate (NRR) of 504 ± 12.3 mg NO3−-N·L−1·d−1. In addition, the two-stage flushing strategy (for the removal of aged sludge) can effectively improve the denitrification capacity, while the denitrification will be inhibited when the influent dissolved oxygen concentration was higher than 4.5 mg L−1. Moreover, the heterotrophs and autotrophs were abundant in the reactors. Over time, the abundance of autotrophs increased while that of heterotrophs decreased. Overall, BSCF could be a promising and economic technology to improve the effluent quality.

The autotrophic community across developmental stages of biocrusts in the Gurbantunggut Desert

Autotrophs, such as cyanobacteria, algae, and prokaryotic non-cyanobacterial autotrophs, play fundamental roles in biocrust development by adhering to soil particles and fixing CO2. The variation in autotrophic community structure, in particular that of prokaryotic non-cyanobacterial autotrophs, and the factors driving this variation during biocrust succession have not been revealed. In this study, we investigated the autotrophic communities in bare sand and three biocrust successional stages in the Gurbantunggut Desert by assessing the cbbL gene, which encodes the form I ribulose1,5-bisphosphate carboxylase/oxygenase (RuBisCO) large subunit. The cbbL gene profiles demonstrated that the abundance of cyanobacteria and algae (form IAB, 105–107 copies g−1 soil) and prokaryotic non-cyanobacterial autotrophs (form IC, ~107 copies g−1 soil) changed significantly as succession progressed. Stepwise multiple regression showed that soil water, organic carbon, total phosphorous, available potassium content and pH explained 95.7% of the variation in cyanobacterial and algal (form IAB) abundance. Soil water content explained 60.1% of the variation in prokaryotic non-cyanobacterial autotroph (form IC) abundance. Variation partitioning analysis showed that the variation in form IC community structure was related to soil nutrients (C, N, and P), ion content (Mg2+, Na+ and Ca2+) and physical soil properties (86.1% explained). The variation in form IAB community structure could be predictable by total potassium, total organic carbon and Na+ and Mg2+ content (37.1% explained). Our results showed significant shifts in both autotrophic abundance and community structure, and highlighted the combined effects of soil factors on autotrophic community variation during biocrust succession.

Presence and abundance of bacteria with the Type VI secretion system in a coastal environment and in the global oceans.

Marine bacteria employ various strategies to maintain their competitive advantage over others in a mixed community. The use of Type VI Secretion Systems (T6SS), a protein secretion apparatus used as a molecular weapon for interbacterial competition and eukaryotic interactions, is one of the competitive strategies that is least studied among heterotrophic bacteria living in the water column. To get an insight into the temporal and spatial distribution of bacteria with T6SS in this portion of the marine environment, we examine the presence and abundance of T6SS-bearing bacteria at both local and global scales through the use of metagenome data from water samples obtained from the coast of Monterey Bay and the TARA Oceans project. We also track the abundance of T6SS-harboring bacteria through a two-year time series of weekly water samples in the same coastal site to examine the environmental factors that may drive their presence and abundance. Among the twenty-one T6SS-bearing bacterial genera examined, we found several genera assume a particle-attached lifestyle, with only a few genera having a free-living lifestyle. The abundance of T6SS-harboring bacteria in both niches negatively correlates with the abundance of autotrophs. Globally, we found that T6SS genes are much more abundant in areas with low biological productivity. Our data suggest that T6SS-harboring bacteria tend to be abundant spatially and temporally when organic resources are limited. This ecological study agrees with the patterns observed from several in vitro studies; that T6SS could be an adaptive strategy employed by heterotrophic bacteria to obtain nutrients or reduce competition when resources are in limited quantity.

Ecological drivers switch from bottom-up to top-down during model microbial community successions.

Bottom-up selection has an important role in microbial community assembly but is unable to account for all observed variance. Other processes like top-down selection (e.g., predation) may be partially responsible for the unexplained variance. However, top-down processes and their interaction with bottom-up selective pressures often remain unexplored. We utilised an in situ marine biofilm model system to test the effects of bottom-up (i.e., substrate properties) and top-down (i.e., large predator exclusion via 100 µm mesh) selective pressures on community assembly over time (56 days). Prokaryotic and eukaryotic community compositions were monitored using 16 S and 18 S rRNA gene amplicon sequencing. Higher compositional variance was explained by growth substrate in early successional stages, but as biofilms mature, top-down predation becomes progressively more important. Wooden substrates promoted heterotrophic growth, whereas inert substrates' (i.e., plastic, glass, tile) lack of degradable material selected for autotrophs. Early wood communities contained more mixotrophs and heterotrophs (e.g., the total abundance of Proteobacteria and Euglenozoa was 34% and 41% greater within wood compared to inert substrates). Inert substrates instead showed twice the autotrophic abundance (e.g., cyanobacteria and ochrophyta made up 37% and 10% more of the total abundance within inert substrates than in wood). Late native (non-enclosed) communities were mostly dominated by autotrophs across all substrates, whereas high heterotrophic abundance characterised enclosed communities. Late communities were primarily under top-down control, where large predators successively pruned heterotrophs. Integrating a top-down control increased explainable variance by 7-52%, leading to increased understanding of the underlying ecological processes guiding multitrophic community assembly and successional dynamics.

Assessing the responses of Sphagnum micro-eukaryotes to climate changes using high throughput sequencing.

Current projections suggest that climate warming will be accompanied by more frequent and severe drought events. Peatlands store ca. one third of the world’s soil organic carbon. Warming and drought may cause peatlands to become carbon sources through stimulation of microbial activity increasing ecosystem respiration, with positive feedback effect on global warming. Micro-eukaryotes play a key role in the carbon cycle through food web interactions and therefore, alterations in their community structure and diversity may affect ecosystem functioning and could reflect these changes. We assessed the diversity and community composition of Sphagnum-associated eukaryotic microorganisms inhabiting peatlands and their response to experimental drought and warming using high throughput sequencing of environmental DNA. Under drier conditions, micro-eukaryotic diversity decreased, the relative abundance of autotrophs increased and that of osmotrophs (including Fungi and Peronosporomycetes) decreased. Furthermore, we identified climate change indicators that could be used as early indicators of change in peatland microbial communities and ecosystem functioning. The changes we observed indicate a shift towards a more “terrestrial” community in response to drought, in line with observed changes in the functioning of the ecosystem.

Toxigenic phytoplankton groups and neurotoxin levels related to two contrasting environmental conditions at the coastal area of Rio de Janeiro (west of South Atlantic)

An assessment of the major pigments and neurotoxins and a description of the phytoplankton community were carried out within the coastal region of Rio de Janeiro State (Brazil), during winter and the following spring of 2018. Overall, six stations were investigated for oceanographic conditions (with CTD casts). Filtered water samples were used to estimate the chlorophyll a (CHL-a), carotenoids (CAR), and phycobiliproteins (PHY) using UV–Vis spectrophotometry, as well as the quantification of saxitoxins (STX) and domoic acid (DA), through High Performance Liquid Chromatography (HPLC). Planktonic organisms were counted using sedimentation chambers of different volumes and an inverted microscope. A cluster analysis, SIMPER, and ANOSIM were applied to the phytoplankton data along with diversity indexes, and non-parametric statistics to phycotoxins and pigments. There was a significant difference between the winter and spring phytoplankton community, associated with the mixed layer depth (r2 = −0.626, p < 0.05) and temperature (r2 = 0.641, p < 0.05). Phytoplankton biomass and C:CHL-a indicated a higher production during the winter than in spring, with the potentially toxic genus Pseudo-nitzschia responsible for 12.79% of autotrophic abundance (SIMPER output). Pigments showed a slight increase in CAR during spring, while PHY remained at trace concentrations. Both the DA and STX were quantified in winter and spring, but with significant differences only for STX between the sampling periods. Among the 71 taxa, 11 were identified as potentially toxic with an emphasis on STX-producing dinoflagellates and cyanobacteria, such as Alexandrium sp., Gymnodinium spp. along with Trichodesmium spp. Season-related environmental variability may be the major driving force modulating the mixed assemblage of species that support different levels of phycotoxins.

Autotrophic microbial community succession from glacier terminus to downstream waters on the Tibetan Plateau.

Glaciers harbour diverse microbes and autotrophic microbes play a key role in sustaining the glacial ecosystems by providing organic carbon. The succession of glacier-originated autotrophic microbes and their effects on downstream aquatic ecosystems remain unknown. We herein investigated the shift of autotrophic microbial communities in waters (not biofilms) along a glacier meltwater transect consisting of a glacier terminus outflow (subglacial), a glacial stream, two glacier-fed lakes (upper and lower) and their outflow on the Tibetan Plateau. The autotrophic community was characterized by cbbL gene using qPCR, T-RFLP and clone library/sequencing methods. The results demonstrated that form IC and ID autotrophic microbes exhibited a much higher abundance than form IAB in all waters along the transect. Form IAB autotrophic abundance in waters gradually decreased, while the form IC exhibited a substantial increase in the upper lake waters, and ID exhibited a substantial increase in the lower lake waters. The water form IC autotrophic community structure exhibited a distinguished shift from the glacier terminus outflow to the stream, while the form ID showed a dramatic shift from the stream to the lower lake. Our results revealed the succession patterns of glacier-originated autotrophic microbial communities and possible effects on downstream aquatic ecosystems.

Spatiotemporal Variability in Autotrophic and Heterotrophic Microbial Plankton Abundances in a Subtropical Estuary (Galveston Bay, Texas)

Williams, A.K. and Quigg, A., 2019. Spatiotemporal variability in autotrophic and heterotrophic microbial plankton abundances in a subtropical estuary (Galveston Bay, Texas). Journal of Coastal Research, 35(2), 434–444. Coconut Creek (Florida), ISSN 0749-0208.The subtropical estuary Galveston Bay (Texas) is influenced by frequent riverine pulses from two important rivers in its northern sector and by ocean tides from the Gulf of Mexico in its southern sector. This study examined combinations of abiotic and biotic factors that synergistically influence spatiotemporal variability of autotrophic and heterotrophic microbial plankton abundances (0.2–20 µm). Potential nutrient limitation observed in situ was supported by in vitro enrichment bioassays. Shifts in the relative in situ abundance of autotrophs and heterotrophs, stained with SYBR Green I and enumerated on a flow cytometer, were significantly related to temperature, total nitrogen, dissolved inorganic nitrogen-to-phosphorus ratios, and total organic carbon concentrations along estuarine gradients. Observations reveal a pattern of serial colimitation of the microbial plankton community. Among the variables tested, nitrogen became the principal limiting factor for growth if phosphorous was available and temperatures were warm. In vitro nutrient enrichment bioassays performed with inorganic nitrogen (as nitrate) and Pi revealed variations in the responses between autotrophic and heterotrophic microbes under nutrient-limiting conditions, particularly during co-occurring phytoplankton blooms. The measured dynamics in planktonic relationships will have important impacts on understanding estuarine nutrient processing.

Desert and steppe soils exhibit lower autotrophic microbial abundance but higher atmospheric CO2 fixation capacity than meadow soils

CO2-fixing by soil autotrophic microbes is as important as by plants in semi-arid and arid ecosystems, such as the Tibetan Plateau grassland. CO2-fixing microbial community characteristics, capacity and their driving environmental factors remain unclear. Here we investigated the autotrophic microbial community in grassland surface soils on the Tibetan Plateau using molecular methods targeting the large subunit gene (cbbL) of ribulose-1, 5-bisphosphate carboxylase/oxygenase. The CO2 fixation capacity was assessed by the 13CO2 probing method. The results showed that soil autotrophic microbial abundance substantially increased from desert, steppe to meadow. The autotrophic abundance significantly increased with enhancing mean annual precipitation (MAP), soil ammonium concentration and aboveground plant biomass (APB). Forms IAB and IC autotrophic microbial communities strongly varied with grassland types. Variation partitioning analysis revealed that the structure variations were mainly explained by MAP and aridity, which explained 4.2% and 2.6% for the IAB community, and 7.6% and 8.5% for the IC community. Desert and steppe soils exhibited significantly higher atmospheric 13CO2 fixation rate than meadow soils (29 versus 18 mg kg−1soil d−1). The 13CO2 fixation rate negatively correlated with APB and soil ammonium concentration, demonstrating the substantially important role of autotrophic microbes in oligotrophic soils. Form IAB autotrophs were phylogenetically affiliated with Cyanobacteria. Form IC autotrophs were affiliated with Rhizobiales and Actinobacteria, the former gradually increased and the latter decreased from desert, steppe to meadow. Our findings offer new insight into the importance of MAP in driving soil autotrophic microbial community and highlight microbial roles in carbon cycling in dryland ecosystems.

Multiscale analysis of autotroph-heterotroph interactions in a high-temperature microbial community

Interactions among microbial community members can lead to emergent properties, such as enhanced productivity, stability, and robustness. Iron-oxide mats in acidic (pH 2–4), high-temperature (> 65 °C) springs of Yellowstone National Park contain relatively simple microbial communities and are well-characterized geochemically. Consequently, these communities are excellent model systems for studying the metabolic activity of individual populations and key microbial interactions. The primary goals of the current study were to integrate data collected in situ with in silico calculations across process-scales encompassing enzymatic activity, cellular metabolism, community interactions, and ecosystem biogeochemistry, as well as to predict and quantify the functional limits of autotroph-heterotroph interactions. Metagenomic and transcriptomic data were used to reconstruct carbon and energy metabolisms of an important autotroph (Metallosphaera yellowstonensis) and heterotroph (Geoarchaeum sp. OSPB) from the studied Fe(III)-oxide mat communities. Standard and hybrid elementary flux mode and flux balance analyses of metabolic models predicted cellular- and community-level metabolic acclimations to simulated environmental stresses, respectively. In situ geochemical analyses, including oxygen depth-profiles, Fe(III)-oxide deposition rates, stable carbon isotopes and mat biomass concentrations, were combined with cellular models to explore autotroph-heterotroph interactions important to community structure-function. Integration of metabolic modeling with in situ measurements, including the relative population abundance of autotrophs to heterotrophs, demonstrated that Fe(III)-oxide mat communities operate at their maximum total community growth rate (i.e. sum of autotroph and heterotroph growth rates), as opposed to net community growth rate (i.e. total community growth rate subtracting autotroph consumed by heterotroph), as predicted from the maximum power principle. Integration of multiscale data with ecological theory provides a basis for predicting autotroph-heterotroph interactions and community-level cellular organization.

DNA-SIP Reveals the Diversity of Chemolithoautotrophic Bacteria Inhabiting Three Different Soil Types in Typical Karst Rocky Desertification Ecosystems in Southwest China.

Autotrophs that inhabit soils receive less attention than their counterparts in other ecosystems, such as deep-sea and subsurface sediments, due to the low abundance of autotrophs in soils with high organic contents. However, the karst rocky desertification region is a unique ecosystem that may have a low level of organic compounds. Therefore, we propose that karst rocky desertification ecosystems may harbor diverse autotrophic microbial communities. In this study, DNA-SIP was employed to identify the chemolithoautotrophic bacteria inhabiting three soil types (i.e., grass, forest, and agriculture) of the karst rocky desertification ecosystems. The results indicated that potential chemolithoautotrophic population was observed in each soil type, even at different time points after amending 13C-NaHCO3, confirming our hypothesis that diverse autotrophs contribute to the carbon cycle in karst soils. Bacteria, such as Ralstonia, Ochrobactrum, Brevibacterium, Acinetobacter, and Corynebacterium, demonstrated their potential to assimilate inorganic carbon and reduce nitrate or thiosulfate as electron acceptors. Putative mixotrophs were identified by DNA-SIP as well, suggesting the metabolic versatility of soil microbiota. A co-occurrence network further indicated that autotrophs and heterotrophs may form associated communities to sustain the ecosystem function. Our current study revealed the metabolic diversity of autotrophic bacteria in soil habitats and demonstrated the potentially important role of chemoautotrophs in karst rocky desertification ecosystems.

Delineation of marine ecosystem zones in the northern Arabian Sea during winter

Abstract. The spatial and temporal variability of marine autotrophic abundance, expressed as chlorophyll concentration, is monitored from space and used to delineate the surface signature of marine ecosystem zones with distinct optical characteristics. An objective zoning method is presented and applied to satellite-derived Chlorophyll a (Chl a) data from the northern Arabian Sea (50–75∘ E and 15–30∘ N) during the winter months (November–March). Principal component analysis (PCA) and cluster analysis (CA) were used to statistically delineate the Chl a into zones with similar surface distribution patterns and temporal variability. The PCA identifies principal components of variability and the CA splits these into zones based on similar characteristics. Based on the temporal variability of the Chl a pattern within the study area, the statistical clustering revealed six distinct ecological zones. The obtained zones are related to the Longhurst provinces to evaluate how these compared to established ecological provinces. The Chl a variability within each zone was then compared with the variability of oceanic and atmospheric properties viz. mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), photosynthetically active radiation (PAR), nitrate and dust optical thickness (DOT) as an indication of atmospheric input of iron to the ocean. The analysis showed that in all zones, peak values of Chl a coincided with low SST and deep MLD. The rate of decrease in SST and the deepening of MLD are observed to trigger the algae bloom events in the first four zones. Lagged cross-correlation analysis shows that peak Chl a follows peak MLD and SST minima. The MLD time lag is shorter than the SST lag by 8 days, indicating that the cool surface conditions might have enhanced mixing, leading to increased primary production in the study area. An analysis of monthly climatological nitrate values showed increased concentrations associated with the deepening of the mixed layer. The input of iron seems to be important in both the open-ocean and coastal areas of the northern and north-western parts of the northern Arabian Sea, where the seasonal variability of the Chl a pattern closely follows the variability of iron deposition.

The impact of dissolved inorganic nitrogen and phosphorous on responses of microbial plankton to the Texas City “Y” oil spill in Galveston Bay, Texas (USA)

Ongoing bioremediation research seeks to promote naturally occurring microbial polycyclic aromatic hydrocarbon (PAH) degradation during and after oil spill events. However, complex relationships among functionally different microbial groups, nutrients and PAHs remain unconstrained. We conducted a surface water survey and corresponding nutrient amendment bioassays following the Texas City “Y” oil spill in Galveston Bay, Texas. Resident microbial groups, defined as either heterotrophic or autotrophic were enumerated by flow cytometry. Heterotrophic abundance was increased by oil regardless of nutrient concentrations. Contrastingly, autotrophic abundance was inhibited by oil, but this reaction was less severe when nutrient concentrations were higher. Several PAH compounds were reduced in nutrient amended treatments relative to controls suggesting nutrient enhanced microbial PAH processing. These findings provide a first-look at nutrient limitation during microbial oil processing in Galveston Bay, an important step in understanding if nutrient additions would be a useful bioremediation strategy in this and other estuarine systems.

The microbial food web in the Doñana marshland: Influence of trophic state and hydrology

We investigated the composition of the microbial food web in the marshland of Doñana National Park (SW Spain). We analysed factors affecting the predominance of autotrophic (A) or heterotrophic (H) microorganisms in a set of 16 marshland water bodies that differ in their hydrological pattern. Autotrophic organisms were predominant in the Doñana marshland, with autotrophs between 0.3 and 25.3 times higher than heterotrophs in biomass. The variance partitioning analysis using the log A:H biomass ratio (A/H) as a response variable revealed that water body spatial position accounted for the largest portion of total variance (16% of unique effects), followed by environmental variables (13%), with a shared variation of 24%. Zooplankton biomass had no significant influence on A/H ratio. The two first axes of RDA analysis were related to soluble reactive phosphate (SRP) and dissolved inorganic nitrogen (DIN) concentrations respectively. Cyanobacteria were predominant in waters with high SRP, while other organisms were distributed in relation to DIN by their size, with small organisms predominating with low DIN and large ones with high DIN. Spatial effects reflect the importance of location with respect to the water source in this marshland, where flooding areas are very much dominated by autotrophs, while confined areas, which are a long way from nutrient sources, have a more balanced abundance of autotrophs and heterotrophs.

The Temporal Dynamics of Coastal Phytoplankton and Bacterioplankton in the Eastern Mediterranean Sea.

This study considers variability in phytoplankton and heterotrophic bacterial abundances and production rates, in one of the most oligotrophic marine regions in the world–the Levantine Basin. The temporal dynamics of these planktonic groups were studied in the coastal waters of the southeastern Mediterranean Sea approximately every two weeks for a total of two years. Heterotrophic bacteria were abundant mostly during late summer and midwinter, and were positively correlated with bacterial production and with N2 fixation. Based on size fractionating, picophytoplankton was abundant during the summer, whereas nano-microphytoplankton predominated during the winter and early spring, which were also evident in the size-fractionated primary production rates. Autotrophic abundance and production correlated negatively with temperature, but did not correlate with inorganic nutrients. Furthermore, a comparison of our results with results from the open Levantine Basin demonstrates that autotrophic and heterotrophic production, as well as N2 fixation rates, are considerably higher in the coastal habitat than in the open sea, while nutrient levels or cell abundance are not different. These findings have important ecological implications for food web dynamics and for biological carbon sequestration in this understudied region.

A 30,000 yr record of land–ocean interaction in the eastern Gulf of Guinea

A 30,000yr dinocyst and pollen record from the eastern equatorial Atlantic (off Cameroon) has been investigated in order to identify land–ocean linkages during the last deglacial transition. A strong correlation between the abundance of Brigantedinium spp. and the Ca/Fe ratio during the last glacial period suggests enhanced marine productivity in association with cool seawater temperatures and nutrient input linked to coastal upwelling and/or a proximal river mouth. Dry conditions are recorded on the adjacent continent with a significant representation of open vegetation indicators and the Afromontane taxon Podocarpus. After 17calkaBP these indicators register a sharp decline as a result of a climatic transition from the dry/cooler conditions of the last glacial period to the wetter/warmer conditions of the deglaciation. Simultaneously, dinocysts show a significant shift from dominant heterotrophs to an increasing abundance of autotrophs, reflecting warmer conditions. Significant changes are observed during the Younger Dryas, with a return to drier conditions and higher salinities. The start of the Holocene is marked by very low-salinity conditions, reflecting optimal monsoonal conditions over west equatorial Africa. The end of the African Humid Period is observed between 6 and 5calkaBP, followed by significant fluctuations in both terrestrial and oceanic proxies.

Microbial food web structure affects bottom‐up effects and elemental stoichiometry in periphyton assemblages

Heterotrophic organisms constitute an important part of the overall biomass of periphyton assemblages, but the role of these components for the propagation of resource enrichment and top‐down control has rarely been addressed systematically. We report results from six laboratory experiments differing in their trophic structure by the inclusion of different functional groups of microbial benthic organisms (i.e., algae, bacteria, flagellates, ciliates, and rotifers). We manipulated the supply of organic carbon, phosphate, light, and the presence of the respective top consumer in full factorial designs. Increased supply of dissolved organic carbon (DOC) increased total periphyton biomass in almost all experiments, whereas increased phosphorus (P) supply increased total biomass only if algae were present. The effects of DOC and P on the ratio of heterotrophic to autotrophic abundance depended on trophic structure, where additional resources enhanced the autotrophic component when the basal heterotrophs were limited by low organic carbon (C) or by consumer pressure. Light had a comparably minor influence on total biomass and relative abundances. Purely heterotrophic biofilms had higher C: P ratios than autotrophic assemblages. Increased P supply decreased periphyton C: P throughout all experiments except for the experiment with the highest trophic complexity, which yielded low C: P ratios across all treatment levels. Our experimental microcosms with artificially constructed periphyton communities revealed the importance of the benthic microbial food‐web structure for both top‐down and bottom‐up propagating effects of consumers and resources, respectively.