Availability Of Inorganic Nutrients Research Articles

Prevalence and Preferred Niche of Small Eukaryotes with Mixotrophic Potentials in the Global Ocean.

Unicellular eukaryotes that are capable of phago-mixotrophy in the ocean compete for inorganic nutrients and light with autotrophs, and for bacterial prey with heterotrophs. In this study, we ask what the overall prevalence of eukaryotic mixotrophs in the vast open ocean is, and how the availability of inorganic nutrients, light, and prey affects their relative success. We utilized the Tara Oceans eukaryotic 18S rRNA gene and environmental context variables dataset to conduct a large-scale field analysis. We also performed isolate-based culture experiments to verify growth and nutritional resource relationships for representative mixotrophic taxa. The field analysis suggested that the overall prevalence of mixotrophs were negatively correlated with nutrient concentrations and positively associated with light availability. Concentrations of heterotrophic bacteria as a single variable also presented a positive correlation with mixotrophic prevalence, but to a lesser extent. On the other hand, the culture experiments demonstrated a taxa-specific relationship between mixotrophic growth and nutrition resources, i.e., the growth of one group was significantly dependent on light availability, while the other group was less affected by light when they received sufficient prey. Both groups were capable of growing efficiently with low inorganic nutrients when receiving sufficient prey and light. Therefore, our field analysis and culture experiments both suggest that phago-mixotrophy for ocean eukaryotes is seemingly an efficient strategy to compensate for nutrient deficiency but unnecessary to compensate for light scarcity. This study collectively revealed a close relationship between abiotic and biotic nutritional resources and the prevalence of trophic strategies, shedding light on the importance of light and nutrients for determining the competitive success of mixotrophs versus autotrophic and heterotrophic eukaryotes in the ocean.

Unlocking the potential of elemental sulfur autotrophic denitrification through inorganic nutrient optimization

Elemental sulfur packed-bed (S0PB) bioreactors are increasingly employed to treat secondary effluents from wastewater treatment plants. Limited availability of inorganic nutrients, such as ammonium, phosphate, and inorganic carbon, significantly determines the growth of autotrophic microorganisms and the denitrification efficiency. This study investigated the constraining thresholds of inorganic nutrients on the elemental sulfur autotrophic denitrification process. In the absence of ammonium, phosphate, and bicarbonate, the denitrification rate decreased. Specifically, the ammonium assimilation process could be replaced by nitrate assimilation pathways, resulting in a 29.5 % decline in denitrification efficiency when the ammonium-N feed was reduced from 2.0 to 0 mg-N/L. However, the absence of phosphate-P and bicarbonate-C led to substantial drops in denitrification efficiency by 66.7 % and 77.5 %, respectively. This was primarily attributed to diminished biomass yield, as evidenced by reductions of 77.8–82.6 % in ATP and 35.7–44.3 % in protein content, along with a 12.4 %–14.7 % decrease in the relative abundances of dominant sulfur-oxidizing bacteria. Correlation analyses established specific thresholds for desired nitrate removal amounts: 0.131 for ammonium-N, 0.053 for phosphate-P, and 0.597 for bicarbonate-C. These thresholds provided valuable guidance for adjusting real-life wastewater through external supplementation when employing a pilot-scale S0PB bioreactor, leading to a significant enhancement of denitrification efficiency. This study emphasizes the significance of considering the concentrations of inorganic nutrients in wastewater when implementing S0PB bioreactors, which is a context that frequently neglects potential negative consequences.

Soil, Humipedon, Forest Life and Management

In recent years, three sections (Humipedon, Copedon and Lithopedon) were recognized in the soil profile. It was then possible to link the first and most biologically active section to the characteristics of the environment and soil genesis. In particular, it is now possible to distinguish organic horizons, mainly produced by arthropods and enchytraeids in cold and acidic or dry and arid environments, from organo-mineral horizons produced by earthworms in more temperate and mesotrophic environments. Each set of horizons can be associated with a humus system or form, with important implications for forestry. Anecic/endogeic earthworms and Mull or Amphi systems are more abundant in the early and late stages of sylvogenesis; by completely recycling litter, earthworms accelerate the availability of organic and inorganic soil nutrients to roots and pedofauna. On the other hand, arthropods and Moder or Tangel systems characterize the intermediate stages of sylvogenesis, where thickening in the organic horizons and the parallel impoverishment/reduction in the underlying organo-mineral horizons are observed. Recognizing the humus system at the right spatial and temporal scale is crucial for the biological management of a forest. This article includes a data review, new data from a doctoral thesis, and recent comparisons of Italian and French investigations.

Hydrogeochemistry of low agricultural soil yield in Sahelian and sub-tropical watersheds, Northern Cameroon

Bound to the north by the Sahara and to the south by the Sudanian savannah, watersheds in the African Sahelian belt supply food and water to an estimated 135 million people. Being one of the Earth's most vulnerable zones to climate change impacts, the Sahel covers a 3.1 million km2 corridor from the Atlantic Ocean in the west to the Red Sea in the east. It is predicted that decadal timescale migration of Sahelian arid conditions southwards, and associated changes in water-rock interaction patterns resulting from desertification and reduction in rainfall trends, would increasingly alter soil nutrients availability. In this pilot study, we developed a hydrogeochemical approach by linking local geology to elemental dynamics, while focusing on nutrient enrichment, depletion, mobility, flux, and exchange between bedrock and groundwater. This approach was successfully applied to two watersheds in Northern Cameroon: the Sahelian Douka Longo sedimentary watershed (SDLSW) and the tropical Bidou igneous watershed (TBIW). Comparative inorganic nutrient budgets and availability suggest that carbonates and plagioclases are prone to weak and intermediate chemical weathering, compared to stronger rates recorded for granite, basalt, trachyte, and sandstone. Collectively, these sources contribute to significant trace element nutrients enrichment of local water bodies within the watersheds. Non-isochemical dissolution produces highly mobile Ca, Mn, Na, Cu, Zn, K, Ni and Fe compared to elements not part of plant nutrients. Acidic groundwater recharged by rainwater through preferential flow pass has a Ca + Mg–NO3 and Ca + Mg–HCO3 chemical signature in the SDLSW and the TBIW, respectively. Both watersheds are characterised by distinct solute flux patterns, with lower annual nutrient loss rates associated with the TBIW. The data indicate that surface water runoff needs to be managed to control nutrient deficiencies and excesses, and that low-yield capacity in both watersheds appear to be partly linked to P, Fe, and Mo deficiencies.

Effect of experimentally increased nutrient availability on the structure, metabolic activities, and potential microbial functions of a maritime Antarctic microbial mat.

The role of competitive interactions based on resource utilisation was explored in a phototrophic microbial mat from Byers Peninsula (Maritime Antarctica). Shotgun metagenomic profiling of the mat showed a taxonomic and functionally diverse microbial community. The heterotrophic bacterial community was dominated by Proteobacteria, where genera typically found in polar habitats, such as Janthinobacterium, Pseudomonas, and Polaromonas, were highly prevalent. Cyanobacteria played the main role as primary producers, accompanied by diatoms and chlorophytes. To test the potential effects of the inorganic nutrient (N and P) availability on this community, a fully factorial nitrate and phosphorus addition experiment was conducted in situ. The mat exhibited a functional and structural response to the nutrient amendments. Compared to the undisturbed mat, phosphorus fertilisation favoured the growth of (non-heterocystous) cyanobacteria relative to that of diatoms, as indicated by changes in the carotenoid pigment biomarkers. Although no mat accretion was visible, fertilisation improved the phototrophic activity, and, mainly, when P was amended, the production of exopolymeric substances was favoured, whereas further changes in the vertical distribution of primary production activity were observed as well. Illumina amplicon sequencing of the 16S rRNA gene also demonstrated changes in the relative abundance of heterotrophic prokaryotes, which were detectable from the phylum to the genus level and mainly related to the amendment of nitrogen. Predictions made on the functional skills of these shifted prokaryotic communities indicated changes in abundance selecting taxa with a metabolic adaptation to the new nutrient scenarios. They mainly consisted of the enhancement of ecological strategies and metabolic regulatory mechanisms related to the uptake and metabolising of either nitrogen or phosphorus, regulated by its availability whether in a balanced way or not. This study is a pioneer in demonstrating how shifts in the regional dynamic of nutrients might alter the metabolic equilibrium of these initially considered homeostatic benthic communities. They can be accordingly considered as taxonomically diverse microbiomes with a functional repertoire still inclined to respond to the biogeochemical alteration of nutrient cycles, although occurring in a cold extreme environment where biological activity is partially restricted by environmental harshness.

Spatial variability of nutrient sources determining phytoplankton Chlorophyll-a concentrations in the Bay of Bengal

Primary production is a key marine ecosystem driver in the Bay of Bengal and is important for the societies and economies of the surrounding countries. Although the availability of inorganic nutrients is known to control primary production in this region, the specific nutrient sources that affect primary production in different parts of the bay have not been identified. In this study, we assess the importance of nutrients from different sources in determining chlorophyll-a concentration, an indicator of primary production, in the Bay of Bengal by conducting multiple linear regression of satellite-derived chlorophyll-a concentration, sea surface height anomaly, and sea surface temperature; modelled dust deposition; and in situ river discharge from 1997 to 2016. River-borne nutrients were important up to approximately 200 km from the coast. Deep-ocean nutrients influenced chlorophyll-a concentrations mainly in the south-western and western bay, whereas wind-borne nutrients were more important in the central and eastern bay. Any attempt to understand the impact of nutrients from a certain source should also consider the potential impacts of other nutrient sources. Although climate impacts on chlorophyll-a concentrations through river discharge were observed in our study, future studies should investigate climate-change impacts through atmospheric aerosols and mesoscale eddies.

Nutrient limitation of primary production in rivers along a land use gradient in the Lake Biwa Basin, Shiga Prefecture, Japan

Benthic microalgae (BMA) biomass and community structure in freshwater lotic systems are often limited by inorganic nutrient availability. We examined nutrient limitation of benthic algae biomass and community structure using nutrient diffusing substrates in four rivers along a land use gradient in the Lake Biwa basin, Japan. Ambient in-stream nutrient concentrations were correlated to catchment land use, with the highest nitrogen and phosphorus concentrations being found in rivers draining more forested catchments. Nutrient limitation of primary producer biomass and nutrient-driven changes in community structure were evident in all four rivers, regardless of in-stream nutrient concentrations and surrounding land use. BMA biomass (measured as chlorophyll log-response ratio) exhibited the greatest nutrient limitation in rivers with higher in-stream nutrient concentrations. The relationship to catchment land use was less clear, with the highest nutrient limitation being observed in the catchment with moderate amounts of agricultural and forested land use. Nutrient additions resulted in a shift from dominance by Bacillariophyceae (diatoms) to a mixed Bacillariophyceae-Chlorophyceae (chlorophyte) community in all rivers and this shift was most pronounced in the forested catchments. Changes within the diatom community with nutrient additions were also observed, although the shifts in diatom community within a river in response to nutrient additions were much smaller than the differences in diatom community composition among rivers. Diatom taxa classified as highly motile increased with nutrient additions in all rivers. Our results suggest that primary producer community in rivers may be sensitive to nutrient inputs even in areas with elevated nutrient concentrations and catchments dominated by agricultural land use. There is likely widespread nutrient limitation in rivers of Japan, across both in-stream nutrient and land use gradients and any increases in nutrient loading will likely stimulate benthic algal growth. Our findings highlight the importance of looking at both biomass and species composition to assess ecosystem-level impacts of elevated nutrient levels.

Bacterial response to spatial gradients of algal-derived nutrients in a porous microplate

Photosynthetic microalgae are responsible for 50% of the global atmospheric CO2 fixation into organic matter and hold potential as a renewable bioenergy source. Their metabolic interactions with the surrounding microbial community (the algal microbiome) play critical roles in carbon cycling, but due to methodological limitations, it has been challenging to examine how community development is influenced by spatial proximity to their algal host. Here we introduce a copolymer-based porous microplate to co-culture algae and bacteria, where metabolites are constantly exchanged between the microorganisms while maintaining physical separation. In the microplate, we found that the diatom Phaeodactylum tricornutum accumulated to cell abundances ~20 fold higher than under normal batch conditions due to constant replenishment of nutrients through the porous structure. We also demonstrate that algal-associated bacteria, both single isolates and complex communities, responded to inorganic nutrients away from their host as well as organic nutrients originating from the algae in a spatially predictable manner. These experimental findings coupled with a mathematical model suggest that host proximity and algal culture growth phase impact bacterial community development in a taxon-specific manner through organic and inorganic nutrient availability. Our novel system presents a useful tool to investigate universal metabolic interactions between microbes in aquatic ecosystems.

Restoration benefits of soil nutrient manipulation and weeding in invaded dry and wet tropical ecosystems in Hawaiʻi

Reducing soil nutrient availability has been proposed as a strategy to favor native vs. non‐native invasive plant species and represents a potential alternative to traditional manual removal or chemical control methods. We implemented a field experiment in invaded dry and wet montane Hawaiian ecosystems to test responses of soil and dominant plant species to three soil nutrient treatments (Control = no nutrient manipulation; Carbon = C substrate added to reduce nutrients; Fertilizer = fertilizer added to increase nutrients) and two non‐native plant treatments (Weeds removed; Weeds present) in a fully factorial experiment in each ecosystem over 18 months. Carbon amendments reduced soil inorganic nutrient availability by 60–70% in dry shrubland and 30–50% in wet forest. Fertilizer amendments increased soil inorganic nutrient availability by >20‐fold. Altered nutrient availability did not impact gross mineralization or nitrification rates in either ecosystem. In dry shrubland, neither C amendments nor weed removal altered growth or reproduction, but fertilizer increased woody growth and forb/grass reproduction in both natives and non‐natives. In wet forest, weed removal but not C amendments increased growth and survival of native woody seedlings, while fertilizer decreased native seedling survival and increased non‐native woody seedling growth. Overall, growth and reproduction of native and non‐natives responded similarly to altered nutrient availability, indicating that for the tropical ecosystems and species examined, manipulating nutrient availability does not favor native versus non‐native invasive plants in the first 18 months. In contrast, weed removal had positive effects on native plant growth, likely mediated through changes in other resources.

Temporal dynamics of carbon sequestration in coastal North Atlantic fjord system as seen through dissolved organic matter characterisation

Fjord systems in higher latitudes are unique coastal water ecosystems that facilitate the study of dissolved organic matter (DOM) dynamics from surface to deeper waters. The current work was undertaken in the Trondheim fjord characterized by North Atlantic waters, and compared DOM fractions from three depths - surface (3 m), intermediate (225 m) and deep (440 m) in four seasons, from late spring to winter in 2017. The high-resolution mass spectrometry data showed that DOM composition varies significantly in different seasons rather than in different depths in the fjord systems. The bacterial community composition was comparable except at spring surface and summer intermediate depths. Bacterial production was minimal below the euphotic layer, even with sufficient availability of inorganic nutrients. The bacterial production rate in the surface waters was about 7 times and over 50 times higher than that of the aphotic zone in the winter and the summer seasons, respectively. The surface heterotrophic microbial communities might have rapidly consumed the available labile DOM, with the production of more refractory DOM limiting bacterial production in aphotic layers. The greater number of CRAM-like formulas determined in the surface waters compared to other depths supports our hypothesis. The refractory DOM sequestered in the water column may either be exported into sediments attached to particulate matter and marine gels, or may escape into the atmosphere as carbon dioxide/monoxide during the photochemical oxidation pathways, suggesting that it is involved in climate change scenarios.

Response of coastal phytoplankton to upwelling induced hydrological changes in the Alappuzha mud bank region, southwest coast of India

The response of phytoplankton community to the co-existing events of coastal upwelling and mud banks in the nearshore waters of Alappuzha (15 m depth), located in the southwest coast of India from April to November 2016, is described based on size-fractionated phytoplankton biomass (chlorophyll a), primary production and community composition. The study region exhibited well-distinct spatio-temporal hydrological changes because of the influence of wind-driven coastal upwelling, prevalent during the southwest monsoon (SWM) period. However, the formation of mud banks, in addition to coastal upwelling, was observed at station M2, which facilitated the substantial increase of water column turbidity and inorganic nutrients (ammonium, phosphate, and silicate) during and after the peak SWM period compared to the non-mud bank reference stations (M1 and M3). The prevailing hydrological changes were complemented the corresponding phytoplankton productivity patterns, in which profound domination of nanophytoplankton (2–20 µm) chlorophyll a and primary production was observed throughout the study region, irrespective of seasons. The SIMPER analysis, based on phytoplankton (mostly >20 µm) species composition data (microscopy), revealed the formation of certain characterizing species, mainly comprised of diatoms and dinoflagellates. The consistent predominance of the nanophytoplankton, established under variable hydrological scenarios, showed that the inorganic nutrient (specifically ammonium) availability was instrumental in defining the widespread growth of nanophytoplankton community compared to the prevailing light levels. The present study thus revealed that even the small-sized phytoplankton community could survive in the nutrient-enriched coastal waters, characterised by the co-existing upwelling and mud banks.

Coral Productivity Is Co-Limited by Bicarbonate and Ammonium Availability.

The nitrogen environment and nitrogen status of reef-building coral endosymbionts is one of the important factors determining the optimal assimilation of phototrophic carbon and hence the growth of the holobiont. However, the impact of inorganic nutrient availability on the photosynthesis and physiological state of the coral holobiont is partly understood. This study aimed to determine if photosynthesis of the endosymbionts associated with the coral Stylophora pistillata and the overall growth of the holobiont were limited by the availability of dissolved inorganic carbon and nitrogen in seawater. For this purpose, colonies were incubated in absence or presence of 4 µM ammonium and/or 6 mM bicarbonate. Photosynthetic performances, pigments content, endosymbionts density and growth rate of the coral colonies were monitored for 3 weeks. Positive effects were observed on coral physiology with the supplementation of one or the other nutrient, but the most important changes were observed when both nutrients were provided. The increased availability of DIC and NH4+ significantly improved the photosynthetic efficiency and capacity of endosymbionts, in turn enhancing the host calcification rate. Overall, these results suggest that in hospite symbionts are co-limited by nitrogen and carbon availability for an optimal photosynthesis.

Considering the rates of growth in two taxa of coral across Pacific islands.

Reef-building coral taxa demonstrate considerable flexibility and diversity in reproduction and growth mechanisms. Corals take advantage of this flexibility to increase or decrease size through clonal expansion and loss of live tissue area (i.e. via reproduction and mortality of constituent polyps). The biological lability of reef-building corals may be expected to map onto varying patterns of demography across environmental contexts which can contribute to geographic variation in population dynamics. Here we explore the patterns of growth of two common coral taxa, corymbose Pocillopora and massive Porites, across seven islands in the central and south Pacific. The islands span a natural gradient of environmental conditions, including a range of pelagic primary production, a metric linked to the relative availability of inorganic nutrients and heterotrophic resources for mixotrophic corals, and sea surface temperature and thermal histories. Over a multi-year sampling interval, most coral colonies experienced positive growth (greater planar area of live tissue in second relative to first time point), though the distributions of growth varied across islands. Island-level median growth did not relate simply to estimated pelagic primary productivity or temperature. However, at locations that experienced an extreme warm-water event during the sampling interval, most Porites colonies experienced net losses of live tissue and nearly all Pocillopora colonies experienced complete mortality. While descriptive statistics of demographics offer valuable insights into trends and variability in colony change through time, simplified models predicting growth patterns based on summarized oceanographic metrics appear inadequate for robust demographic prediction. We propose that the complexity of life history strategies among colonial reef-building corals introduces unique demographic flexibility for colonies to respond to a wide breadth of environmental conditions.

Viability of pico- and nanophytoplankton in the Baltic Sea during spring

Phytoplankton cell death is an important process in marine food webs, but the viability of natural phytoplankton communities remains unexplored in many ecosystems. In this study, we measured the viability of natural pico- and nanophytoplankton communities in the central and southern parts of the Baltic Sea (55°21′ N, 17°06′ E–60°18′ N, 19°14′ E) during spring (4th–15th April 2016) to assess differences among phytoplankton groups and the potential relationship between cell death and temperature, and inorganic nutrient availability. Cell viability was determined by SYTOX Green cell staining and flow cytometry at a total of 27 stations representing differing hydrographic regimes. Three general groups of phytoplankton (picocyanobacteria, picoeukaryotes, and nanophytoplankton) were identified by cytometry using pigment fluorescence and light scatter characteristics. The picocyanobacteria and picoeukaryotes had significantly higher cell viability than the nanophytoplankton population at all depths throughout the study area. Viability correlated positively with the photosynthetic efficiency (Fv/Fm, maximum quantum yield of photosystem II) as measured on the total phytoplankton community. However, an anticipated correlation with dissolved organic carbon was not observed. We found that the abiotic factors suggested to affect phytoplankton viability in other marine ecosystems were not as important in the Baltic Sea, and other biotic processes, e.g. processes related to species succession could have a more pronounced role.

Cover crops as modifying agents of microbiological soil attribute

Carbon sources are exuded and deposited by different soil cover plants. They promote growth, diversity and enhancement of soil microbial community functionality, due to organic matter degradation by participating in major biochemical cycles and the availability of inorganic nutrients to plants. In this way, it is necessary to evaluate the microbiological attributes of the soil after cover cropping, which allows for surveying and monitoring the soil quality, thereby enabling rapid responses in relation to managing changes in the soil. Thus, the objective of this study was to evaluate soil microbiological attributes and soybean grain yield under the influence of different cover crops. The experiment was installed in the year 2015. The treatments were constituted by the following vegetation coverages: sorghum, millet, Urochloa ruziziensis, forage turnip, Urochloa brizantha, crambe and fallow area, with cover crops sown in succession to the soybean crop for three years prior to the date of installation of the experiment .The evaluated parameters were soil microbial biomass carbon, soil basal respiration, metabolic quotient, enzymatic activity of acid phosphatase and soil β-glucosidase, plant phytomass produced by the different cover crops and soybean yield in each area. The use of cover crops promotes higher soybean yield. The microbial activity and its efficiency were modulated according to the type of cover crop used. Soil under sorghum mulch provided lower microbial efficiency. The U. ruziziensis plant residues remain for less time on the soil. The results show that U. brizantha may be the most suitable for its use as a soil cover plant, providing improvements in its attributes.

Environmental stimuli drive a transition from cooperation to competition in synthetic phototrophic communities.

Phototrophic communities of photosynthetic algae or cyanobacteria and heterotrophic bacteria or fungi are pervasive throughout the environment1. How interactions between members contribute to the resilience and affect the fitness of phototrophic communities is not fully understood2,3. Here, we integrated metatranscriptomics, metabolomics and phenotyping with computational modelling to reveal condition-dependent secretion and cross-feeding of metabolites in a synthetic community. We discovered that interactions between members are highly dynamic and are driven by the availability of organic and inorganic nutrients. Environmental factors, such as ammonia concentration, influenced community stability by shifting members from collaborating to competing. Furthermore, overall fitness was dependent on genotype and streamlined genomes improved growth of the entire community. Our mechanistic framework provides insights into the physiology and metabolic response to environmental and genetic perturbation of these ubiquitous microbial associations.

Nutrient starvation impairs the trophic plasticity of reef‐building corals under ocean warming

Abstract Global warming of the world's oceans is driving reef‐building corals towards their upper thermal limit, inducing bleaching, nutrient starvation and mortality. In addition, corals are predicted to experience large fluctuations in seawater nutrient concentrations, following water column stratification or eutrophication problems, which can further alter their nutritional capacities and ultimately their resilience to global change. We investigated the effect of thermal stress and dissolved inorganic nutrient (DINUT) availability on the auto‐ and heterotrophic nutritional capacities of corals. In particular, we assessed the effect of nitrogen enrichment or DINUT depletion (both in nitrogen and in phosphorus) on the assimilation of heterotrophic nutrients and on the heat‐stress tolerance of the reef‐building coral Stylophora pistillata. Here, we show that DINUT depletion enhanced coral bleaching under thermal stress and more importantly, significantly impaired rates of heterotrophic nutrient assimilation, inducing coral starvation. In contrast, corals grown under nitrogen enrichment maintained high rates of heterotrophic nutrient assimilation and avoided bleaching, although nutrient uptake rates were lowered. We therefore observed a positive coupling between auto‐ and heterotrophy within the coral–dinoflagellate symbiosis, indicating that heterotrophic processes require a minimum of autotrophically acquired nutrients to be functional. These findings show that the trophic plasticity of corals directly depends on the availability of dissolved inorganic nutrients in seawater. The lack of a shift towards greater heterotrophy under DINUT depletion may lead to substantial modifications of the role that feeding plays in the response of reef‐building corals to climate change. A plain language summary is available for this article.

Controls on the Fate of Dissolved Organic Carbon Under Contrasting Upwelling Conditions

To understand controls on the production and remineralization of recalcitrant dissolved organic carbon produced in association with positive net community production, we simulated upwelling systems of different intensities by combining and incubating whole seawater collected from different depths in the Florida Strait (27˚N, ~79˚W). The natural microbial communities in the treatments grew under controlled light and temperature for 15 days (i.e., the autotrophic phase); they were subsequently incubated for 35 days in the dark heterotrophic phase. We analyzed the phytoplankton composition and pigment fluorescence intensity during the light phase, and dissolved organic and inorganic variables during both phases. Initial high or low availability of inorganic nutrients controlled phytoplankton growth and the magnitude of NCP. In the strong upwelling treatment, with higher initial inorganic nutrients, 25% of NCP accumulated as DOC after 15 days; however, this material was in turn fully remineralized during the dark phase. In contrast, low nutrients in the weak upwelling treatment not only limited NCP, but also the fraction of NCP accumulated as DOC (11%). Surprisingly, most of this fraction resisted microbial remineralization in the dark phase, suggesting that upwellings of different intensities affect the quality of dissolved organic matter produced, thereby affecting the timing and location of its remineralization and, hence, its prospects for export to the deep ocean.

Herbivore Effects on Ecosystem Process Rates in a Low-Productive System

Mammalian herbivores shape the structure and function of many nutrient-limited or low-productive terrestrial ecosystems through modification of plant communities and plant–soil feedbacks. In the tundra biome, mammalian herbivores may both accelerate and decelerate plant biomass growth, microbial activity and nutrient cycling, that is, ecosystem process rates. Selective foraging and associated declines of palatable species are known to be major drivers of plant–soil feedbacks. However, declines in dominant plants of low palatability often linked with high herbivore densities may also modify ecosystem process rates, yet have received little attention. We present data from an island experiment with a 10-year vole density manipulation, to test the hypothesis that herbivores accelerate process rates by decreasing the relative abundance of poorly palatable plants to palatable ones. We measured plant species abundances and community composition, nitrogen contents of green plant tissues and multiple soil and litter variables under high and low vole density. Corroborating our hypothesis, periodic high vole density increased ecosystem process rates in low-productive tundra. High vole density was associated with both increasing relative abundance of palatable forbs over unpalatable evergreen dwarf shrubs and higher plant N content both at species and at community level. Changes in plant community composition, in turn, explained variation in microbial activity in litter and soil inorganic nutrient availability. We propose a new conceptual model with two distinct vole–plant–soil feedback pathways. Voles may drive local plant–soil feedbacks that either increase or decrease ecosystem process rates, in turn promoting heterogeneity in vegetation and soils across tundra landscapes.

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

AbstractSubalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.