Temperature Adaptation of Aquatic Bacterial Community Growth Is Faster in Response to Rising than to Falling Temperature.

Bacteria are key organisms in energy and nutrient cycles, and predicting the effects of temperature change on bacterial activity is important in assessing global change effects. A changing in situ temperature will affect the temperature adaptation of bacterial growth in lake water, both long term in response to global change, and short term in response to seasonal variations. The rate of adaptation may, however, depend on whether temperature is increasing or decreasing, since bacterial growth and turnover scale with temperature. Temperature adaptation was studied for winter (in situ temperature 2.5 °C) and summer communities (16.5 °C) from a temperate lake in Southern Sweden by exposing them to a temperature treatment gradient between 0 and 30 °C in ~ 5 °C increments. This resulted mainly in a temperature increase for the winter and a decrease for the summer community. Temperature adaptation of bacterial community growth was estimated as leucine incorporation using a temperature Sensitivity Index (SI, log growth at 35 °C/4 °C), where higher values indicate adaptation to higher temperatures. High treatment temperatures resulted in higher SI within days for the winter community, resulting in an expected level of community adaptation within 2 weeks. Adaptation for the summer community was also correlated to treatment temperature, but the rate of adaption was slower. Even after 5 weeks, the bacterial community had not fully adapted to the lowest temperature conditions. Thus, during periods of increasing temperature, the bacterial community will rapidly adapt to function optimally, while decreasing temperature may result in long periods of non-optimal functioning.

Browning of freshwaters: Consequences to ecosystem services, underlying drivers, and potential mitigation measures.

Browning of surface waters, as a result of increasing dissolved organic carbon and iron concentrations, is a widespread phenomenon with implications to the structure and function of aquatic ecosystems. In this article, we provide an overview of the consequences of browning in relation to ecosystem services, outline what the underlying drivers and mechanisms of browning are, and specifically focus on exploring potential mitigation measures to locally counteract browning. These topical concepts are discussed with a focus on Scandinavia, but are of relevance also to other regions. Browning is of environmental concern as it leads to, e.g., increasing costs and risks for drinking water production, and reduced fish production in lakes by limiting light penetration. While climate change, recovery from acidification, and land-use change are all likely factors contributing to the observed browning, managing the land use in the hydrologically connected parts of the landscape may be the most feasible way to counteract browning of natural waters.

Afforestation driving long-term surface water browning.

Increase in surface water color (browning), caused by rising dissolved organic carbon (DOC) and iron concentrations, has been widely reported and studied in the last couple of decades. This phenomenon has implications to aquatic ecosystem function and biogeochemical carbon cycling. While recovery from acidification and changes in climate-related variables, such as precipitation and length of growing season, are recognized as drivers behind browning, land-use change has received less attention. In this study, we include all of the above factors and aim to discern their individual and combined contribution to water color variation in an unprecedentedly long (1940-2016) and highly resolved dataset (~20 times per month), from a river in southern Sweden. Water color showed high seasonal variability and a marked long-term increase, particularly in the latter half of the dataset (~1980). Short-term and seasonal variations were best explained by precipitation, with temperature playing a secondary role. All explanatory variables (precipitation, temperature, S deposition, and land-use change) contributed significantly and together predicted 75% of the long-term variation in water color. Long-term change was best explained by a pronounced increase in Norway spruce (Picea abies Karst) volume-a measure of land-use change and a proxy for buildup of organic soil layers-and by change in atmospheric S deposition. When modeling water color with a combination of explanatory variables, Norway spruce showed the highest contribution to explaining long-term variability. This study highlights the importance of considering land-use change as a factor behind browning and combining multiple factors when making predictions in water color and DOC.

Bacterioplankton Responses to Increased Organic Carbon and Nutrient Loading in a Boreal Estuary-Separate and Interactive Effects on Growth and Respiration.

Increases in the terrestrial export of dissolved organic carbon (C) to rivers may be associated with additional loading of organic nitrogen (N) and phosphorus (P) to the coastal zone. However, little is known about how these resources interact in the regulation of heterotrophic bacterioplankton metabolism in boreal coastal ecosystems. Here, we measured changes in bacterioplankton production (BP) and respiration (BR) in response to full-factorial (C, N, and P) enrichment experiments at two sites within the Öre estuary, northern Sweden. The BR was stimulated by single C additions and further enhanced by combined additions of C and other nutrients. Single addition of N or P had no effect on BR rates. In contrast, BP was primarily limited by P at the site close to the river mouth and did not respond to C or N additions. However, at the site further away from the near the river mouth, BP was slightly stimulated by single additions of C. Possibly, the natural inflow of riverine bioavailable dissolved organic carbon induced local P limitation of BP near the river mouth, which was then exhausted and resulted in C-limited BP further away from the river mouth. We observed positive interactions between all elements on all responses except for BP at the site close to the river mouth, where P showed an independent effect. In light of predicted increases in terrestrial P and C deliveries, we expect future increases in BP and increases of BR of terrestrially delivered C substrates at the Öre estuary and similar areas.

Labile carbon 'primes' fungal use of nitrogen from submerged leaf litter.

Microbial decomposers colonising submerged leaf litter are in close spatial proximity with periphytic algae and can use carbon (C) exudates released during photosynthesis. We investigated whether labile C delivered as algal exudates could affect the microbial colonisation and decomposition of leaf litter. Using microcosms, we submerged leaf litter in pond water and monitored fungal and bacterial growth over time and tested the effect of algal photosynthetic exudates by comparing microcosms in light and dark. In order to experimentally assign the effect of algal products to labile C delivery and test for a C driven mechanism, we ran a parallel experiment with microcosms in the dark where we mimicked the delivery of algal labile C by continuously adding glucose. Labile C delivered as algal exudates or glucose resulted in a dominance of fungal decomposers over bacteria, and stimulated the acquisition of more N-rich OM fractions from litter during periods of active fungal growth. Our results highlight that labile C stimulates fungal decomposers and increases N removal from leaf litter. Since fungal necromass is more resistant to degradation than bacterial, we expect that a fungal-dominated litter degradation might contribute to more protected C pools.

Salinity Effects on Iron Speciation in Boreal River Waters.

Previous studies report high and increasing iron (Fe) concentrations in boreal river mouths. This Fe has shown relatively high stability to salinity-induced aggregation in estuaries. The aim of this study was to understand how the speciation of Fe affects stability over salinity gradients. For Fe to remain in suspension interactions with organic matter (OM) are fundamental and these interactions can be divided in two dominant phases: organically complexed Fe, and colloidal Fe (oxy)hydroxides, stabilized by surface interactions with OM. The stability of these two Fe phases was tested using mixing experiments with river water and artificial seawater. Fe speciation of river waters and salinity-induced aggregates was determined by synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy. The relative contribution of the two Fe phases varied widely across the sampled rivers. Moreover, we found selective removal of Fe (oxy)hydroxides by aggregation at increasing salinity, while organically complexed Fe was less affected. However, Fe-OM complexes were also found in the aggregates, illustrating that the control of Fe stability is not explained by the prevalence of the respective Fe phases alone. Factors such as colloid size and the chemical composition of the OM may also impact the behavior of Fe species.

Experimental insights into the importance of aquatic bacterial community composition to the degradation of dissolved organic matter.

Bacteria play a central role in the cycling of carbon, yet our understanding of the relationship between the taxonomic composition and the degradation of dissolved organic matter (DOM) is still poor. In this experimental study, we were able to demonstrate a direct link between community composition and ecosystem functioning in that differently structured aquatic bacterial communities differed in their degradation of terrestrially derived DOM. Although the same amount of carbon was processed, both the temporal pattern of degradation and the compounds degraded differed among communities. We, moreover, uncovered that low-molecular-weight carbon was available to all communities for utilisation, whereas the ability to degrade carbon of greater molecular weight was a trait less widely distributed. Finally, whereas the degradation of either low- or high-molecular-weight carbon was not restricted to a single phylogenetic clade, our results illustrate that bacterial taxa of similar phylogenetic classification differed substantially in their association with the degradation of DOM compounds. Applying techniques that capture the diversity and complexity of both bacterial communities and DOM, our study provides new insight into how the structure of bacterial communities may affect processes of biogeochemical significance.

Dissolved Organic Nitrogen Inputs from Wastewater Treatment Plant Effluents Increase Responses of Planktonic Metabolic Rates to Warming.

Increased anthropogenic pressures on coastal marine ecosystems in the last century are threatening their biodiversity and functioning. Global warming and increases in nutrient loadings are two major stressors affecting these systems. Global warming is expected to increase both atmospheric and water temperatures and increase precipitation and terrestrial runoff, further increasing organic matter and nutrient inputs to coastal areas. Dissolved organic nitrogen (DON) concentrations frequently exceed those of dissolved inorganic nitrogen in aquatic systems. Many components of the DON pool have been shown to supply nitrogen nutrition to phytoplankton and bacteria. Predictions of how global warming and eutrophication will affect metabolic rates and dissolved oxygen dynamics in the future are needed to elucidate their impacts on biodiversity and ecosystem functioning. Here, we experimentally determine the effects of simultaneous DON additions and warming on planktonic community metabolism in the Baltic Sea, the largest coastal area suffering from eutrophication-driven hypoxia. Both bacterioplankton community composition and metabolic rates changed in relation to temperature. DON additions from wastewater treatment plant effluents significantly increased the activation energies for community respiration and gross primary production. Activation energies for community respiration were higher than those for gross primary production. Results support the prediction that warming of the Baltic Sea will enhance planktonic respiration rates faster than it will for planktonic primary production. Higher increases in respiration rates than in production may lead to the depletion of the oxygen pool, further aggravating hypoxia in the Baltic Sea.

Phylogeography of the freshwater raphidophyte Gonyostomum semen confirms a recent expansion in northern Europe by a single haplotype.

Gonyostmum semen is a freshwater raphidophyte that has increased in occurrence and abundance in several countries in northern Europe since the 1980s. More recently, the species has expanded rapidly also in north-eastern Europe, and it is frequently referred to as invasive. To better understand the species history, we have explored the phylogeography of G. semen using strains from northern Europe, United States, and Japan. Three regions of the ribosomal RNA gene (small subunit [SSU], internal transcribed spacer [ITS] and large subunit [LSU]) and one mitochondrial DNA marker (cox1) were analyzed. The SSU and partial LSU sequences were identical in all strains, confirming that they belong to the same species. The ITS region differentiated the American from the other strains, but showed high intra-strain variability. In contrast, the mitochondrial marker cox1 showed distinct differences between the European, American, and Japanese strains. Interestingly, only one cox1 haplotype was detected in European strains. The overall low diversity and weak geographic structure within northern European strains supported the hypothesis of a recent invasion of new lakes by G. semen. Our data also show that the invasive northern European lineage is genetically distinct from the lineages from the other continents. Finally, we concluded that the mitochondrial cox1 was the most useful marker in determining large-scale biogeographic patterns in this species.

Importance of boreal rivers in providing iron to marine waters.

This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters--the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

Population genetic structure of a microalgal species under expansion.

Biological invasions often cause major perturbations in the environment and are well studied among macroorganisms. Less is known about invasion by free-living microbes. Gonyostomum semen (Raphidophyceae) is a freshwater phytoplankton species that has increased in abundance in Northern Europe since the 1980's and has expanded its habitat range. In this study, we aimed to determine the genetic population structure of G. semen in Northern Europe and to what extent it reflects the species' recent expansion. We sampled lakes from 12 locations (11 lakes) in Norway, Sweden and Finland. Multiple strains from each location were genotyped using Amplified Fragment Length Polymorphism (AFLP). We found low differentiation between locations, and low gene diversity within each location. Moreover, there was an absence of genetic isolation with distance (Mantel test, p = 0.50). According to a Bayesian clustering method all the isolates belonged to the same genetic population. Together our data suggest the presence of one metapopulation and an overall low diversity, which is coherent with a recent expansion of G. semen.

Genetic diversity within and genetic differentiation between blooms of a microalgal species.

The field of genetic diversity in protists, particularly phytoplankton, is under expansion. However, little is known regarding variation in genetic diversity within populations over time. The aim of our study was to investigate intrapopulation genetic diversity and genetic differentiation in the freshwater bloom-forming microalga Gonyostomum semen (Raphidophyceae). The study covered a 2-year period including all phases of the bloom. Amplified fragment length polymorphism (AFLP) was used to determine the genetic structure and diversity of the population. Our results showed a significant differentiation between samples collected during the two blooms from consecutive years. Also, an increase of gene diversity and a loss of differentiation among sampling dates were observed over time within a single bloom. The latter observations may reflect the continuous germination of cysts from the sediment. The life cycle characteristics of G. semen, particularly reproduction and recruitment, most likely explain a high proportion of the observed variation. This study highlights the importance of the life cycle for the intraspecific genetic diversity of microbial species, which alternates between sexual and asexual reproduction.

Effect of acid deposition on quantity and quality of dissolved organic matter in soil-water.

The aim of this study was to explore how acid deposition may affect the concentration and quality of dissolved organic matter (DOM) in soil-water. This was done by a small-scale acidification experiment during two years where 0.5 × 0.5 m(2) plots were artificially irrigated with water with different sulfuric acid content, and soil-water was sampled using zero-tension lysimeters under the O-horizon. The DOM was characterized using absorbance, fluorescence, and size exclusion chromatography analyses. Our results showed lower mobility of DOM in the high acid treatment. At the same time, there was a significant change in the DOM quality. Soil-water in the high acid treatment exhibited DOM that was less colored, less hydrophobic, less aromatic, and of lower molecular weight, compared to the low acid treatment. This supports the hypothesis that reduction in sulfur deposition is an important driver behind the ongoing brownification of surface waters in many regions.

Regional invariance among microbial communities.

Microbial ecology has focused much on causes of between-site variation in community composition. By analysing five data-sets each of aquatic bacteria and phytoplankton, we demonstrated that microbial communities show a large degree of similarity in community composition and that abundant taxa were widespread, a typical pattern for many metazoan metacommunities. The regional abundance of taxa explained on average 85 and 41% of variation in detection frequency and 58 and 31% of variation in local abundances for bacteria and phytoplankton, respectively. However, regional abundance explained less variation in local abundances with increasing environmental variation between sites within data-sets. These findings indicate that the studies of microbial assemblages need to consider similarities between communities to better understand the processes underlying the assembly of microbial communities. Finally, we propose that the degree of regional invariance can be linked to the evolution of microbes and the variation in ecosystem functions performed by microbial communities.

Influence of dissolved organic matter source on lake bacterioplankton structure and function--implications for seasonal dynamics of community composition.

It has been suggested that autochthonous (internally produced) organic carbon and allochthonous (externally produced) organic carbon are utilized by phylogenetically different bacterioplankton. We examined the relationship between the source of organic matter and the structure and function of lake bacterial communities. Differences and seasonal changes in bacterial community composition in two lakes differing in their source of organic matter were followed in relation to environmental variables. We also performed batch culture experiments with amendments of various organic substrates, namely fulvic acids, leachates from algae, and birch and maple leaves. Differences in bacterial community composition between the lakes, analysed by terminal restriction fragment length polymorphism, correlated with variables related to the relative loading of autochthonous and allochthonous carbon (water colour, dissolved organic carbon, nutrients, and pH). Seasonal changes correlated with temperature, chlorophyll and dissolved organic carbon in both lakes. The substrate amendments led to differences in both structure and function, i.e. production, respiration and growth yield, of the bacterial community. In conclusion, our results suggest that the source of organic matter influences community composition both within and among lakes and that there may be a coupling between the structure and function of the bacterial community.

Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs.

Ecosystems are supported by organic carbon from two distinct sources. Endogenous carbon is produced by photosynthesis within an ecosystem by autotrophic organisms. Exogenous carbon is produced elsewhere and transported into ecosystems. Consumers may use exogenous carbon with consequent influences on population dynamics, predator-prey relationships and ecosystem processes. For example, exogenous inputs provide resources that may enhance consumer abundance beyond levels supported by within-system primary production. Exogenous fluxes of organic carbon to ecosystems are often large, but this material is recalcitrant and difficult to assimilate, in contrast to endogenously produced organic matter, which is used more easily. Here we show, by the experimental manipulation of dissolved inorganic (13)C in two lakes, that internal primary production is insufficient to support the food webs of these ecosystems. Additions of NaH(13)CO(3) enriched the (13)C content of dissolved inorganic carbon, particulate organic carbon, zooplankton and fish. Dynamics of (13)C indicate that 40-55% of particulate organic carbon and 22-50% of zooplankton carbon are derived from terrestrial sources, showing that there is significant subsidy of these ecosystems by organic carbon produced outside their boundaries.