Susanna Hietanen

Community Structure of the Bacteria Associated with Nodularia sp. (Cyanobacteria) Aggregates in the Baltic Sea

The community structure of the bacteria associated with Nodularia spumigena (Mertens) cyanobacterial aggregates in the Baltic Sea was studied with temperature gradient gel electrophoresis (TGGE), using a 16S rRNA gene fragment as a target. Various developmental stages of the aggregates and free-floating cyanobacterial filaments were sampled to reveal possible changes in associated microbial community structure during development and senescence of the aggregates. The microbial community structures of all samples differed, and the communities of young and decaying aggregates were separated by cluster analysis of the TGGE fingerprint data. Sequencing of the TGGE fragments indicated the presence of bacteria from the α-, β-, and γ-proteobacterial groups, as well as members of Cytophaga–Flexibacter–Bacteroides lineages and gram-positive Actinobacteria spp. The majority of the Nodularia-associated sequences were not closely related to previously reported 16S rDNA sequences from the Baltic Sea or any other environment. The structure of the bacterial assemblage reflects the environmental changes associated with the succession and decay of the cyanobacterial aggregates. In addition, the sequence data suggest that the N. spumigena (Mertens) blooms in the Baltic Sea may host thus far uncharacterized bacterial species.

Denitrification in the River Estuaries of the Northern Baltic Sea

Abstract Estuaries have been suggested to have an important role in reducing the nitrogen load transported to the sea. We measured denitrification rates in six estuaries of the northern Baltic Sea. Four of them were river mouths in the Bothnian Bay (northern Gulf of Bothnia), and two were estuary bays, one in the Archipelago Sea (southern Gulf of Bothnia) and the other in the Gulf of Finland. Denitrification rates in the four river mouths varied between 330 and 905 μmol N m−2 d−1. The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 μmol N m−2 d−1 to 910 μmol N m−2 d−1 and from 230 μmol N m−2 d−1 to 320 μmol N m−2 d−1, respectively. Denitrification removed 3.6–9.0% of the total nitrogen loading in the river mouths and in the estuary bay in the Gulf of Finland, where the residence times were short. In the estuary bay with a long residence time, in the Archipelago Sea, up to 4.5% of nitrate loading and 19% of nitrogen loading were removed before entering the sea. According to our results, the sediments of the fast-flowing rivers and the estuary areas with short residence times have a limited capacity to reduce the nitrogen load to the Baltic Sea.

Silicate Release from Sand-Manipulated Sediment Cores: Biogenic or Adsorbed Si?

The influence of an addition of either sand and/or spring-bloom algae on the efflux of nutrients from intact sediment cores from the Baltic Sea was studied in a flow-through experiment. The addition of sand significantly increased the efflux of silicon (Si) from sediment, but the algal addition did not. The effects on phosphorus (P) were not as clear, and fluxes of nitrogen (NH4 and NO2 + 3) remained relatively unaffected by the additions. The small effect of the algal addition was caused by the short time-period covered by the experiment and possibly by adsorption of released Si by the sediment. A follow-up bottle experiment showed that despite the apparently lower content of easily available Si and biogenic silica, BSi, in the sand, the sand-induced Si efflux was caused by release of Si from the sand itself, rather than by indirectly increasing the dissolution of BSi present in the sediment.

Spatial and temporal dynamics of ammonia oxidizers in the sediments of the Gulf of Finland, Baltic Sea

The diversity and dynamics of ammonia-oxidizing bacteria (AOB) and archaea (AOA) nitrifying communities in the sediments of the eutrophic Gulf of Finland (GoF) were investigated. Using clone libraries of ammonia monooxygenase (amoA) gene fragments and terminal restriction fragment length polymorphism (TRFLP), we found a low richness of both AOB and AOA. The AOB amoA phylogeny matched that of AOB 16S ribosomal genes from the same samples. AOA communities were characterized by strong spatial variation while AOB communities showed notable temporal patterns. At open sea sites, where transient anoxic conditions prevail, richness of both AOA and AOB was lowest and communities were dominated by organisms with gene signatures unique to the GoF. Given the importance of nitrification as a link between the fixation of nitrogen and its removal from aquatic environments, the low diversity of ammonia-oxidizing microbes across the GoF could be of relevance for ecosystem resilience in the face of rapid global environmental changes.

Aeration-Induced Changes in Temperature and Nitrogen Dynamics in a Dimictic Lake.

Low levels of oxygen (O) in the hypolimnion layer of lakes are harmful to benthic animals and fish; they may also adversely affect nutrient cycles. Artificial aeration is often used in lake management to counteract these problems, but the effects of aeration on nitrogen (N) cycling are not known. We studied the effects of hypolimnetic aeration on N dynamics and temperature in a eutrophic lake by comparing continuous and pulsed aeration with a nonaerated station. Aeration decreased the accumulation of NH-N deep in the lake (20-33 m) by supplying O for nitrification, which in turn provided substrate for denitrification and promoted N removal. Aeration also increased the temperature in the hypolimnion. Denitrification rate was highest in the nonaerated deep areas (average, 7.62 mg N m d) due to very high rates during spring turnover of the water column, demonstrating that natural turnover provides O for nitrification. During stratification, denitrification was highest at the continuously aerated station (4.06 mg N m d) and lowest at the nonaerated station (3.02 mg N m d). At the periodically aerated station, aeration pauses did not restrict the increase in temperature but resulted in accumulation of NH-N and decreased the contribution of denitrification as a nitrate reduction process. Our findings demonstrate that hypolimnetic aeration can substantially affect N cycling in lakes and that the effect depends on the aeration strategy. Because N is one of the main nutrients controlling eutrophication, the effects of aeration methods on N removal should be considered as part of strategies to manage water quality in lakes.

Biogeochemistry: The depths of nitrogen cycling

Breakdown of dissolved organic nitrogen in the ocean had been thought to be the preserve of microbes at the surface. The discovery that these microbes are not up to the task calls for a reassessment of the biogeochemistry of this nitrogen pool.

Measuring nitrification in sediments – comparison of two techniques and three 15NO measurement methods

Nitrification is a crucial process in sediment nitrogen cycling. We compared two 15N tracer-based nitrification measurement techniques (isotope pairing technique (IPT) combined with 15N nitrate pool dilution and 15N ammonium oxidation) and three different 15N analyses from bottom water nitrate (ammonia diffusion, denitrifier and SPINMAS) in a sediment mesocosm. The 15N nitrate pool dilution technique combined with IPT can be used to quantify the in situ nitrification, but the minimum detection limit for the total nitrification is higher than that in the 15N ammonium oxidation technique. The 15N ammonium oxidation technique, however, is not applicable for sediments that have high ammonium content. If nitrate concentration and the amount of 15N label in the sample are low, the 15N nitrate analysis should be done with the denitrifier method. In higher 15N concentrations, the less sensitive SPINMAS method can also be applied. The ammonia diffusion method is not suitable for bottom water 15N nitrate analyses.

Particulate organic matter controls benthic microbial N retention andN removal in contrasting estuaries of the Baltic Sea

Estuaries worldwide are known to act as “filters” of land-derived N loads, yet their variable environmental settings can affect microbial nitrogen (N) retention and removal and thus the coastal filter function. We investigated microbial N-retention (nitrification, ammonium assimilation) and N-removal (denitrification, anammox) in the aphotic benthic systems (here defined as: bottom boundary layer [BBL] and sediment) of two Baltic Sea estuaries that differ in riverine N loads, trophic state, bottom 20 topography, and sediment type. Contrary to our expectations, nitrification rates (5 – 227 nmol L d) in the BBL neither differed between the eutrophied Vistula estuary and the oligotrophic Öre estuary, nor between seasons. Ammonium assimilation rates were slightly higher in the oligotrophic Öre estuary in spring but did not differ between estuaries in summer (9 – 704 nmol L d). In the sediment, no anammox was found in either estuary and denitrification rates were higher in the eutrophied (349 ± 117 μmol N m d) than in the oligotrophic estuary (138 ± 47 μmol N m d). Irrespective of their 25 differences, in both estuaries the quality of the mainly phytoplankton-derived particulate organic matter (POM) evaluated by means of C:N and POC:Chl.a ratios seemed to control N-cycling processes through the availability of particulate organic N and C as substrate sources. Our data suggest, that in stratified estuaries, phytoplankton-derived POM is an essential link between riverine N loads and benthic N cycling and may function as a temporary N reservoir via long particle residence time Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-450 Manuscript under review for journal Biogeosciences Discussion started: 26 October 2018 c

A bioreactor approach to investigate the linkage between methane oxidation and nitrate/nitrite reduction in the pelagic oxic-anoxic transition zone of the central Baltic Sea

Evidence of aerobic methane oxidation coupled to denitrification has been provided for different freshwater environments, whereas the significance of this process for the marine realm has not been adequately investigated. The goal of this study was to investigate the methane-related reduction of nitrate/nitrite in a marine environment (salinity 8.5). A water sample was collected from the oxic-anoxic transition zone of the Gotland Deep (central Baltic Sea) and the microorganisms contained therein were cultivated in a bioreactor under hypoxic conditions (0.5 µM O2). To enrich the microorganisms involved in the coupled process the bioreactor was continuously sparged with methane as the sole energy and carbon source and simultaneously supplied with a nutrient solution rich in nitrate and nitrite. The bioreactor experiment showed a relationship between the turnover of methane and the concomitant concentration decrease of nitrite and nitrate at the early stage of the experiment. This relationship indicates the role of methanotrophs, which may support heterotrophic denitrifiers by the release of organic compounds as an energy source. Besides, a mixture of uncultured microorganisms, aerobic methanotrophic and heterotrophic denitrifying bacteria were identified in the enrichment culture. Microbial incorporation of nitrite and methane was proven on the cellular and gene levels via 15NO2- / 13CH4 incubation experiments and subsequent analyses with nano secondary ion mass spectrometry (NanoSIMS) and stable isotope probing (SIP). The NanoSIMS showed the incorporation of 15N in almost all the bacteria and in 9% of those there was a concomitant enrichment in 13C. The relatively low abundance of methane-consuming bacteria in the bioreactor was further reflected in specific fatty acids indicative for type I methanotrophic bacteria. Based on pmoA gene analyses, this bacterium is different from the one that was identified as the only key player of methane oxidation in previous studies in the Gotland Deep, indicating the existence of other subordinate methanotrophic bacteria at that site. The results provide the first indications for the predisposition of a methane-related reduction of nitrate/nitrite under hypoxic conditions in the marine realm, supporting the assumption of an interaction between methanotrophic and denitrifying bacteria which hitherto has only been described for fresh water environments.