ALBERT

Description: The effects of temperature and pCO2 on the development of bacterial communities were investigated in mesocosm experiments with Baltic Sea water, during a diatom bloom in autumn and during a cyanobacterial bloom in summer. We measured prokaryotic cell numbers and activity and used 454 pyrosequencing to compare the development of the bacterial community composition (BCC) and to identify the major bacterial taxa. Organic matter dynamics were assessed to differentiate between direct effects and indirect phytoplankton induced effects. The development of BCC followed well-known bloom dynamics. A principle coordinate analysis (PCoA) of bacterial OTUs (operational taxonomic units) revealed that phytoplankton succession and temperature were the major bacterial community structuring variables whereas only a very weak impact of pCO2 was found. This was corroborate by the trends in bacterial bulk parameters. However, significant effects of pCO2 on the relative abundance of individual dominant OTUs occurred. Our results suggest that increasing pCO2 only slightly affects the development of overall bacterial community composition but impacts specific bacterial groups which might influence also particular organic matter degradation processes.

Description: Oxygen-deficient waters in the ocean, generally referred to as oxygen minimum zones (OMZ), are expected to expand as a consequence of global climate change. Poor oxygenation is promoting microbial loss of inorganic nitrogen (N) and increasing release of sediment-bound phosphate (P) into the water column. These intermediate water masses, nutrient-loaded but with an N deficit relative to the canonical N:P Redfield ratio of 16:1, are transported via coastal upwelling into the euphotic zone. To test the impact of nutrient supply and nutrient stoichiometry on production, partitioning and elemental composition of dissolved (DOC, DON, DOP) and particulate (POC, PON, POP) organic matter, three nutrient enrichment experiments were conducted with natural microbial communities in shipboard mesocosms, during research cruises in the tropical waters of the southeast Pacific and the northeast Atlantic. Maximum accumulation of POC and PON was observed under high N supply conditions, indicating that primary production was controlled by N availability. The stoichiometry of microbial biomass was unaffected by nutrient N:P supply during exponential growth under nutrient saturation, while it was highly variable under conditions of nutrient limitation and closely correlated to the N:P supply ratio, although PON:POP of accumulated biomass generally exceeded the supply ratio. Microbial N:P composition was constrained by a general lower limit of 5:1. Channelling of assimilated P into DOP appears to be the mechanism responsible for the consistent offset of cellular stoichiometry relative to inorganic nutrient supply and nutrient drawdown, as DOP build-up was observed to intensify under decreasing N:P supply. Low nutrient N:P conditions in coastal upwelling areas overlying O2-deficient waters seem to represent a net source for DOP, which may stimulate growth of diazotrophic phytoplankton. These results demonstrate that microbial nutrient assimilation and partitioning of organic matter between the particulate and the dissolved phase are controlled by the N:P ratio of upwelled nutrients, implying substantial consequences for nutrient cycling and organic matter pools in the course of decreasing nutrient N:P stoichiometry.

Description: Increasing concentrations of atmospheric carbon dioxide are projected to lead to an increase in sea surface temperatures, potentially impacting marine ecosystems and biogeochemical cycling. Here we conducted an indoor mesocosm experiment with a natural plankton community taken from the Baltic Sea in summer. We induced a plankton bloom via nutrient addition and followed the dynamics of the different carbon and nitrogen pools for a period of one month at temperatures ranging from 9.5 °C to 17.5 °C, representing a range of ± 4 °C relative to ambient temperature. The uptake of dissolved inorganic carbon (DIC) and the net build-up of both particulate (POC) and dissolved organic carbon (DOC) were all enhanced at higher temperatures and almost doubled over a temperature gradient of 8 °C. Furthermore, elemental ratios of carbon and nitrogen (C:N) in both particulate and dissolved organic matter increased in response to higher temperatures, both reaching very high C:N ratios of 〉30 at +4 °C. Altogether, these observations suggest a pronounced increase in excess carbon fixation in response to elevated temperatures. Most of these findings are contrary to results from similar experiments conducted with plankton populations sampled in spring, revealing large uncertainties in our knowledge of temperature sensitivities of key processes in marine carbon cycling. Since a major difference to previous mesocosm experiments was the dominant phytoplankton species, we hypothesize that species composition might play an important role in the response of biogeochemical cycling to increasing temperatures.

Description: Iron is an essential micronutrient often limiting the growth of marine microorganisms in wide areas of the world’s oceans. In high concentrations, iron, by contrast, is potentially toxic and usually leads to irreversible cell encrustation followed by cell death. To counteract both, microorganisms have evolved the strategy of producing organic iron-binding molecules, so called iron-ligands, enabling them to improve the bioavailability and uptake of iron as well as to mitigate its potentially toxic effects. Hydrothermal vents are among the major sources of iron in the oceans. These dynamic habitats host a variety of metabolically highly specialized and versatile microbes that not only have to cope with partially high iron concentrations but may also be able to mediate the availability of inorganic hydrothermal iron by actively producing iron-ligands. However, hardly any information exists to-date describing the impact of increasing iron concentrations on hydrothermal plume microbial communities and their potential to form iron-ligands. We therefore set up microcosm experiments with hydrothermal plume material in artificial seawater along an iron gradient ranging from 0 to 10 mM. We found that the microbial community at low iron concentrations (0.1 to 100 μM) differs significantly from that found in the original non-treated plume sample, allowing a certain group of Epsilonproteobacteria to become dominant (up to 93% of the overall community). The microbial community detected at 10 mM is by contrast more similar to that found in the original plume sample and consists mainly of one gammaproteobacterial group (up to 97% of the overall community). We further analyzed these results in the context of ligand concentrations and structural diversity and found indications for microbially mediated iron-ligand formation. This is the first holistic experimental approach linking studies of hydrothermal vent microbial community composition with the geochemistry involved in organic iron-ligand formation.