From microstructures to macroscale carbon export : influences of marine snow composition on settling velocity and microbial colonization

Sinking marine aggregates are an integral component of the marine biological carbon pump. The specific focus of this thesis was on techniques to study the composition and sinking behavior of single aggregates collected in situ. Thin-sectioning of aggregates in soft and hard embedding media was used to examine the structure and colonization patterns of aggregates at high spatial resolution. This enabled detailed characterization of the aggregate matrix and localization of selected bacterial colonizers. To study the effect of aggregate composition on sinking velocity, I used Particle Image Velocimetry to analyze the fluid flow around settling aggregates in a flow chamber. Aggregate flow fields strongly resembled those of impermeable, porous spheres which suggested that Stokes law is applicable to settling marine aggregates. The influence of aggregate composition on export flux was studied in a laboratory experiment through incorporation of plastic microfibers into settling aggregates. Microfiber addition decreased potential export flux by 30-60% and, if confirmed in situ, may substantially reduce the efficiency of the biological carbon pump

The Arctic picoeukaryote Micromonas pusilla benefits synergistically from warming and ocean acidification

In the Arctic Ocean, climate change effects such as warming and ocean acidification (OA) are manifesting faster than in other regions. Yet, we are lacking a mechanistic understanding of the interactive effects of these drivers on Arctic primary producers. In the current study, one of the most abundant species of the Arctic Ocean, the prasinophyte Micromonas pusilla, was exposed to a range of different pCO2 levels at two temperatures representing realistic current and future scenarios for nutrient-replete conditions. We observed that warming and OA synergistically increased growth rates at intermediate to high pCO2 levels. Furthermore, elevated temperatures shifted the pCO2 optimum of biomass production to higher levels. Based on changes in cellular composition and photophysiology, we hypothesise that the observed synergies can be explained by beneficial effects of warming on carbon fixation in combination with facilitated carbon acquisition under OA. Our findings help to understand the higher abundances of picoeukaryotes such as M. pusilla under OA, as has been observed in many mesocosm studies

Woody encroachment and forest degradation in sub-Saharan Africa's woodlands and savannas 1982-2006

We review the literature and find 16 studies from across Africa's savannas and woodlands where woody encroachment dominates. These small-scale studies are supplemented by an analysis of long-term continent-wide satellite data, specifically the Normalized Difference Vegetation Index (NDVI) time series from the Global Inventory Modeling and Mapping Studies (GIMMS) dataset. Using dry-season data to separate the tree and grass signals, we find 4.0% of non-rainforest woody vegetation in sub-Saharan Africa (excluding West Africa) significantly increased in NDVI from 1982 to 2006, whereas 3.52% decreased. The increases in NDVI were found predominantly to the north of the Congo Basin, with decreases concentrated in the Miombo woodland belt. We hypothesize that areas of increasing dry-season NDVI are undergoing woody encroachment, but the coarse resolution of the study and uncertain relationship between NDVI and woody cover mean that the results should be interpreted with caution; certainly, these results do not contradict studies finding widespread deforestation throughout the continent. However, woody encroachment could be widespread, and warrants further investigation as it has important consequences for the global carbon cycle and land–climate interactions

A vast icefish breeding colony discovered in the Antarctic

A breeding colony of notothenioid icefish (Neopagetopsis ionah, Nybelin 1947) of globally unprecedented extent has been discovered in the southern Weddell Sea, Antarctica. The colony was estimated to cover at least �240 km2 of the eastern flank of the Filchner Trough, comprised of fish nests at a density of 0.26 nests per square meter, representing an estimated total of �60 million active nests and associated fish biomass of >60,000 tonnes. The majority of nests were each occupied by 1 adult fish guarding 1,735 eggs (±433 SD). Bot- tom water temperatures measured across the nesting colony were up to 2�C warmer than the surrounding bottom waters, indicating a spatial correlation between the modified Warm Deep Water (mWDW) upflow onto the Weddell Shelf and the active nesting area. Historical and concurrently collected seal movement data indicate that this concentrated fish biomass may be utilized by predators such as Weddell seals (Lep- tonychotes weddellii, Lesson 1826). Numerous degraded fish carcasses within and near the nesting colony suggest that, in death as well as life, these fish provide input for local food webs and influence local biogeo- chemical processing. To our knowledge, the area surveyed harbors the most spatially expansive continuous fish breeding colony discovered to date globally at any depth, as well as an exceptionally high Antarctic sea- floor biomass. This discovery provides support for the establishment of a regional marine protected area in the Southern Ocean under the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) umbrella

Eddy Study to Understand Physical-Chemical-Biological Coupling and the Biological Carbon Pump as a Function of Eddy Type off West Africa, Cruise No. M160, 23.11.2019 - 20.12.2019, Mindelo (Cabo Verde) - Mindelo (Cabo Verde)

Cruise M160 is part of concerted MOSES/REEBUS Eddy Study featuring three major research expeditions (M156, M160, MSM104). It aims to develop both a qualitative and quantitative understanding of the role of physical-chemical-biological coupling in eddies for the biological pump. The study is part of the MOSES “Ocean Eddies” event chain, which follows three major hypotheses to be addressed by the MOSES/REEBUS field campaigns: (1) Mesoscale and sub-mesoscale eddies play an important role in transferring energy along the energy cascade from the large-scale circulation to dissipation at the molecular level. (2) Mesoscale and sub-mesoscale eddies are important drivers in determining onset, magnitude and characteristics of biological productivity in the ocean and contribute significantly to global primary production and particle export and transfer to the deep ocean. (3) Mesoscale and sub-mesoscale eddies are important for shaping extreme biogeochemical environments (e.g., pH, oxygen) in the oceans, thus acting as a source/sink function for greenhouse gases. In contrast to the other two legs, MOSES Eddy Study II during M160 did not include any benthic work but focused entirely on the pelagic dynamics within eddies. It accomplished a multi-disciplinary, multi-parameter and multi-platform study of two discrete cyclonic eddies in an unprecedented complexity. The pre-cruise search for discrete eddies suitable for detailed study during M160 had already started a few months prior to the cruise. Remote sensing data products (sea surface height, sea surface temperature, ocean color/chlorophyll a) were used in combination with eddy detection algorithms and numerical modelling to identify and track eddies in the entire eddy field off West Africa. In addition, 2 gliders and 1 waveglider had been set out from Mindelo/Cabo Verde for pre-cruise mapping of the potential working area north of the Cabo Verdean archipelago. At the start of M160, a few suitable eddies – mostly of cyclonic type – had been identified, some of which were outside the safe operation range of the motorglider plane. As technical problems delayed the flight operations, the first eddy (center at 14.5°N/25°W) for detailed study was chosen to the southwest of the island of Fogo. It was decided to carry out a first hydrographic survey there followed by the deployment of a suite of instruments (gliders, waveglider, floats, drifter short-term mooring). Such instrumented, we left this first eddy and transited – via a strong anticyclonic feature southwest of the island of Santiago – to the region northeast of the island of Sal, i.e. in the working range of the glider plane. During the transit, a full suite of underway measurements as well as CTD/RO section along 22°W (16°-18.5°N) were carried in search for sub-surface expressions of anticyclonic eddy features. In the northeast, we had identified the second strong cyclonic eddy (center at 18°N/22.5°W) which was chosen for detailed study starting with a complete hydrographic survey (ADCP, CTD/RO, other routine station work). After completion of the mesoscale work program, we identified a strong frontal region at the southwestern rim of the cyclonic eddy, which was chosen for the first sub-mesoscale study with aerial observation component. There, the first dye release experiment was carried out which consisted of the dye release itself followed by an intense multi-platforms study of the vertical and horizontal spreading of the initial dye streak. This work was METEOR-Berichte, Cruise M160, Mindelo – Mindelo, 23.11.2019 4 – 20.12.2019 supported and partly guided by aerial observation of the research motorglider Stemme, which was still somewhat compromised by technical issues and meteorological conditions (high cloud cover, Saharan dust event). Nevertheless, this first dye release experiment was successful and showed rapid movement of the dynamic meandering front. After completion of work on this second eddy and execution of a focused sampling program at the Cape Verde Ocean Observation, RV METEOR returned to the first eddy for continuation of the work started there in the beginning of the cruise. This was accompanied by a relocation of the airbase of Stemme from the international airport of Sal to the domestic airport of Fogo. The further execution of the eddy study at this first eddy, which again included a complete hydrographic survey followed by a mesoscale eddy study with dye release, was therefore possible with aerial observations providing important guidance for work on RV METEOR. Overall, M160 accomplished an extremely intense and complex work program with 212 instrument deployments during station work, 137 h of observation with towed instruments and a wide range of underway measurements throughout the cruise. Up to about 30 individually tracked platforms (Seadrones, glider, wavegliders, drifters, floats) were in the water at the same time providing unprecedented and orchestrated observation capabilities in an eddy. All planned work components were achieved and all working groups acquired the expected numbers of instrument deployments and sampling opportunities

Einfluss von Mikrostrukturen in Marine Snow auf Sinkverhalten und mikrobielle Besiedlung

Sinking marine aggregates are an integral component of the marine biological carbon pump. The specific focus of this thesis was on techniques to study the composition and sinking behavior of single aggregates collected in situ. Thin-sectioning of aggregates in soft and hard embedding media was used to examine the structure and colonization patterns of aggregates at high spatial resolution. This enabled detailed characterization of the aggregate matrix and localization of selected bacterial colonizers. To study the effect of aggregate composition on sinking velocity, I used Particle Image Velocimetry to analyze the fluid flow around settling aggregates in a flow chamber. Aggregate flow fields strongly resembled those of impermeable, porous spheres which suggested that Stokes law is applicable to settling marine aggregates. The influence of aggregate composition on export flux was studied in a laboratory experiment through incorporation of plastic microfibers into settling aggregates. Microfiber addition decreased potential export flux by 30-60% and, if confirmed in situ, may substantially reduce the efficiency of the biological carbon pump

Aggregate turnover at the base of the euphotic zone – Influences of diatom morphology on carbon export

The world’s oceans play an important role in sequestering carbon over timescales of 105 years and more. For the carbon to be transferred from the atmosphere to the deep ocean where it is stored, it first has to be fixed in the form of particulate organic matter. This is mostly done by microalgae like diatoms, which fix CO2 through photosynthesis. When particle abundances are high (for example during a phytoplankton bloom), diatoms and other organic and inorganic matter aggregate and sink through the water column to the deep sea. Only a small fraction of the carbon fixed by phytoplankton reaches the seafloor because it is getting degraded by microorganisms and higher animals while it is sinking through the water column. Therefore, understanding the factors that influence degradation and carbon release is important for predicting and analyzing local as well as global carbon fluxes. One factor that has been neglected so far is the effect of phytoplankton morphology on carbon export. For this thesis, I studied two species of diatoms exhibiting common but contrasting morphologies: chain-forming and non-chain-forming, and studied the aggregation process and other down-stream factors affected by aggregate morphology: size, sinking velocity, carbon and nitrogen content, and microbial degradation rate. The process of aggregate formation and the appearance and structure of the respective aggregates strongly suggested that in chain-forming diatoms physical aggregation via the entangling of chains is the predominating process, whereas in non-chain-forming diatoms, TEP was the main factor causing aggregation. This had large influences on the settling behavior and the carbon content, thus affecting the amount of bacterial colonization and aggregate degradation

Using cryosectioning to examine bacterial colonization dynamics in marine snow

Due to their high organic matter content, marine snow particles are hotspots for microbial activity. The heterogeneous composition of marine snow makes microbial dynamics and microbe-substrate interactions hard to examine using standard filtration and microscopy. As spatial information is crucial to better understand these interactions, we have developed cryosectioning of frozen embedded marine snow as new tool for high-resolution 3D visualization of individual aggregates. We used this method on in situ collected marine snow to conduct a series of incubations where we compared the colonization potential of a) motile Marinobacter adhaerens and their aflagellate mutants and b) bacteria extracted from two different water depths. Surprisingly, we observed attachment and penetration for M. adhaerens with and without flagella, suggesting that bacterial motility is not the only controlling factor for aggregate colonization. Our method and findings shed new light on the role of special adaptations of aggregate-associated microorganisms and pave the way for future research on specialized microbe-substrate interactions and sequential degradation of organic compounds