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

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

Embedding and slicing of intact in situ collected marine snow

The biological carbon pump is largely driven by the formation and sinking of marine snow. Because of their high organic matter content, marine snow aggregates are hotspots for microbial activity, and microbial organic matter degradation plays an important role in the attenuation of carbon fluxes to the deep sea. Our inability to examine and characterize microscale distributions of compounds making up the aggregate matrix, and of possible niches inside marine snow, has hindered our understanding of the basic processes governing marine carbon export and sequestration. To address this issue, we have adapted soft‐embedding and sectioning to study the spatial structure and components of marine aggregates at high resolution. Soft‐embedding enables rapid quantitative sampling of undisturbed marine aggregates from the water column and from sediment traps, followed by spatially resolved staining and characterization of substrates of the aggregate matrix and the microorganisms attached to it. Particular strengths of the method include in situ embedding in sediment traps and successful fluorescence in situ hybridization (FISH)‐probe labeling, supporting studies of microbial diversity and ecology. The high spatial resolution achieved by thin‐sectioning of soft‐embedded aggregates offers the possibility for improved understanding of the composition and structure of marine snow, which directly influence settling velocity, microbial colonization and diversity, degradation rates, and carbon content. Our method will help to elucidate the small‐scale processes underlying large‐scale carbon cycling in the marine environment, which is especially relevant in the context of rising anthropogenic CO2 emissions and global change

Hard and soft plastic resin embedding for single-cell element uptake investigations of marine-snow-associated microorganisms using nano-scale secondary ion mass spectrometry

Marine snow aggregates are microhabitats for diverse microbial communities with various active metabolic pathways. Rapid recycling and symbiotic transfer of nutrients within aggregates poses a significant challenge for accurately assessing aggregate‐associated turnover rates. Although single‐cell uptake measurements are well‐established for free‐living microorganisms, suitable methods for cells embedded in marine snow are currently lacking. Comparable cell‐specific measurements within sinking pelagic aggregates would have the potential to address core questions regarding aggregate‐associated fluxes. However, the capacity to perform microscale studies is limited by the difficulty of sampling and preserving the fragile aggregate structure. Furthermore, the application of nano‐scale secondary ion mass spectrometry (NanoSIMS) to aggregates is complicated by technical requirements related to vacuum and ablation resistance. Here, we present a NanoSIMS‐optimized method for fixation, embedding, and sectioning of marine snow. Stable isotope labeling of laboratory‐generated aggregates enabled visualization of label incorporation into prokaryotic and eukaryotic cells embedded in the aggregate structure. The current method is also amenable to various staining procedures, including transparent exopolymer particles, Coomassie stainable particles, nucleic acids, and eukaryotic cytoplasm. We demonstrate the potential for using structural stains to generate three‐dimensional (3D) models of marine snow and present a simplified calculation of porosity and fractal dimension. This multipurpose method enables combined investigations of 3D aggregate structure, spatial microbial distribution, and single‐cell activity within individual aggregates and provides new possibilities for future studies on microbial interactions and elemental uptake within marine snow

Krill and salp faecal pellets contribute equally to the carbon flux at the Antarctic Peninsula

This study was part of the project “Population shift and ecosystem response – krill vs. salps” funded by the Lower Saxony Ministry of Science and Culture (MWK) lead by B.M. M.H.I., C.M.F., C.K. and S.S. were supported by the HGF Young Investigator Group SeaPump “Seasonal and regional food web interactions with the biological pump”, VH-NG-1000. CMF was additionally supported by the AWI Strategy Fund project EcoPump. MHI was additionally supported by the DFG Research Center of Excellence “The Ocean Floor – Earth’s Uncharted Interface”: EX-2077-390741603.Krill and salps are important for carbon flux in the Southern Ocean, but the extent of their contribution and the consequences of shifts in dominance from krill to salps remain unclear. We present a direct comparison of the contribution of krill and salp faecal pellets (FP) to vertical carbon flux at the Antarctic Peninsula using a combination of sediment traps, FP production, carbon content, microbial degradation, and krill and salp abundances. Salps produce 4-fold more FP carbon than krill, but the FP from both species contribute equally to the carbon flux at 300 m, accounting for 75% of total carbon. Krill FP are exported to 72% to 300 m, while 80% of salp FP are retained in the mixed layer due to fragmentation. Thus, declining krill abundances could lead to decreased carbon flux, indicating that the Antarctic Peninsula could become a less efficient carbon sink for anthropogenic CO2 in future.Publisher PDFPeer reviewe