How microbial food web interactions shape the arctic ocean bacterial community revealed by size fractionation experiments

In the Arctic, seasonal changes are substantial, and as a result, the marine bacterial community composition and functions differ greatly between the dark winter and light-intensive summer. While light availability is, overall, the external driver of the seasonal changes, several internal biological interactions structure the bacterial community during shorter timescales. These include specific phytoplankton–bacteria associations, viral infections and other top-down controls. Here, we uncover these microbial interactions and their effects on the bacterial community composition during a full annual cycle by manipulating the microbial food web using size fractionation. The most profound community changes were detected during the spring, with ‘mutualistic phytoplankton’—Gammaproteobacteria interactions dominating in the pre-bloom phase and ‘substrate-dependent phytoplankton’—Flavobacteria interactions during blooming conditions. Bacterivores had an overall limited effect on the bacterial community composition most of the year. However, in the late summer, grazing was the main factor shaping the community composition and transferring carbon to higher trophic levels. Identifying these small-scale interactions improves our understanding of the Arctic marine microbial food web and its dynamics

Winter−spring transition in the subarctic Atlantic: microbial response to deep mixing and pre-bloom production

In temperate, subpolar and polar marine systems, the classical perception is that diatoms initiate the spring bloom and thereby mark the beginning of the productive season. Contrary to this view, we document an active microbial food web dominated by pico- and nanoplankton prior to the diatom bloom, a period with excess nutrients and deep convection of the water column. During repeated visits to stations in the deep Iceland and Norwegian basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the succession and dynamics of photosynthetic and heterotrophic microorganisms. We observed that the early phytoplankton production was followed by a decrease in the carbon:nitrogen ratio of the dissolved organic matter in the deep mixed stations, an increase in heterotrophic prokaryote (bacteria) abundance and activity (indicated by the high nucleic acid:low nucleic acid bacteria ratio), and an increase in abundance and size of heterotrophic protists. The major chl a contribution in the early winter-spring transition was found in the fraction 50 µm) were stimulated by deep mixing later in the period, while picophytoplankton were unaffected by mixing; both physical and biological reasons for this development are discussed herein

Glacier retreat alters downstream fjord ecosystem structure and function in Greenland

The melting of the Greenland Ice Sheet is accelerating, with glaciers shifting from marine to land termination and potential consequences for fjord ecosystems downstream. Monthly samples in 2016 in two fjords in southwest Greenland show that subglacial discharge from marine-terminating glaciers sustains high phytoplankton productivity that is dominated by diatoms and grazed by larger mesozooplankton throughout summer. In contrast, melting of land-terminating glaciers results in a fjord ecosystem dominated by bacteria, picophytoplankton and smaller zooplankton, which has only one-third of the annual productivity and half the CO2 uptake compared to the fjord downstream from marine-terminating glaciers.publishedVersio

Biological transformation of Arctic dissolved organic matter in a NE Greenland fjord

Arctic waters are often enriched with terrestrial dissolved organic matter (DOM) characterized by having elevated visible wavelength fluorescence (commonly termed humic-like). Here, we have identified the sources of fluorescent DOM (FDOM) in a high Arctic fjord (Young Sound, NE Greenland) influenced by glacial meltwater. The biological transformation of FDOM was further investigated using plankton community size-fractionation experiments. The intensity of ultraviolet fluorescence (commonly termed amino acid-like) was highly variable and positively correlated to bacterial production and mesozooplankton grazing. The overall distribution of visible FDOM in the fjord was hydrographically driven by the high-signal intrusion of Arctic terrestrial DOM from shelf waters and dilution with glacial runoff in the surface waters. However, the high-intensity visible FDOM that accumulated in subsurface waters in summer was not solely linked to allochthonous sources. Our data indicate that microbial activity, in particular, protist bacterivory, to be a source. A decrease in visible FDOM in subsurface waters was concurrent with an increase in bacterial abundance, indicating an active bacterial uptake or modification of this DOM fraction. This was confirmed by net-loss of visible FDOM in experiments during summer when bacterial activity was high. The degradation of visible FDOM appeared to be associated with bacteria belonging to the order Alteromonadales mainly the genus Glaciecola and the SAR92 clade. The findings provide new insight into the character of Arctic terrestrial DOM and the biological production and degradation of both visible and UV wavelength organic matter in the coastal Arctic.publishedVersio

Heterogeneous distribution of plankton within the mixed layer and its implications for bloom formation in tropical seas

Intensive sampling at the coastal waters of the central Red Sea during a period of thermal stratification, prior to the main seasonal bloom during winter, showed that vertical patches of prokaryotes and microplankton developed and persisted for several days within the apparently density uniform upper layer. These vertical structures were most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioural aggregation. Simulating a mixing event by adding nutrient-rich deep water abruptly triggered dense phytoplankton blooms in the nutrient-poor environment of the upper layer. These findings suggest that vertical structures within the mixed layer provide critical seeding stocks that can rapidly exploit nutrient influx during mixing, leading to winter bloom formation

Microbial dynamics in high latitude ecosystems. Responses to mixing, runoff and seasonal variation a rapidly changing environment

The pronounced warming at high latitude alters a range of physical conditions i.e. the magnitude of runoff, sea-ice extent and strength of stratification and thus affect the biological systems. As microorganisms form the living base of the pelagic food web and are the major drives of biogeochemical processing it is critical to understand their response to these changes. This Ph.D. project focuses on the smallest (<2μm) and most abundant microorganisms, heterotrophic bacterioplankton (bacteria and Archaea) and autotrophic picophytoplankton, and the factors regulating their abundance, diversity and activity in the Arctic-Subarctic Atlantic Ocean. The study covers hydrographic regimes off and around Iceland, Norway (including Svalbard) and East Greenland (60-83°N), and combines field observations and experiments during different seasons. The main aim is to elucidate the three following topics: 1) Challenges phytoplankton face related to high seasonality and low light conditions 2) Bioavailability of dissolved organic matter (DOM) to bacterial communities and their response to an increase in terrestrial loading 3) Importance of top-down control by heterotrophic nanoflagellates (HNF) on both pico-sized phytoplankton and bacteria. My study underpins that picophytoplankton are important contributors to primary production, especially during the winter-spring transition (Paper I and III) and autumn (Paper V). They boosted the growth of heterotrophic microorganisms before the onset of the diatom spring bloom in the Subarctic Atlantic (Paper I) and dominated the phytoplankton biomass in the high turbid parts of a NE Greenland fjord influenced by glacial meltwater (Paper V). Picophytoplankton were better adapted to low light conditions and demonstrated higher growth rates, than larger phytoplankton (Paper I, II, III, V). In the Polar-influenced water near Greenland, Synechococcus were negligible, while in the Atlantic influenced waters picoeukaryotes and Synechococcus were often equally abundant and the latter dominated on several occasions during autumn and winter (Paper I and III). Unexpectedly, abundances of Synechococcus were as high at 65°N as at 79°N, and molecular analysis suggests the presence of new clades specially adapted to Arctic conditions. Bacteria were generally rather carbon- than nutrient limited, and their abundance increased rapidly in response to the pre-bloom picophytoplankton production of labile carbon in both the Arctic and Subarctic (Paper I and V). In NE Greenland the terrestrial DOM supplied from the Greenland ice sheet proved to be highly bioavailable compared to the autochthonous fjord DOM (Paper IV). The in situ changes in DOM, which were examined via fluorescence signal of different DOM components (FDOM), surprisingly demonstrated that the highest net-growth of bacteria was not coupled to the labile glacial runoff in the surface, but rather to sub-surface the humic-DOM, commonly considered to be refractory. This may be explained by the presence of specific dominating taxa of bacteria that had the ability to degrade humic-DOM (Paper V). Across regions, HNF exerted strong control of picophytoplankton and bacteria (Paper I, II, III, V). HNF grew significantly faster than microzooplankton and were therefore less affected by mixing and relatively more important grazers than their micro-sized counterparts in well-mixed water columns (Paper I and II). HNF larger than 5μm controlled picophytoplankton particularly in the early productive season, while small HNF (3-5μm) mainly kept bacteria in check in autumn (Paper III, V). In conclusion, the studies underline that pico-sized plankton play a fundamental part in the carbon transfer in high latitude ecosystems both as primary producers and via the microbial loop. Picophytoplankton appeared better adapted than larger phytoplankton to low light conditions, and bacteria were capable of degrading terrestrial derived DOM, however, these abilities are highly community specific. The data suggest that a change in mixing patters will affect the microbial food structure and that shifts in coastal microbial community composition should be anticipated with increased runoff

Pre-bloom dynamics of the Subpolar North Atlantic microbial food web

The aim of this study was to investigate the microbial planktonic food web in the pre- bloom period of the Subpolar North Atlantic Ocean. Through repeated visits to the Icelandic Basin, Norwegian Basin, and on the Shetland Shelf in the period March 28th to May 1st, we recorded the abundance of all the functional groups of both autotrophic and heterotrophic microbes. At the first visits, all stations were characterized by low concentrations of chlorophyll a (0.1-0.5 µg l^{-1}) and a low abundance of heterotrophic bacteria (2-3.4 × 10^5 cells ml^{-1}), heterotrophic nanoflagellates (22-84 cells ml^{-1}), ciliates (1-2 cells ml^{-1}), and heterotrophic dinoflagellates (0.1-0.3 cells ml^{-1}) within the upper mixed layer. Following the abundance of heterotrophic protists generally increased; 2- fold for bacteria and up to 5-fold for heterotrophic nanoflagellates. An initial dominance of pico eukaryotes within the phytoplankton community was observed in late winter. This was followed, however, by a significant decrease during the pre-bloom period, despite high nutrient concentrations and increasing light intensity. The decrease of pico eukaryote was concurrent with an increase of heterotrophic nanoflagellates, hence grazing pressure. The microbial trophic interactions were analysed further via grazing experiments, performed with water sampled at the Icelandic Basin. These revealed heterotrophic nanoflagellate removal rates of 10-20 % of bacterial standing stock d^{-1} and as high as 30- 50 % of the standing stock of pico phytoplankton d^{-1} in the euphotic zone. We conclude that heterotrophic nanoflagellates in the pre-bloom can satisfy up to half of their carbon demand by herbivory, and thus the strong focus of heterotrophic nanoflagellates' role, as being mainly bacterivorous, should be revised. We document that the pre-bloom is a productive period with carbon entering the ocean food web largely via the microbial food web. Thus, not only the seasonal changes of physical condition, but also the microbial dynamics in the pre-bloom phase, are central in setting the scene for the spring bloom

Abundance of bacteria and virus during the METEOR cruise M87/1

In temperate, subpolar and polar marine systems, the classical perception that bacteria are carbon limited by end of winter and respond in activity and abundance to the production of new carbon during the diatom spring bloom and post bloom. Contrary to this view, we here document an strong increase in bacterial abundance and activity (latter measured by increasing high nuclei acid (HNA) to low nuclei acid (LNA) bacteria ratio) during the winter-spring transition, where phytoplankton smaller than 10 µm dominate. Further DNA-virus were enumerated and revealed the virus to bacteria ratio (VBR) to be decreasing during winter-spring transition, indicating that the virus did not increase in number accordingly to bacteria. During repeated visits to stations in the deep Icelandic and the Norwegian Basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the abundance of bacteria and the succession of HNA:LNA bacteria and VBR. Water samples were collected from CTD rosette .10 L Niskin bottles and fixed in glutaraldehyde (final conc. 5%), flash frozen in liquid Nitrogen and stored at -80°C until analysis