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Proteorhodopsins are globally abundant photoproteins found in bacteria in the photic zone of the ocean. Although their function as proton pumps with energy-yielding potential has been demonstrated, the ecological role of proteorhodopsins remains largely unexplored. Here, we report the presence and function of proteorhodopsin in a member of the widespread genus Vibrio, uncovered through whole-genome analysis. Phylogenetic analysis suggests that the Vibrio strain AND4 obtained proteorhodopsin through lateral gene transfer, which could have modified the ecology of this marine bacterium. We demonstrate an increased long-term survival of AND4 when starved in seawater exposed to light rather than held in darkness. Furthermore, mutational analysis provides the first direct evidence, to our knowledge, linking the proteorhodopsin gene and its biological function in marine bacteria. Thus, proteorhodopsin phototrophy confers a fitness advantage to marine bacteria, representing a novel mechanism for bacterioplankton to endure frequent periods of resource deprivation at the ocean's surface.

We investigated the role of N 2 -fixation by the colony-forming cyanobacterium,  Aphanizomenon  spp., for the plankton community and N-budget of the N-limited Baltic Sea during summer by using stable isotope tracers combined with novel secondary ion mass spectrometry, conventional mass spectrometry and nutrient analysis. When incubated with  15 N 2 ,  Aphanizomenon  spp. showed a strong  15 N-enrichment implying substantial  15 N 2 -fixation. Intriguingly,  Aphanizomenon  did not assimilate tracers of  15 NH 4 +  from the surrounding water. These findings are in line with model calculations that confirmed a negligible N-source by diffusion-limited NH 4 +  fluxes to  Aphanizomenon  colonies at low bulk concentrations (<250 nm) as compared with N 2 -fixation within colonies. No N 2 -fixation was detected in autotrophic microorganisms <5 μm, which relied on NH 4 + uptake from the surrounding water.  Aphanizomenon  released about 50% of its newly fixed N 2  as NH 4 + . However, NH 4 +  did not accumulate in the water but was transferred to heterotrophic and autotrophic microorganisms as well as to diatoms ( Chaetoceros  sp.) and copepods with a turnover time of ~5 h. We provide direct quantitative evidence that colony-forming Aphanizomenon  releases about half of its recently fixed N 2  as NH 4 + , which is transferred to the prokaryotic and eukaryotic plankton forming the basis of the food web in the plankton community. Transfer of newly fixed nitrogen to diatoms and copepods furthermore implies a fast export to shallow sediments via fast-sinking fecal pellets and aggregates. Hence, N 2 -fixing colony-forming cyanobacteria can have profound impact on ecosystem productivity and biogeochemical processes at shorter time scales (hours to days) than previously thought.

The ecosystems of coastal and enclosed seas are under increasing anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and the Black and Baltic Seas as well known examples. We use an ecosystem model (Ecopath with Ecosim, EwE) to show that reduced top-down control (seal predation) and increased bottom-up forcing (eutrophication) can largely explain the historical dynamics of the main fish stocks (cod, herring and sprat) in the Baltic Sea between 1900 and 1980. Based on these results and the historical fish stock development we identify two major ecological transitions. A shift from seal to cod domination was caused by a virtual elimination of marine mammals followed by a shift from an oligotrophic to a eutrophic state. A third shift from cod to clupeid domination in the late 1980s has previously been explained by overfishing of cod and climatic changes. We propose that the shift from an oligotrophic to a eutrophic state represents a true regime shift with a stabilizing mechanism for a hysteresis phenomenon. There are also mechanisms that could stabilize the shift from a cod to clupeid dominated ecosystem, but there are no indications that the ecosystem has been pushed that far yet. We argue that the shifts in the Baltic Sea are a consequence of human impacts, although variations in climate may have influenced their timing, magnitude and persistence.

Algae that bloom unnoticed Data obtained by RV Alexander von Humboldt in waters off the Namibian coast reveal that an area of about 7,000 km 2 of African coastal shelf, covered by sulphidic water, was detoxified by the action of bacteria: the biologically harmful sulphide was oxidized to non-toxic colloidal sulphur and sulphate. Eutrophication of coastal waters, often due to human activity, can lead algal blooms causing severe oxygen depletion and the episodic occurrence of hydrogen sulphide with disastrous consequences for the ecosystem. The discovery that sulphide can be completely consumed by bacteria in subsurface waters, and can thus be overlooked by remote sensing or monitoring of shallow coastal waters, suggests that sulphidic bottom waters on continental shelves may be more common than was thought, so could have an important but previously neglected effect on benthic communities. Eutrophication of coastal waters can cause sulphide blooms, which are toxic to marine life. It is shown that these blooms can be rapidly detoxified by sulphide-oxidizing bacteria. This finding suggests that sulphide blooms may occur more frequently than previously appreciated and that the responsible bacterial groups are important to protect coastal ecosystems. Coastal waters support ∼90 per cent of global fisheries and are therefore an important food reserve for our planet 1 . Eutrophication of these waters, due to human activity, leads to severe oxygen depletion and the episodic occurrence of hydrogen sulphide—toxic to multi-cellular life—with disastrous consequences for coastal ecosytems 2 , 3 , 4 , 5 . Here we show that an area of ∼7,000 km 2 of African shelf, covered by sulphidic water, was detoxified by blooming bacteria that oxidized the biologically harmful sulphide to environmentally harmless colloidal sulphur and sulphate. Combined chemical analyses, stoichiometric modelling, isotopic incubations, comparative 16S ribosomal RNA, functional gene sequence analyses and fluorescence in situ hybridization indicate that the detoxification proceeded by chemolithotrophic oxidation of sulphide with nitrate and was mainly catalysed by two discrete populations of γ- and ε-proteobacteria. Chemolithotrophic bacteria, accounting for ∼20 per cent of the bacterioplankton in sulphidic waters, created a buffer zone between the toxic sulphidic subsurface waters and the oxic surface waters, where fish and other nekton live. This is the first time that large-scale detoxification of sulphidic waters by chemolithotrophs has been observed in an open-ocean system. The data suggest that sulphide can be completely consumed by bacteria in the subsurface waters and, thus, can be overlooked by remote sensing or monitoring of shallow coastal waters. Consequently, sulphidic bottom waters on continental shelves may be more common than previously believed, and could therefore have an important but as yet neglected effect on benthic communities.

Microorganisms have been repeatedly discovered in environments that do not support their metabolic activity. Identifying and quantifying these misplaced organisms can reveal dispersal mechanisms that shape natural microbial diversity. Using endospore germination experiments, we estimated a stable supply of thermophilic bacteria into permanently cold Arctic marine sediment at a rate exceeding 10ɸ spores per square meter per year. These metabolically and phylogenetically diverse Firmicutes show no detectable activity at cold in situ temperatures but rapidly mineralize organic matter by hydrolysis, fermentation, and sulfate reduction upon induction at 50°C. The closest relatives to these bacteria come from warm subsurface petroleum reservoir and ocean crust ecosystems, suggesting that seabed fluid flow from these environments is delivering thermophiles to the cold ocean. These transport pathways may broadly influence microbial community composition in the marine environment.

To test the hypothesis whether high molecular weight dissolved organic matter (HMW-DOM) in a high latitude marginal sea is dominated by terrestrial derived matter, 10 stations were sampled along the salinity gradient of the central and northern Baltic Sea and were analyzed for concentrations of dissolved organic carbon as well as δ13C values of HMW-DOM. Different end-member-mixing models were applied to quantify the influence of terrestrial DOM and to test for conservative versus non-conservative behavior of the terrestrial DOM in the different Baltic Sea basins. The share of terrestrial DOM to the total HMW-DOM was calculated for each station, ranging from 43 to 83%. This shows the high influence of terrestrial DOM inputs for the Baltic Sea ecosystem. The data also suggest that terrestrial DOM reaching the open Baltic Sea is not subject to substantial removal anymore. However compared to riverine DOM concentrations, our results indicate that substantial amounts of HMW-DOM (> 50%) seem to be removed near the coastline during estuarine mixing. A budget approach yielded residence times for terrestrial DOM of 2.8, 3.0, and 4.5 yr for the Bothnian Bay, the Bothnian Sea and the Baltic Proper.

Seabirds concentrate nutrients from large marine areas on their nesting islands. The high nutrient load may cause runoff into surrounding waters and affect marine communities in similar ways to those reported from marine fertilization experiments. In order to test if cormorant colonies affect algae and invertebrates in surrounding coastal waters, we collectedFucus vesiculosusfronds, its epiphytic algae, and associated invertebrate fauna near abandoned and active cormorant nesting islands as well as reference islands without nesting cormorants in the Stockholm archipelago in the northern Baltic Sea, Sweden. First, we showed, with δ15N analyses, that ornithogenic nitrogen provided a significant nitrogen source for algae and invertebrate consumers near islands with high nest density. Second, the nitrogen and phosphorus content of algae near active cormorant islands with high nest density was elevated, and epiphytic algae increased relative toF. vesiculosus. Third, 3 of 5 invertebrate taxa (Jaera albifrons, Gammarusspp., and Chironomidae) showed increased biomasses near islands with high nest density; but, contrary to former fertilization studies, onlyJ. albifronsincreased in abundance compared to reference islands. We conclude that runoff from seabird colonies has a profound effect on primary producers and some consumers in the surrounding water, but only if the colonies exceed a certain nest density. Thus, seabirds not only affect marine communities via top-town forces as commonly assumed, but also via bottom-up forces by concentrating nutrients around their nesting islands. Consequently, seabird islands can be seen as natural fertilization experiments and give important insights to the effects of eutrophication of marine systems.

This study explores: (1) whether the abundance of macroinvertebrates differs between macrophytes differing in both morphological complexity and tolerance to nutrient enrichment; (2) whether the distribution of invertebrates between macrophytes is due to active habitat choice; and (3) whether invertebrates prefer structurally complex to simple macrophytes. Macroinvertebrate abundance was compared between two common soft-bottom plants of the Baltic Sea that are tolerant to eutrophication, Myriophyllum spicatum and Potamogeton pectinatus, and one common plant that is sensitive to eutrophication, Chara baltica. Both field sampling and habitat choice experiments were conducted. We recorded higher total macroinvertebrate abundance on the structurally complex M. spicatum than on the more simply structured P. pectinatus and C. baltica, but found no difference in macroinvertebrate abundance between P. pectinatus and C. baltica. In accordance with the field results, our experiment indicated that the crustacean Gammarus oceanicus actively chose M. spicatum over the other macrophytes. Besides, we found that G. oceanicus actively preferred complex to simply structured artificial plants, indicating that the animal distribution was at least partly driven by differences in morphological complexity between plant species. In contrast, the gastropod Theodoxus fluviatilis did not make an active habitat choice between the plants. Our findings suggest that human-induced changes in vegetation composition can affect the faunal community. Increased abundance of structurally complex macrophytes, for example, M. spicatum, can result in increased abundance of macroinvertebrates, particularly mobile arthropods that may actively choose a more structurally complex macrophyte.

This study investigates the effect of filtrates from an allelopathic dinoflagellate, Alexandrium tamarense, on four microbial food webs that have been manipulated experimentally from natural seawater by modifying the availability of resources in the form of dissolved organic carbon with additions of peptone, and by altering the grazing pressure with size fractionation. Bacterial production was generally not affected by allelochemicals, but bacteria showed higher net growth in all food webs when allelochemicals were added, whereas heterotrophic nanoflagellates > 7 μm and ciliates were constrained in all food webs. Allelochemicals had the largest negative effects on microbial communities with low grazing pressure. In food webs with high grazing pressure and additional resources, phytoplankton and small nanoflagellates were positively affected by the addition of allelochemicals, suggesting that those were interfering with trophic interactions in the microbial communities. By the lysis of organisms sensitive towards allelochemicals, resources are made available and grazing pressure on certain microorganisms is reduced. However, the intensity of these interactions is modulated by both the availability of resources and the biomass of grazers in the initial food web.

In the relatively simple Baltic Sea ecosystem, zooplankton-feeding spratSprattus sprattusis a major food source for breeding seabirds and piscivorous fish, and an important resource for commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing and recruitment failure of codGadus morhua, which is the main fish predator of sprat, have affected natural-history patterns in a piscivorous seabird, the common guillemotUria aalge, in a complex way. As the sprat stock increased, leading to lower energy content of fish, common guillemot chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the effects differ depending on the management strategy.