Dissolved extracellular polymeric substances (dEPS) dynamics and bacterial growth during sea ice formation in an ice tank study

Extracellular polymeric substances (EPS) are known to help microorganisms to survive under extreme conditions in sea ice. High concentrations of EPS are reported in sea ice from both poles; however, production and dynamics of EPS during sea ice formation have been little studied to date. This investigation followed the production and partitioning of existing and newly formed dissolved organic matter (DOM) including dissolved carbohydrates (dCHO), dissolved uronic acids (dUA) and dissolved EPS (dEPS), along with bacterial abundances during early stages of ice formation. Sea ice was formed from North Sea water with (A) ambient DOM (NSW) and (B) with additional algal-derived DOM (ADOM) in a 6d experiment in replicated mesocosms. In ADOM seawater, total bacterial numbers (TBN) increased throughout the experiment, whereas bacterial growth occurred for 5d only in the NSW seawater. TBN progressively decreased within developing sea ice but with a 2-fold greater decline in NSW compared to ADOM ice. There were significant increases in the concentrations of dCHO in ice. Percentage contribution of dEPS was highest (63%) in the colder, uppermost parts in ADOM ice suggesting the development of a cold-adapted community, producing dEPS possibly for cryo-protection and/or protection from high salinity brines. We conclude that in the early stages of ice formation, allochthonous organic matter was incorporated from parent seawater into sea ice and that once ice formation had established, there were significant changes in the concentrations and composition of dissolved organic carbon pool, resulting mainly from the production of autochthonous DOM by the bacteria.     

ALGAL ASSEMBLAGES IN ANTARCTIC PACK ICE AND IN ICE-EDGE PLANKTON1

A significant amount of the primary production in the Southern Ocean and other ice-covered oceans takes place in localized ice edge plankton blooms. The dynamics of these blooms appear to be closely related to seasonal melting of sea ice. Algal cells released from the ice are a possible source of ice edge planktonic assemblages, but evidence for this “seeding” has been equivocal. We compared algal assemblages in ice and water in the Weddell Sea during the austral spring of 1983 at a receding ice edge with a well-developed ice edge bloom. The high degree of similarity between ice and water column assemblages, the spatial and temporal patterns in the distribution and abundances of species, and preliminary evidence for the viability and growth of ice-associated species provide evidence for seeding from sea ice of some species in Antarctica.     

Sea ice bacterial growth rate, growth efficiency and preference for inorganic nitrogen sources in the Baltic Sea

Seasonal Baltic Sea ice is structurally similar to polar sea ice and provides habitats for diverse ice organism assemblages that are integral to the biogeochemistry and ecology of the sea during winter. Temperature and inorganic nitrogen sources have been suggested to control bacterial growth, with increasing dependence on ammonium at low temperatures. To study the bacterial growth and preference for the nitrogen source, we conducted experiments at 0 and 4°C, using ammonium and nitrate as nitrogen sources at two coastal fast-ice stations in the Gulf of Finland and in the Gulf of Bothnia during three successive winters. The two study sites differ markedly in relation to the allochthonous dissolved organic matter supply from the catchment area. High levels of bacterial growth were recorded at both study sites, with community generation times of 15–37 h. The measured bacterial growth efficiencies of 20–58% suggest that the Baltic sea ice brines provide a rich medium for bacterial growth and efficient functioning of bacteria-based food webs. Our experiments with sea ice samples showed a preference for ammonium at both temperatures and high potential growth in both types of nitrogen supplies. No major differences in phosphorus depletion rates were found at the two temperatures, but rates were always highest when ammonium was added to the experiments. These experiments point out that ice maturity, presumably through changes in bacterial community structure, impacts nitrogen processes and that these processes are pronounced prior to melting of the ice.     

Microbial loop malfunctioning in the annual sea ice at Terra Nova Bay (Antarctica)

We investigated organic carbon quantity and biochemical composition, prokaryotic abundance, biomass and carbon production in the annual and platelet sea ice of Terra Nova Bay (Antarctica), as well as the downward fluxes of organic matter released by melting ice during early spring. Huge amounts of biopolymeric C accumulated in the bottom layer of the ice column concomitantly with the early spring increase in sympagic algal biomass. Such organic material, mostly accounted for by autotrophic biomass, was characterised by a high food quality and was rapidly exported to the sea bottom during sea ice melting. Prokaryote abundance (up to 1.3 × 10⁹ cells L⁻¹) and extracellular enzymatic activities (up to 24.3 μM h⁻¹ for amino-peptidase activity) were extremely high, indicating high rates of organic C degradation in the bottom sea ice. Despite this, prokaryote C production values were very low (range 5–30 ng C L⁻¹ h⁻¹), suggesting that most of the degraded organic C was not channelled into prokaryote biomass. In the platelet ice, we found similar organic C concentrations, prokaryote abundance and biomass values and even higher extracellular enzymatic activities, but values of prokaryote C production (range 800–4,200 ng C L⁻¹ h⁻¹) were up to three orders of magnitude higher than in the intact bottom sea ice. Additional field and laboratory experiments revealed that the dissolved organic material derived from algae accumulating in the bottom sea ice significantly reduced prokaryote C production, suggesting the presence of a potential allopathic control of sympagic algae on prokaryote growth. This article belongs to a special topic: Five articles on Sea-ice communities in Terra Nova Bay (Ross Sea), coordinated by L. Guglielmo and V. Saggiomo, appear in this issue of Polar Biology. The studies were conducted in the frame of the National Program of Research in Antarctica (PNRA) of Italy.     

闽江口-平潭海域有机解磷菌多样性及群落特征

有机磷的微生物矿化是海洋磷循环的重要环节,开展有机解磷菌研究有助于揭示富营养化海域有机磷矿化的微生物驱动机制。以phoX为标志基因,采用Illumina高通量测序技术分析了闽江口-平潭海域2019年4月(春季)及7月(夏季)有机解磷菌的多样性及群落特征。结果表明: 表层海水样品中有机解磷菌的Shannon指数介于3.21～7.91,各站位多样性春季均大于夏季;沉积物样品中有机解磷菌的Shannon指数介于2.04～8.70,各站位多样性夏季大于春季。春季各站位表层海水有机解磷菌的Shannon指数均高于沉积物,而夏季则相反。表层海水中检测到9个门类的有机解磷菌,主要为变形菌门和蓝细菌门;沉积物中则检测到12个门类,主要为变形菌门和拟杆菌门。在属水平上,有机解磷菌的群落组成呈现时空变异特征。表层海水样品中,春季以雷辛格氏菌属、褐杆菌属、深海球菌属和假单胞菌属等属为主,夏季则以聚球藻属、Halioglobus属、玫瑰变色菌属、褐杆菌属、亚硫杆菌属和生丝单胞菌属等属为主。在沉积物样品中,春季主要菌属包括雷辛格氏菌属、褐杆菌属、弧菌属和亚硫杆菌属;夏季主要菌属包括固氮螺丝菌属、氨基杆菌属、Sulfurifustis属、伯克氏菌属和Thiohalobacter属。此外,在表层海水和沉积物样品中均检测到大量未分类的有机解磷菌。冗余分析显示,溶解氧、水温、pH、可溶性无机氮、亚硝态氮、硝态氮等对闽江口-平潭海域表层海水有机解磷菌群落分布影响较大。表层海水和沉积物中存在的丰富有机解磷菌可能在该海域的磷循环中发挥重要作用。     

Ice algal assemblages and vertical export of organic matter from sea ice in the Barents Sea and Nansen Basin (Arctic Ocean)

Algal communities and export of organic matter from sea ice were studied in the offshore marginal ice zone (MIZ) of the northern Barents Sea and Nansen Basin of the Arctic Ocean north of Svalbard by means of ice cores and short-term deployed sediment traps. The observations cover a total of ten stations within the drifting pack ice, visited over a period of 3 years during the period of ice melt in May and July. Maximum flux of particulate organic carbon and chlorophyll a from the ice at 1 m depth (1,537 mg C m⁻² per day and 20 mg Chl a m⁻² per day) exceeded the flux at 30 m by a factor of 2 during spring, a pattern that was reversed later in the season. Although diatoms dominated the ice-associated algal biomass, flagellates at times revealed similarly high biomass and typically dominated the exported algal carbon. Importance of flagellates to the vertical flux increased as melting progressed, whereas diatoms made the highest contribution during the early melting stage. High export of ice-derived organic matter and phytoplankton took place simultaneously in the offshore MIZ, likely as a consequence of ice drift dynamics and the mosaic structure of ice-covered and open water characteristic of this region.     

The biota of Antarctic pack ice in the Weddell sea and Antarctic Peninsula regions

Summary Pack ice surrounding Antarctica supports rich and varied populations of microbial organisms. As part of the Antarctic Marine Ecosystem Research in the Ice Edge Zone (AMERIEZ) studies, we have examined this community during the late spring, autumn, and winter. Although organisms are found throughout the ice, the richest concentrations often occur in the surface layer. The ice flora consists of diatoms and flagellates. Chrysophyte cysts (archaeomonads) of unknown affinity and dinoflagellate cysts are abundant and may serve as overwintering stages in ice. The ice fauna includes a variety of heterotrophic flagellates, ciliates, and micrometazoa. The abundance of heterotrophs indicates an active food web within the ice community. Ice may serve as a temporary habitat or refuge for many of the microbial forms and some of these appear to provide an inoculum for planktonic populations when ice melts. Larger consumers, such as copepods and the Antarctic krill, Euphausia superba are often found on the underside of ice floes and within weathered floes. The importance of the ice biota as a food resource for these pelagic consumers is unknown.     

Effects of Antarctic sea ice biota on seeding as studied in aquarium experiments

Summary The potential seeding impact of sea ice microbial communities was studied during late austral winter early spring 1988 in the Weddell Sea, Antarctica. Experiments were performed in seawater aquariums with natural seawater and seawater enriched with crushed ice. Algal, protozoan and bacterial cell numbers were followed, as well as nutrients and DOC levels. The results showed a potential seeding effect of sea ice communities to the water column. However, the type of ice communities differed greatly from each other and the effect of such seeding will be patchy. In our experiments seeding of seawater by ice rich in algae, flagellates and/or particulate organic carbon lead to the development of communities dominated either by diatoms or bacteria.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Aggregation of algae released from melting sea ice: implications for seeding and sedimentation

Summary Factors influencing the fate of ice algae released from melting sea ice were studied during a R V Polarstern cruise (EPOS Leg 2) to the northwestern Weddell Sea. The large-scale phytoplankton distribution patterns across the receding ice edge and small-scale profiling of the water column adjacent to melting ice floes indicated marked patchiness on both scales. The contribution of typical ice algae to the phytoplankton was not significant. In experiments simulating the conditions during sea ice melting, ice algae revealed a strong propensity to form aggregates. Differences in the aggregation potential were found for algal assemblages collected from the ice interior and the infiltration layer. Although all algal species collected from the ice were also found in aggregates, the species composition of dispersed and aggregated algae differed significantly. Aggregates were of a characteristic structure consisting of monospecific microaggregates which are likely to have formed in the minute brine pockets and channels within the ice. Sinking rates of aggregates were three orders of magnitude higher than those of dispersed ice algae. These observations, combined with the negligible seeding effect of ice algae found during this study, suggest that ice algae released from the melting sea ice are subject to rapid sedimentation. High grazing pressure at the ice edge of the investigation area is another factor eliminating ice algae released during melting.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Nutrient availability as major driver of phytoplankton-derived dissolved organic matter transformation in coastal environment

Incubation experiments were performed to examine the processing of fresh autochthonous dissolved organic matter (DOM) produced by coastal plankton communities in spring and autumn. The major driver of observed DOM dynamics was the seasonally variable inorganic nutrient status and characteristics of the initial bulk DOM, whereas the characteristics of the phytoplankton community seemed to have a minor role. Net accumulation of dissolved organic carbon (DOC) during the 18-days experiments was 3.4 and 9.2 µmol l^?1 d^?1 in P-limited spring and N-limited autumn, respectively. Bacterial bioassays revealed that the phytoplankton-derived DOC had surprisingly low proportions of biologically labile DOC, 12.6% (spring) and 17.5% (autumn). The optical characteristics of the DOM changed throughout the experiments, demonstrating continuous heterotrophic processing of the DOM pool. However, these temporal changes in optical characteristics of the DOM pool were not the same between seasons, indicating seasonally variable environmental drivers. Nitrogen and phosphorus availability is likely the main driver of these seasonal differences, affecting both phytoplankton extracellular release of DOM and its heterotrophic degradation by bacteria. These findings underline the complexity of the DOM production and consumption by the natural planktonic community, and show the importance of the prevailing environmental conditions regulating the DOM pathways.     

海水富营养化对海洋细菌影响的研究进展

综述了海水富营养化对海洋细菌影响的研究进展。随着海水富营养化程度的增加,海洋细菌数量或生物量增加;反硝化细菌、大肠菌群尤其是厌氧性的硫酸盐还原菌和产甲烷菌等典型细菌生理群数量增加;浮游细菌群落结构随富营养化递增趋于简单,物种多样性降低;富营养化也明显导致细菌群落正常功能活性的紊乱。海水富营养化对细菌群落的结构和功能有着深远的影响。     

Persistence of bacterial and archaeal communities in sea ice through an Arctic winter

The structure of bacterial communities in first‐year spring and summer sea ice differs from that in source seawaters, suggesting selection during ice formation in autumn or taxon‐specific mortality in the ice during winter. We tested these hypotheses by weekly sampling (January–March 2004) of first‐year winter sea ice (Franklin Bay, Western Arctic) that experienced temperatures from ?9°C to ?26°C, generating community fingerprints and clone libraries for Bacteria and Archaea. Despite severe conditions and significant decreases in microbial abundance, no significant changes in richness or community structure were detected in the ice. Communities of Bacteria and Archaea in the ice, as in under‐ice seawater, were dominated by SAR11 clade Alphaproteobacteria and Marine Group I Crenarchaeota, neither of which is known from later season sea ice. The bacterial ice library contained clones of Gammaproteobacteria from oligotrophic seawater clades (e.g. OM60, OM182) but no clones from gammaproteobacterial genera commonly detected in later season sea ice by similar methods (e.g. Colwellia, Psychrobacter). The only common sea ice bacterial genus detected in winter ice was Polaribacter. Overall, selection during ice formation and mortality during winter appear to play minor roles in the process of microbial succession that leads to distinctive spring and summer sea ice communities.     

Aminopeptidase Activity in Marine Chroococcoid Cyanobacteria

Synechococci are important primary producers in the ocean and can also utilize some components of the dissolved organic matter (DOM). The readily utilizable DOM in seawater is mainly polymeric (e.g., protein, polysaccharide) or phosphorylated and requires hydrolysis prior to uptake. We examined whether synechococci express ectoenzymes to hydrolyze DOM components and considered the possible significance of ectohydrolases for Synechococcus ecology and organic matter cycling in the sea. Five strains of non-nitrogen-fixing synechococci in axenic cultures were tested for enzyme activities with fluorogenic substrates. All strains show ectocellular aminopeptidase activity, but other enzymes were undetectable. The aminopeptidase level was in the range determined for five marine heterotrophic bacterial isolates tested for comparison. Aminopeptidase was not secreted into the medium; the majority (74%; tested in WH 7803) was cell surface bound, and a small fraction was periplasmic. The periplasmic activity was not released by cold osmotic shock of WH 7803. Phenylmethylsulfonyl fluoride and EDTA, inhibitors of serine and metalloproteases, strongly or completely inhibited WH 7803 aminopeptidase. The enzyme seemed constitutive; per-cell activity did not change during incubations in unenriched seawater, bovine serum albumin, or nitrate-replete mineral medium. In natural planktonic assemblages in the Southern California Bight, aminopeptidase activity was correlated with Synechococcus abundance as well as the abundance of other bacteria. Ectocellular aminopeptidase may be common in marine synechococci and play roles in their nitrogen nutrition, particularly in low-nitrate and low-light environments. Since synechococci are much less abundant than heterotrophic bacteria in seawater, the impact of Synechococcus aminopeptidase on proteolysis in the sea is likely to be episodic and restricted to specialized microenvironments.     

HETEROTROPHY AND PHOTOHETEROTROPHY BY ANTARCTIC MICROALGAE: LIGHT-DEPENDENT INCORPORATION OF AMINO ACIDS AND GLUCOSE 1

Diatoms isolated from the benthic, planktonic and sea ice microbial communities in Mc Murdo Sound, Antarctica assimilated ambient concentrations of dissolved amino acids and glucose in both the light and dark. Uptake of amino acids but not glucose was influenced by the iucubation irradiance and amino acid uptake rates were up to 250 times greater than those of glucose. Amino acids were incorporated into proteins and other complex polymers and the rates of assimilation and patterns of polymer synthesis were similar to those of the light-saturated photosynthetic incorporation of inorganic carbon. This suggests that these diatoms can use exogenous amino acids to synthesize the essential macromolecules for heterotrophic growth. The assimilation of dissolved organic substrates could supplement light-limited growth during the austral spring and summer as well as potentially support the heterotrophic growth of these diatoms throughout the aphotic polar winter.     

环梅山岛海域春季浮游古菌群落空间分布特征研究

【目的】海洋浮游古菌是生物地球化学循环的关键驱动者,但其在近岸海域的水平空间分布特征还未被充分了解。本研究以与陆地紧密相连的环梅山岛海域为例研究浮游古菌在海陆过渡带的水平分布模式。【方法】利用16S rRNA基因扩增子测序,以期从优势类群分布、群落组成变化和物种共现模式3个层面揭示梅山湾潟湖区和临近海域春季浮游古菌的空间分布特征。【结果】该海域浮游古菌在原核生物群落中的相对丰度为0.6%–26.5%,向海侧古菌丰度显著高于潟湖区。浮游古菌群落由奇古菌门Marine Group(MG)I和广古菌门MGII主导,MGI的物种组成较为单一,而MGII的系统发育多样性较高。古菌群落的空间分布受同质扩散、环境选择和非主导过程(包括生态漂变)的共同塑造,其中环境选择主要由悬浮颗粒物、硝酸盐、溶解氧、水温和铵盐驱动。通过网络分析发现MGI与红杆菌科细菌呈广泛的负相关,MGII则普遍与SAR11、SAR116和SAR86等异养细菌类群呈正相关。【结论】本研究初步揭示了环梅山岛海域春季浮游古菌群落的空间分布特征及其调控因子,拓展了对古菌在海陆过渡带分布规律的认识。     

Bacterioplankton community shifts in an arctic lake correlate with seasonal changes in organic matter source

Seasonal shifts in bacterioplankton community composition in Toolik Lake, a tundra lake on the North Slope of Alaska, were related to shifts in the source (terrestrial versus phytoplankton) and lability of dissolved organic matter (DOM). A shift in community composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes, occurred at 4 degrees C in near-surface waters beneath seasonal ice and snow cover in spring. This shift was associated with an annual peak in bacterial productivity ([(14)C]leucine incorporation) driven by the large influx of labile terrestrial DOM associated with snow meltwater. A second shift occurred after the flux of terrestrial DOM had ended in early summer as ice left the lake and as the phytoplankton community developed. Bacterioplankton communities were composed of persistent populations present throughout the year and transient populations that appeared and disappeared. Most of the transient populations could be divided into those that were advected into the lake with terrestrial DOM in spring and those that grew up from low concentrations during the development of the phytoplankton community in early summer. Sequencing of DNA in DGGE bands demonstrated that most bands represented single ribotypes and that matching bands from different samples represented identical ribotypes. Bacteria were identified as members of globally distributed freshwater phylogenetic clusters within the alpha- and beta-Proteobacteria, the Cytophaga-Flavobacteria-Bacteroides group, and the ACTINOBACTERIA:     

Terrestrial dissolved organic matter inflow drives temporal dynamics of the bacterial community of a subarctic estuary (northern Baltic Sea)

Climate change is projected to cause increased inflow of terrestrial dissolved organic matter to coastal areas in northerly regions. Estuarine bacterial community will thereby receive larger loads of organic matter and inorganic nutrients available for microbial metabolism. The composition of the bacterial community and its ecological functions may thus be affected. We studied the responses of bacterial community to inflow of terrestrial dissolved organic matter in a subarctic estuary in the northern Baltic Sea, using a 16S rRNA gene metabarcoding approach. Betaproteobacteria dominated during the spring river flush, constituting ~ 60% of the bacterial community. Bacterial diversity increased as the runoff decreased during summer, when Verrucomicrobia, Betaproteobacteria, Bacteroidetes, Gammaproteobacteria and Planctomycetes dominated the community. Network analysis revealed that a larger number of associations between bacterial populations occurred during the summer than in spring. Betaproteobacteria and Bacteroidetes populations appeared to display similar correlations to environmental factors. In spring, freshly discharged organic matter favoured specialists, while in summer a mix of autochthonous and terrestrial organic matter promoted the development of generalists. Our study indicates that increased inflows of terrestrial organic matter-loaded freshwater to coastal areas would promote specialist bacteria, which in turn might enhance the transformation of terrestrial organic matter in estuarine environments.     

Origin, biochemical composition and vertical flux of particulate organic matter under the pack ice in Terra Nova Bay (Ross Sea, Antarctica) during late summer 1995

Water samples and particulate materials settling under the pack ice were collected in an ice-covered area near the Terra Nova Bay Italian Station during late summer 1995, in order to study short-term changes in the biochemical composition of particulate organic matter. At the end of the study period the phytoplankton biomass increase (up to >3.0 μg chlorophyll-a l⁻¹) was probably related to the intrusion under the pack ice of chlorophylls-enriched surface waters coming from the near ice-free area. Such increase was associated also with a notable increase in particulate organic matter concentrations, as well as in particulate organic matter vertical fluxes (up to >100 mg C m⁻² day⁻¹). Proteins were the most abundant biochemical class of particulate organic matter (on average about 49%), followed by lipids (29%) and carbohydrates (22%). By contrast, organic matter collected in the sediment trap was characterized by the dominance of lipids (about 55% of the total biopolymeric carbon flux) over carbohydrates (28%) and proteins (17%). The hydrolizable particulate biopolymeric carbon accounted for about 23% of total biopolymeric carbon. This value was about one-half of that found in ice-free waters, suggesting that the suspended particulate organic material under the pack ice was less digestible than in ice-free waters or was already partially digested. Despite this, and the decay of labile organic compounds in the sediment trap during the deployment, material settling towards the sea bottom under the pack ice in Terra Nova Bay, owing to its high lipid content, might represent an important high-quality food source for benthic consumers. Finally, assuming as possible the intrusion under sea ice of primary organic matter-enriched waters, we hypothesize the occurrence of a “fertilization” effect deriving from ice-melting areas towards under-ice waters, supplying the latter with an additional rate of primary organic matter. Accepted: 18 February 1999     

THE BOTTOM-ICE MICROALGAL COMMUNITY FROM ANNUAL ICE IN THE INSHORE WATERS OF EAST ANTARCTICA1

The structure, productivity and heterotrophic potential of an extensive microalgal community growing on the underside of sea ice near the Australian Antarctic Station of Casey, are described. Underwater observations made near the Australian Antarctic stations of Davis and Mawson are also reported. This community develops during September, is largely suspended from the bottom surface of annual sea ice and often extends into the underlying water column as conspicuous strands up to 15 cm long. The algal community structure in the strands is dominated by an unidentified tube diatom belonging to the Amphipleura/Berkeleya group and chains of a species of Entomoneis cf. Amphiprora paludosa var. hyperborea (Grunow) Cleve. Unlike previously described bottom ice environments, a brash ice layer under the hard sea ice is absent. Living cells, predominantly Nitzschia frigida Grunow, also occur in microbrine channels in the bottom 3 cm of the ice. Maximal primary production rates of 81 μg C · L^-1· h^-1 occurred during November, then began declining near the end of December. Minimal rates (2.8 μg C · L^-1· h^-1) were reached in mid-January and coincided with changes in the physical structure of the sea ice and in the stability of the water column. An abundant epibacterial community associated with the microalgal strands assimilated ³H-labelled amino acids suggesting significant heterotrophic recycling of dissolved organic matter. Turnover times of assimilated amino acids in the bottom ice community averaged 55 h during November while negligible turnover of these substrates occurred in the water column 1.5 m below the ice. These bottom ice communities have higher primary productivity than typical brash ice communities; they are also accessible to marine herbivores and so may be more important to the Antarctic marine food chain than previously supposed.     

Size‐spectrum based differential response of phytoplankton to nutrient and iron‐organic matter combinations in microcosm experiments in a Chilean Patagonian Fjord

The Patagonian fjords have been recognized as a major region of relatively high primary productivity systems during spring–summer bloom periods, where iron‐organic matter forms may be essential complexes involved in key growth processes connected to the carbon and nitrogen cycles. We used two dissolved organic matter (DOM) types, marine polysaccharide and siderophore, as a model to understand how they affect the bioavailability of Fe to phytoplankton and bacteria and to assess their ecological role in fjord systems. A 10‐day microcosm study was performed in the Comau Fjord during summer conditions (March 2012). Pico‐, nano‐, and microphytoplankton abundance, total chlorophyll‐a and bacteria abundance, and bacterial secondary production estimates were analyzed in five treatments: (i) control (no additions), (ii) only nutrients (NUT: PO₄, NO₃, Si), (iii) nutrients + Fe(II), (iv) polysaccharide (natural diatoms extracted: 1–3 beta Glucan), and (v) Hexandentate Desferroxiamine B (DFB, siderophore). Our results showed that while DFB reduced Fe bioavailability for almost all phytoplankton assemblages in the fjord, polysaccharide did not have effects on the iron bioavailability. At Nutrients + Fe and Polysaccharide treatments, chlorophyll‐a concentration abruptly increased from 0.9 to 20 mg m^?3 during the first 4–6 days of the experimental period. Remarkably, at the Nutrients + Fe treatment, the development of the bloom was accompanied by markedly high abundances of Synechococcus, picoeukaryotes, and autotrophic nanoflagellates within the first 4 days of the experiment. Our study indicated that small plankton (phytoplankton <20 μm and bacteria) were the first to respond to dissolved Nutrients + Fe compared to large sized micro‐phytoplankton cells (>20 μm). This could be at least partially attributed to biological utilization of Fe (2 to 3 nM) by <20 μm phytoplankton and bacteria through the interaction with organic ligands released by bacteria that eventually could increase solubility of the Fe dissolved fraction thus having a positive effect on the small‐sized phytoplankton community.