Trace metals are required as cofactors in metalloproteins that are essential in microbial metabolism and growth. The microbial requirements of diverse metals and the capabilities of prokaryotic taxa to acquire these metals remain poorly understood. We present here results from metagenomic observations over an entire productive season in the region off Kerguelen Island (Indian Sector of the Southern Ocean). We observed seasonal...

Biogeochemical cycles constitute Earth’s life support system and distinguish our planet from others in this solar system. Microorganisms are the primary drivers of these cycles. Understanding the controls on marine microbial dynamics and how microbes will respond to environmental change is essential for building and assessing model-based forecasts and generating robust projections of climate change impacts on ocean productivit...

Iron (Fe) governs the cycling of organic carbon in large parts of the Southern Ocean. The strategies of diverse microbes to acquire the different chemical forms of Fe under seasonally changing organic carbon regimes remain, however, poorly understood. Here, we report high‐resolution seasonal metagenomic observations from the region off Kerguelen Island (Indian Sector of the Southern Ocean) where natural Fe‐fertilization induce...

As marine microorganisms and environmental conditions coevolved over geological timescales, metals have been incorporated into all essential metabolic processes. In the modern ocean, metals are present from trace amounts limiting microbial growth to toxic concentrations. Dissolved trace metals are a major bioavailable reservoir. However, the acquisition of metals from marine particles remains largely unexplored. Here, we combi...

The Southern Ocean hosts a large diversity of diatoms that play a major role in carbon fluxes. How the seasonal dynamics in the abundance of specific taxa in surface waters are linked to their contribution to carbon export remains, however, poorly understood. We present here synchronized observations from autonomous samplers deployed in the mixed layer (42 m) and at depth (300 m) during an entire productive season (October 201...

Summary Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains we...

Spatial and seasonal dynamics of microbial loop fluxes were investigated in contrasting productivity regimes in the Indian sector of the Southern Ocean. Observations carried out in late summer (February–March 2018; project MOBYDICK) revealed higher microbial biomasses and fluxes in the naturally iron‐fertilized surface waters of Kerguelen island in comparison to surrounding off‐plateau waters. Differences were most pronounced...

Summary The Southern Ocean remains one of the least explored marine environments. The investigation of temporal microbial dynamics has thus far been hampered by the limited access to this remote ocean. We present here high‐resolution seasonal observations of the prokaryotic community composition during phytoplankton blooms induced by natural iron fertilization. A total of 18 seawater samples were collected by a moored remote a...

Summary Iron (Fe) is a limiting nutrient in large regions of the ocean, but the strategies of prokaryotes to cope with this micronutrient are poorly known. Using a gene‐specific approach from metatranscriptomics data, we investigated seven Fe‐related metabolic pathways in microbial communities from high nutrient low chlorophyll and naturally Fe‐fertilized waters in the Southern Ocean. We observed major differences in the contr...

Summary The interplay among microorganisms profoundly impacts biogeochemical cycles in the ocean. Culture‐based work has illustrated the diversity of diatom–prokaryote interactions, but the question of whether these associations can affect the spatial distribution of microbial communities is open. Here, we investigated the relationship between assemblages of diatoms and of heterotrophic prokaryotes in surface waters of the Ind...

Summary To better understand the functional responses in prokaryotes to dissolved organic matter ( DOM ), we compared the transcriptional pattern of natural prokaryotic communities grown in continuous cultures on seawater amended with phytoplankton‐derived DOM . Metatranscriptomic reads were classified taxonomically (by genomic binning) and functionally (using K yoto Encyclopedia of G enes and G enomes), and the relative gene...

We investigated the metabolic response to iron (Fe) limitation of two bacterial strains of Alteromonas macleodii , isolated from a coastal and an oceanic marine environment. Bacteria were grown under Fe‐limited and Fe‐replete conditions, and comparative analyses of cellular properties and total proteomes were conducted. Respiration was reduced by a factor of two in both strains, but the growth rate of the oceanic strain was le...

We investigated the contribution of distinct bacterial groups to bulk abundance and leucine incorporation during a spring phytoplankton bloom induced by natural iron fertilization in the Southern Ocean (Kerguelen Ocean and Plateau Compared Study, January–February 2005). Oligonucleotide probes were designed to target five operational taxonomic units (OTUs) at a narrow phylogenetic level (≥ 99% identity of the 16S ribosomal ribo...

Summary One of the first comparisons of a natural iron fertilized bloom with a high‐nutrient low‐chlorophyll (HNLC) site was undertaken during the Kerguelen ocean and plateau compared study (KEOPS) cruise. To understand better the bacteria–phytoplankton relationship in the context of natural iron fertilization, bacterial diversity and activity was investigated in the bloom and in the adjacent HNLC region by 16S rDNA clone libr...

The photo‐ and bioreactive components of dissolved organic matter (DOM) from three different environments were determined during long‐term decomposition experiments. Terrigenous DOM was collected from a black‐water system, plankton DOM was harvested from phytoplankton cultures, and lake water served as a DOM source with both terrigenous and plankton components. Photomineralization accounted for the removal of 46 and 7% of terr...

Diurnal dynamics of dissolved organic matter (DOM) fluorescence and hydrogen peroxide (H 2 O 2 ) concentrations were followed in the upper 100 m of the water column at five stations across the subtropical Atlantic Ocean in July and August 1996. The 10% levels of surface solar radiation for the ultraviolet (UV) B range (at 305‐ and 320‐nm wavelengths) were at 16 and 23 m in depth and for the UVA range (at 340‐ and 380‐nm wavele...

The effect of surface solar radiation on the optical properties and the biological availability of dissolved organic carbon (DOC) and its humic and nonhumic component was investigated in the northern Adriatic Sea (44°52'N 13°52'E) and the coastal North Sea (53°00'N 4°45'E). In the northern Adriatic Sea, humic substances of primarily autochthonous origin contributed, on average, 15% to the bulk DOC, whereas in the coastal North...

The effect of ultraviolet radiation on the bioavailability of dissolved organic matter (DOM) to marine bacterioplankton was investigated in the Mediterranean Sea (Strait of Gibraltar, South and North Aegean Sea) and in the Caribbean Sea off Curacao. Surface‐water samples (collected between 1 and 85 m in depth) exposed to solar radiation did not show a distinct pattern in subsequent bacterial growth. However, samples collected...