Isolation of an aerobic sulfur oxidizer from the SUP05/Arctic96BD-19 clade

Bacteria from the uncultured SUP05/Arctic96BD-19 clade of gamma proteobacterial sulfur oxidizers (GSOs) have the genetic potential to oxidize reduced sulfur and fix carbon in the tissues of clams and mussels, in oxygen minimum zones and throughout the deep ocean (>200 m). Here, we report isolation of the first cultured representative from this GSO clade. Closely related cultures were obtained from surface waters in Puget Sound and from the deep chlorophyll maximum in the North Pacific gyre. Pure cultures grow aerobically on natural seawater media, oxidize sulfur, and reach higher final cell densities when glucose and thiosulfate are added to the media. This suggests that aerobic sulfur oxidation enhances organic carbon utilization in the oceans. The first isolate from the SUP05/Arctic96BD-19 clade was given the provisional taxonomic assignment ‘Candidatus: Thioglobus singularis'', alluding to the clade''s known role in sulfur oxidation and the isolate''s planktonic lifestyle.     

Sulfur oxidizers dominate carbon fixation at a biogeochemical hot spot in the dark ocean

Bacteria and archaea in the dark ocean (>200 m) comprise 0.3–1.3 billion tons of actively cycled marine carbon. Many of these microorganisms have the genetic potential to fix inorganic carbon (autotrophs) or assimilate single-carbon compounds (methylotrophs). We identified the functions of autotrophic and methylotrophic microorganisms in a vent plume at Axial Seamount, where hydrothermal activity provides a biogeochemical hot spot for carbon fixation in the dark ocean. Free-living members of the SUP05/Arctic96BD-19 clade of marine gamma-proteobacterial sulfur oxidizers (GSOs) are distributed throughout the northeastern Pacific Ocean and dominated hydrothermal plume waters at Axial Seamount. Marine GSOs expressed proteins for sulfur oxidation (adenosine phosphosulfate reductase, sox (sulfur oxidizing system), dissimilatory sulfite reductase and ATP sulfurylase), carbon fixation (ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO)), aerobic respiration (cytochrome c oxidase) and nitrogen regulation (PII). Methylotrophs and iron oxidizers were also active in plume waters and expressed key proteins for methane oxidation and inorganic carbon fixation (particulate methane monooxygenase/methanol dehydrogenase and RuBisCO, respectively). Proteomic data suggest that free-living sulfur oxidizers and methylotrophs are among the dominant primary producers in vent plume waters in the northeastern Pacific Ocean.     

Heterotrophic carbon metabolism and energy acquisition in Candidatus Thioglobus singularis strain PS1, a member of the SUP05 clade of marine Gammaproteobacteria

A hallmark of the SUP05 clade of marine Gammaproteobacteria is the ability to use energy obtained from reduced inorganic sulfur to fuel autotrophic fixation of carbon using RuBisCo. However, some SUP05 also have the genetic potential for heterotrophic growth, raising questions about the roles of SUP05 in the marine carbon cycle. We used genomic reconstructions, physiological growth experiments and proteomics to characterize central carbon and energy metabolism in Candidatus Thioglobus singularis strain PS1, a representative from the SUP05 clade that has the genetic potential for autotrophy and heterotrophy. Here, we show that the addition of individual organic compounds and 0.2 μm filtered diatom lysate significantly enhanced the growth of this bacterium. This positive growth response to organic substrates, combined with expression of a complete TCA cycle, heterotrophic pathways for carbon assimilation, and methylotrophic pathways for energy conversion demonstrate strain PS1's capacity for heterotrophic growth. Further, our inability to verify the expression of RuBisCO suggests that carbon fixation was not critical for growth. These results highlight the metabolic diversity of the SUP05 clade that harbours both primary producers and consumers of organic carbon in the oceans and expand our understanding of specific pathways of organic matter oxidation by the heterotrophic SUP05.     

Bacterial community structure associated with a dimethylsulfoniopropionate-producing North Atlantic algal bloom

The bacteria associated with oceanic algal blooms are acknowledged to play important roles in carbon, nitrogen, and sulfur cycling, yet little information is available on their identities or phylogenetic affiliations. Three culture-independent methods were used to characterize bacteria from a dimethylsulfoniopropionate (DMSP)-producing algal bloom in the North Atlantic. Group-specific 16S rRNA-targeted oligonucleotides, 16S ribosomal DNA (rDNA) clone libraries, and terminal restriction fragment length polymorphism analysis all indicated that the marine Roseobacter lineage was numerically important in the heterotrophic bacterial community, averaging >20% of the 16S rDNA sampled. Two other groups of heterotrophic bacteria, the SAR86 and SAR11 clades, were also shown by the three 16S rRNA-based methods to be abundant in the bloom community. In surface waters, the Roseobacter, SAR86, and SAR11 lineages together accounted for over 50% of the bacterial rDNA and showed little spatial variability in abundance despite variations in the dominant algal species. Depth profiles indicated that Roseobacter phylotype abundance decreased with depth and was positively correlated with chlorophyll a, DMSP, and total organic sulfur (dimethyl sulfide plus DMSP plus dimethyl sulfoxide) concentrations. Based on these data and previous physiological studies of cultured Roseobacter strains, we hypothesize that this lineage plays a role in cycling organic sulfur compounds produced within the bloom. Three other abundant bacterial phylotypes (representing a cyanobacterium and two members of the alpha Proteobacteria) were primarily associated with chlorophyll-rich surface waters of the bloom (0 to 50 m), while two others (representing Cytophagales and delta Proteobacteria) were primarily found in deeper waters (200 to 500 m).     

Distribution of Roseobacter RCA and SAR11 lineages and distinct bacterial communities from the subtropics to the Southern Ocean

We assessed the composition of the bacterioplankton in the Atlantic sector of the Southern Ocean in austral fall and winter and in New Zealand coastal waters in summer. The various water masses between the subtropics/Agulhas–Benguela boundary region and the Antarctic coastal current exhibited distinct bacterioplankton communities with the highest richness in the polar frontal region, as shown by denaturing gradient gel electrophoresis of 16S rRNA gene fragments. The SAR11 clade and the Roseobacter clade‐affiliated (RCA) cluster were quantified by real‐time quantitative PCR. SAR11 was detected in all samples analysed from subtropical waters to the coastal current and to depths of > 1000 m. In fall and winter, this clade constituted < 3% to 48% and 4–28% of total bacterial 16S rRNA genes respectively, with highest fractions in subtropical to polar frontal regions. The RCA cluster was only present in New Zealand coastal surface waters not exceeding 17°C, in the Agulhas–Benguela boundary region (visited only during the winter cruise), in subantarctic waters and in the Southern Ocean. In fall, this cluster constituted up to 36% of total bacterial 16S rRNA genes with highest fractions in the Antarctic coastal current and outnumbered the SAR11 clade at most stations in the polar frontal region and further south. In winter, the RCA cluster constituted lower proportions than the SAR11 clade and did not exceed 8% of total bacterial 16S rRNA genes. In fall, the RCA cluster exhibited significant positive correlations with latitude and ammonium concentrations and negative correlations with concentrations of nitrate, phosphate, and for near‐surface samples also with chlorophyll a, biomass production of heterotrophic prokaryotes and glucose turnover rates. The findings show that the various water masses between the subtropics and the Antarctic coastal current harbour distinct bacterioplankton communities. They further indicate that the RCA cluster, despite the narrow sequence similarity of > 98% of its 16S rRNA gene, is an abundant component of the heterotrophic bacterioplankton in the Southern Ocean, in particular in its coldest regions.     

Ubiquitous Dissolved Inorganic Carbon Assimilation by Marine Bacteria in the Pacific Northwest Coastal Ocean as Determined by Stable Isotope Probing

In order to identify bacteria that assimilate dissolved inorganic carbon (DIC) in the northeast Pacific Ocean, stable isotope probing (SIP) experiments were conducted on water collected from 3 different sites off the Oregon and Washington coasts in May 2010, and one site off the Oregon Coast in September 2008 and March 2009. Samples were incubated in the dark with 2 mM ¹³C-NaHCO₃, doubling the average concentration of DIC typically found in the ocean. Our results revealed a surprising diversity of marine bacteria actively assimilating DIC in the dark within the Pacific Northwest coastal waters, indicating that DIC fixation is relevant for the metabolism of different marine bacterial lineages, including putatively heterotrophic taxa. Furthermore, dark DIC-assimilating assemblages were widespread among diverse bacterial classes. Alphaproteobacteria, Gammaproteobacteria, and Bacteroidetes dominated the active DIC-assimilating communities across the samples. Actinobacteria, Betaproteobacteria, Deltaproteobacteria, Planctomycetes, and Verrucomicrobia were also implicated in DIC assimilation. Alteromonadales and Oceanospirillales contributed significantly to the DIC-assimilating Gammaproteobacteria within May 2010 clone libraries. 16S rRNA gene sequences related to the sulfur-oxidizing symbionts Arctic96BD-19 were observed in all active DIC assimilating clone libraries. Among the Alphaproteobacteria, clones related to the ubiquitous SAR11 clade were found actively assimilating DIC in all samples. Although not a dominant contributor to our active clone libraries, Betaproteobacteria, when identified, were predominantly comprised of Burkholderia. DIC-assimilating bacteria among Deltaproteobacteria included members of the SAR324 cluster. Our research suggests that DIC assimilation is ubiquitous among many bacterial groups in the coastal waters of the Pacific Northwest marine environment and may represent a significant metabolic process.     

Ammonia-oxidizing archaea release a suite of organic compounds potentially fueling prokaryotic heterotrophy in the ocean

Ammonia-oxidizing archaea (AOA) constitute a considerable fraction of microbial biomass in the global ocean, comprising 20%–40% of the ocean's prokaryotic plankton. However, it remains enigmatic to what extent these chemolithoautotrophic archaea release dissolved organic carbon (DOC). A combination of targeted and untargeted metabolomics was used to characterize the exometabolomes of three model AOA strains of the Nitrosopumilus genus. Our results indicate that marine AOA exude a suite of organic compounds with potentially varying reactivities, dominated by nitrogen-containing compounds. A significant fraction of the released dissolved organic matter (DOM) consists of labile compounds, which typically limit prokaryotic heterotrophic activity in open ocean waters, including amino acids, thymidine and B vitamins. Amino acid release rates corresponded with ammonia oxidation activity and the three Nitrosopumilus strains predominantly released hydrophobic amino acids, potentially as a result of passive diffusion. Despite the low contribution of DOC released by AOA (~0.08%–1.05%) to the heterotrophic prokaryotic carbon demand, the release of physiologically relevant metabolites could be crucial for microbes that are auxotrophic for some of these compounds, including members of the globally abundant and ubiquitous SAR11 clade.     

A metaproteomic assessment of winter and summer bacterioplankton from Antarctic Peninsula coastal surface waters

A metaproteomic survey of surface coastal waters near Palmer Station on the Antarctic Peninsula, West Antarctica, was performed, revealing marked differences in the functional capacity of summer and winter communities of bacterioplankton. Proteins from Flavobacteria were more abundant in the summer metaproteome, whereas winter was characterized by proteins from ammonia-oxidizing Marine Group I Crenarchaeota. Proteins prevalent in both seasons were from SAR11 and Rhodobacterales clades of Alphaproteobacteria, as well as many lineages of Gammaproteobacteria. The metaproteome data were used to elucidate the main metabolic and energy generation pathways and transport processes occurring at the microbial level in each season. In summer, autotrophic carbon assimilation appears to be driven by oxygenic photoautotrophy, consistent with high light availability and intensity. In contrast, during the dark polar winter, the metaproteome supported the occurrence of chemolithoautotrophy via the 3-hydroxypropionate/4-hydroxybutyrate cycle and the reverse tricarboxylic acid cycle of ammonia-oxidizing archaea and nitrite-oxidizing bacteria, respectively. Proteins involved in nitrification were also detected in the metaproteome. Taurine appears to be an important source of carbon and nitrogen for heterotrophs (especially SAR11), with transporters and enzymes for taurine uptake and degradation abundant in the metaproteome. Divergent heterotrophic strategies for Alphaproteobacteria and Flavobacteria were indicated by the metaproteome data, with Alphaproteobacteria capturing (by high-affinity transport) and processing labile solutes, and Flavobacteria expressing outer membrane receptors for particle adhesion to facilitate the exploitation of non-labile substrates. TonB-dependent receptors from Gammaproteobacteria and Flavobacteria (particularly in summer) were abundant, indicating that scavenging of substrates was likely an important strategy for these clades of Southern Ocean bacteria. This study provides the first insight into differences in functional processes occurring between summer and winter microbial communities in coastal Antarctic waters, and particularly highlights the important role that ‘dark'' carbon fixation has in winter.     

Resourceful heterotrophs make the most of light in the coastal ocean

The carbon cycle in the coastal ocean is affected by how heterotrophic marine bacterioplankton obtain their energy. Although it was previously thought that these organisms relied on the organic carbon in seawater for all of their energy needs, several recent discoveries now suggest that pelagic bacteria can depart from a strictly heterotrophic lifestyle by obtaining energy through unconventional mechanisms that are linked to the penetration of sunlight into surface waters. These newly discovered mechanisms involve the harvesting of energy, either directly from light or indirectly from inorganic compounds that are formed when dissolved organic carbon absorbs light. In coastal systems, these mixed metabolic strategies have implications for how efficiently organic carbon is retained in the marine food web and how climatically important gases are exchanged between the ocean and the atmosphere.     

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system. Received: 17 June 1997 / Accepted 25 September 1997     

Bacterial Community Structure Associated with a Dimethylsulfoniopropionate-Producing North Atlantic Algal Bloom

The bacteria associated with oceanic algal blooms are acknowledged to play important roles in carbon, nitrogen, and sulfur cycling, yet little information is available on their identities or phylogenetic affiliations. Three culture-independent methods were used to characterize bacteria from a dimethylsulfoniopropionate (DMSP)-producing algal bloom in the North Atlantic. Group-specific 16S rRNA-targeted oligonucleotides, 16S ribosomal DNA (rDNA) clone libraries, and terminal restriction fragment length polymorphism analysis all indicated that the marine Roseobacter lineage was numerically important in the heterotrophic bacterial community, averaging >20% of the 16S rDNA sampled. Two other groups of heterotrophic bacteria, the SAR86 and SAR11 clades, were also shown by the three 16S rRNA-based methods to be abundant in the bloom community. In surface waters, the Roseobacter, SAR86, and SAR11 lineages together accounted for over 50% of the bacterial rDNA and showed little spatial variability in abundance despite variations in the dominant algal species. Depth profiles indicated that Roseobacter phylotype abundance decreased with depth and was positively correlated with chlorophyll a, DMSP, and total organic sulfur (dimethyl sulfide plus DMSP plus dimethyl sulfoxide) concentrations. Based on these data and previous physiological studies of cultured Roseobacter strains, we hypothesize that this lineage plays a role in cycling organic sulfur compounds produced within the bloom. Three other abundant bacterial phylotypes (representing a cyanobacterium and two members of the α Proteobacteria) were primarily associated with chlorophyll-rich surface waters of the bloom (0 to 50 m), while two others (representing Cytophagales and δ Proteobacteria) were primarily found in deeper waters (200 to 500 m).     

Metabolic flexibility of SUP05 under low DO growth conditions

Chemoautotrophic bacteria from the SUP05 clade often dominate anoxic waters within marine oxygen minimum zones (OMZs) where they use energy gained from the oxidation of reduced sulfur to fuel carbon fixation. Some of these SUP05 bacteria are facultative aerobes that can use either nitrate or oxygen as a terminal electron acceptor making them ideally suited to thrive at the boundaries of OMZs where they experience fluctuations in dissolved oxygen (DO). SUP05 metabolism in these regions, and therefore the biogeochemical function of SUP05, depends largely on their sensitivity to oxygen. We evaluated growth and quantified differences in gene expression in Ca. T. autotrophicus strain EF1 from the SUP05 clade under high DO (22 μM), anoxic, and low DO (3.8 μM) concentrations. We show that strain EF1 cells respire oxygen and nitrate and that cells have higher growth rates, express more genes, and fix more carbon when oxygen becomes available for aerobic respiration. Evidence that facultatively aerobic SUP05 are more active and respire nitrate when oxygen becomes available at low concentrations suggests that they are an important source of nitrite across marine OMZ boundary layers.     

Basin-scale patterns in the abundance of SAR11 subclades, marine Actinobacteria (OM1), members of the Roseobacter clade and OCS116 in the South Atlantic

Bacterioplankton are major biogeochemical agents responsible for mediating the flux of dissolved organic matter (DOM) and subsequent cycling of nutrients in the oceans. Most information about the composition of bacterioplankton communities has come from studies along well-defined biogeochemical gradients in the northern hemisphere. This study extends observations of spatial and temporal dynamics for SAR11, Actinobacteria and OCS116 in the North Atlantic by demonstrating distinct spatial variability in the abundance and distribution of these and other lineages across the South Atlantic gyre and in the Benguela upwelling system. We identified shifts in SAR11, Actinobacteria, OCS116, SAR86, SAR116 and members of the Roseobacter clade along basin-scale gradients in nutrients, chlorophyll and dissolved organic carbon (DOC). Distinct SAR11 subclades dominated the western and eastern regions of the gyre, and Actinobacteria, OCS116 and members of the Roseobacter lineages were most abundant at the deep chlorophyll maxima. SAR86 and SAR116 accounted for a significant fraction of coastal and open ocean communities, respectively, and members of the gamma sulfur oxidizer (GSO) clade persisted in the Benguela upwelling system. These data suggest that distinct communities are partitioned along basin-scale biogeochemical gradients, that SAR11 community structure varies across the gyre and that Actinobacteria, OCS116, and members of the Roseobacter clade are closely associated with phytoplankton in the gyre.     

Transporter genes expressed by coastal bacterioplankton in response to dissolved organic carbon

Coastal ocean bacterioplankton control the flow of dissolved organic carbon (DOC) from terrestrial and oceanic sources into the marine food web, and regulate the release of inorganic carbon to atmospheric and offshore reservoirs. While the fate of the chemically complex coastal DOC reservoir has long been recognized as a critical feature of the global carbon budget, it has been problematic to identify both the compounds that serve as major conduits for carbon flux and the roles of individual bacterioplankton taxa in mediating that flux. Here we analyse random libraries of expressed genes from a coastal bacterial community to identify sequences representing DOC‐transporting proteins. Predicted substrates of expressed transporter genes indicated that carboxylic acids, compatible solutes, polyamines and lipids may be key components of the biologically labile DOC pool in coastal waters, in addition to canonical bacterial substrates such as amino acids, oligopeptides and carbohydrates. Half of the expressed DOC transporter sequences in this coastal ocean appeared to originate from just eight taxa: Roseobacter, SAR11, Flavobacteriales and five orders of γ‐Proteobacteria. While all major taxa expressed transporter genes for some DOC components (e.g. amino acids), there were indications of specialization within the bacterioplankton community for others (e.g. carbohydrates, carboxylic acids and polyamines). Experimental manipulations of the natural DOC pool that increased the concentration of phytoplankton‐ or vascular plant‐derived compounds invoked a readily measured response in bacterial transporter gene expression. This highly resolved view of the potential for carbon flux into heterotrophic bacterioplankton cells identifies possible bioreactive components of the coastal DOC pool and highlights differing ecological roles in carbon turnover for the resident bacterial taxa.     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.

     

The presence of the glycolysis operon in SAR11 genomes is positively correlated with ocean productivity

Bacteria in the SAR11 clade are highly abundant in marine surface waters, but currently little is known about the carbon compounds that support these large heterotrophic populations. To better understand the carbon requirements of these organisms, we conducted a multiphasic exploration of carbohydrate utilization among SAR11 isolates from the Northeast Pacific Ocean and the Sargasso Sea. A comparison of three SAR11 genomes showed they all lacked a recognizable PTS system, the oxidative portion of the pentose phosphate shunt (zwf^‐, pgl^‐), genes for the Embden–Meyerhoff–Parnas (pfk^‐, pyk^‐) and Entner–Doudoroff (eda^‐) pathways of glycolysis. Strain HTCC7211, isolated from an ocean gyre, was missing other glycolysis genes as well. Growth assays, radioisotopes, metagenomics and microarrays were used to test the hypothesis that these isolates might be limited in their abilities to transport and oxidize exogenous carbohydrates. Galactose, fucose, rhamnose, arabinose, ribose and mannose could not serve as carbon sources for the isolates tested. However, differences in glucose utilization were detected between coastal and ocean gyre isolates, with the coastal isolates capable of transporting, incorporating and oxidizing glucose while the open ocean isolate could not. Subsequent microarray analysis of a coastal isolate suggested that an operon encoding a variant of the Entner–Doudoroff pathway is likely responsible for the observed differences in glucose utilization. Metagenomic analysis indicated this operon is more commonly found in coastal environments and is positively correlated with chlorophyll a concentrations. Our results indicated that glycolysis is a variable metabolic property of SAR11 metabolism and suggest that glycolytic SAR11 are more common in productive marine environments.     

The Temporal Dynamics of Coastal Phytoplankton and Bacterioplankton in the Eastern Mediterranean Sea

This study considers variability in phytoplankton and heterotrophic bacterial abundances and production rates, in one of the most oligotrophic marine regions in the world–the Levantine Basin. The temporal dynamics of these planktonic groups were studied in the coastal waters of the southeastern Mediterranean Sea approximately every two weeks for a total of two years. Heterotrophic bacteria were abundant mostly during late summer and midwinter, and were positively correlated with bacterial production and with N₂ fixation. Based on size fractionating, picophytoplankton was abundant during the summer, whereas nano-microphytoplankton predominated during the winter and early spring, which were also evident in the size-fractionated primary production rates. Autotrophic abundance and production correlated negatively with temperature, but did not correlate with inorganic nutrients. Furthermore, a comparison of our results with results from the open Levantine Basin demonstrates that autotrophic and heterotrophic production, as well as N₂ fixation rates, are considerably higher in the coastal habitat than in the open sea, while nutrient levels or cell abundance are not different. These findings have important ecological implications for food web dynamics and for biological carbon sequestration in this understudied region.