Efficient carbon and nitrogen transfer from marine diatom aggregates to colonizing bacterial groups

Bacterial degradation of sinking diatom aggregates is key for the availability of organic matter in the deep-ocean. Yet, little is known about the impact of aggregate colonization by different bacterial taxa on organic carbon and nutrient cycling within aggregates. Here, we tracked the carbon (C) and nitrogen (N) transfer from the diatom Leptocylindrus danicus to different environmental bacterial groups using a combination of C-13 and N-15 isotope incubation (incubated for 72 h), CARD-FISH and nanoSIMS single-cell analysis. Pseudoalteromonas bacterial group was the first colonizing diatom-aggregates, succeeded by the Alteromonas group. Within aggregates, diatom-attached bacteria were considerably more enriched in C-13 and N-15 than non-attached bacteria. Isotopic mass balance budget indicates that both groups showed comparable levels of diatom C in their biomass, accounting for 19 +/- 7% and 15 +/- 11%, respectively. In contrast to C, bacteria of the Alteromonas groups showed significantly higher levels of N derived from diatoms (77 +/- 28%) than Pseudoalteromonas (47 +/- 17%), suggesting a competitive advantage for Alteromonas in the N-limiting environments of the deep-sea. Our results imply that bacterial succession within diatom aggregates may largely impact taxa-specific C and N uptake, which may have important consequences for the quantity and quality of organic matter exported to the deep ocean

Cytoklepty in the plankton: A host strategy to optimize the bioenergetic machinery of endosymbiotic algae

Endosymbioses have shaped the evolutionary trajectory of life and remain ecologically important. Investigating oceanic photosymbioses can illuminate how algal endosymbionts are energetically exploited by their heterotrophic hosts and inform on putative initial steps of plastid acquisition in eukaryotes. By combining three-dimensional subcellular imaging with photophysiology, carbon flux imaging, and transcriptomics, we show that cell division of endosymbionts (Phaeocystis) is blocked within hosts (Acantharia) and that their cellular architecture and bioenergetic machinery are radically altered. Transcriptional evidence indicates that a nutrient-independent mechanism prevents symbiont cell division and decouples nuclear and plastid division. As endosymbiont plastids proliferate, the volume of the photosynthetic machinery volume increases 100-fold in correlation with the expansion of a reticular mitochondrial network in close proximity to plastids. Photosynthetic efficiency tends to increase with cell size, and photon propagation modeling indicates that the networked mitochondrial architecture enhances light capture. This is accompanied by 150-fold higher carbon uptake and up-regulation of genes involved in photosynthesis and carbon fixation, which, in conjunction with a ca.15-fold size increase of pyrenoids demonstrates enhanced primary production in symbiosis. Mass spectrometry imaging revealed major carbon allocation to plastids and transfer to the host cell. As in most photosymbioses, microalgae are contained within a host phagosome (symbiosome), but here, the phagosome invaginates into enlarged microalgal cells, perhaps to optimize metabolic exchange. This observation adds evidence that the algal metamorphosis is irreversible. Hosts, therefore, trigger and benefit from major bioenergetic remodeling of symbiotic microalgae with potential consequences for the oceanic carbon cycle. Unlike other photosymbioses, this interaction represents a so-called cytoklepty, which is a putative initial step toward plastid acquisition

Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses

Many diatoms that inhabit low-nutrient waters of the open ocean live in close association with cyanobacteria. Some of these associations are believed to be mutualistic, where N2-fixing cyanobacterial symbionts provide N for the diatoms. Rates of N2 fixation by symbiotic cyanobacteria and the N transfer to their diatom partners were measured using a high-resolution nanometer scale secondary ion mass spectrometry approach in natural populations. Cell-specific rates of N2 fixation (1.15–71.5 fmol N per cell h−1) were similar amongst the symbioses and rapid transfer (within 30 min) of fixed N was also measured. Similar growth rates for the diatoms and their symbionts were determined and the symbiotic growth rates were higher than those estimated for free-living cells. The N2 fixation rates estimated for Richelia and Calothrix symbionts were 171–420 times higher when the cells were symbiotic compared with the rates estimated for the cells living freely. When combined, the latter two results suggest that the diatom partners influence the growth and metabolism of their cyanobacterial symbionts. We estimated that Richelia fix 81–744% more N than needed for their own growth and up to 97.3% of the fixed N is transferred to the diatom partners. This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production. Further, this is the first demonstration of N transfer from an N2 fixer to a unicellular partner. These symbioses are important models for molecular regulation and nutrient exchange in symbiotic systems

Molecular characterization of symbiotic associations between chemoautotrophic sulfur-oxidizing microorganisms and nematodes in shallow marine sediments

Shallow marine sediments are preferred environments for nematodes harboring endo- or ectosymbiotic microorganisms. In most cases, the symbionts are autotrophic, sulfur-oxidizing bacteria. In the present work molecular biological methods such as comparative 16S rRNA and 18S rRNA sequence analysis, fluorescence in situ hybridization (FISH) and slot blot hybridization with group specific rRNA-targeted oligonucleotide probes were used to investigate and characterize these symbiotic associations and their natural habitat. Thus we investigated the specificity of the symbiotic association between ectosymbiotic bacteria and gut-bearing nematodes of the genus Leptonemella from intertidal sandy sediments near the island of Sylt (Wadden Sea) and we determined the identity and phylogeny of the endosymbionts of mouthless nematode, Astomonema sp. from coral reef sediments in the Bahamas. Also the microbial diversity and community structure of the intertidal Sylt sandy sediments, the preferred habitat of the Leptonemella spp. was determined and characterized over depth (0 to 12 cm) and seasons (March, July, October)

Molekulare Charakterisierung symbiotischer Beziehungen zwischenchemoautotrophen schwefeloxidierenden Mikroorganismen und Nematoden inmarinen Sedimenten

Shallow marine sediments are preferred environments for nematodes harboring endo- or ectosymbiotic microorganisms. In most cases, the symbionts are autotrophic, sulfur-oxidizing bacteria. In the present work molecular biological methods such as comparative 16S rRNA and 18S rRNA sequence analysis, fluorescence in situ hybridization (FISH) and slot blot hybridization with group specific rRNA-targeted oligonucleotide probes were used to investigate and characterize these symbiotic associations and their natural habitat. Thus we investigated the specificity of the symbiotic association between ectosymbiotic bacteria and gut-bearing nematodes of the genus Leptonemella from intertidal sandy sediments near the island of Sylt (Wadden Sea) and we determined the identity and phylogeny of the endosymbionts of mouthless nematode, Astomonema sp. from coral reef sediments in the Bahamas. Also the microbial diversity and community structure of the intertidal Sylt sandy sediments, the preferred habitat of the Leptonemella spp. was determined and characterized over depth (0 to 12 cm) and seasons (March, July, October)

In Situ Analysis of a Silver Nanoparticle-Precipitating Shewanella Biofilm by Surface Enhanced Confocal Raman Microscopy.

Shewanella oneidensis MR-1 is an electroactive bacterium, capable of reducing extracellular insoluble electron acceptors, making it important for both nutrient cycling in nature and microbial electrochemical technologies, such as microbial fuel cells and microbial electrosynthesis. When allowed to anaerobically colonize an Ag/AgCl solid interface, S. oneidensis has precipitated silver nanoparticles (AgNp), thus providing the means for a surface enhanced confocal Raman microscopy (SECRaM) investigation of its biofilm. The result is the in-situ chemical mapping of the biofilm as it developed over time, where the distribution of cytochromes, reduced and oxidized flavins, polysaccharides and phosphate in the undisturbed biofilm is monitored. Utilizing AgNp bio-produced by the bacteria colonizing the Ag/AgCl interface, we could perform SECRaM while avoiding the use of a patterned or roughened support or the introduction of noble metal salts and reducing agents. This new method will allow a spatially and temporally resolved chemical investigation not only of Shewanella biofilms at an insoluble electron acceptor, but also of other noble metal nanoparticle-precipitating bacteria in laboratory cultures or in complex microbial communities in their natural habitats