ALBERT

Description: Understanding the evolution of the free-living, cyanobacterial, diazotroph Trichodesmium is of great importance because of its critical role in oceanic biogeochemistry and primary production. Unlike the other 〉150 available genomes of free-living cyanobacteria, only 63.8% of the Trichodesmium erythraeum (strain IMS101) genome is predicted to encode protein, which is 20–25%...

Description: Glutamine synthetase (GS), a key enzyme in biological nitrogen assimilation, is regulated in multiple ways in response to varying nitrogen sources and levels. Here we show a small regulatory RNA, NsiR4 (nitrogen stress-induced RNA 4), which plays an important role in the regulation of GS in cyanobacteria. NsiR4 expression in...

Description: N2 fixation fuels ~ 50 % of new primary production in the oligotrophic South Pacific Ocean. The VAHINE experiment has been designed to track the fate of diazotroph derived nitrogen (DDN) and carbon within a coastal lagoon ecosystem in a comprehensive way. For this, large-volume (~ 50 m3) mesocosms were deployed in the New Caledonia lagoon and were intentionally fertilized with dissolved inorganic phosphorus (DIP) to stimulate N2 fixation. This study examined the temporal dynamics of the prokaryotic community together with the evolution of biogeochemical parameters for 23 consecutive days in one of these mesocosms (M1) and in the Nouméa lagoon using MiSeq 16S rRNA gene sequencing. We observed clear successions within M1, some of which were not mirrored in the lagoon. The dominating classes in M1 were alpha- and gammaproteobacteria, cyanobacteria (mainly Synechococcus), eukaryotic microalgae, on days 10 and 14 Marine Group II euryarchaea, on days 12–23 also Flavobacteriia. Enclosure led to significant changes in the M1 microbial community, probably initiated by the early decay of Synechococcus and diatoms. However, we did not detect a pronounced bottle effect with a copiotroph-dominated community. The fertilization with ~ 0.8 μM DIP on day 4 did not have directly observable effects on the overall community within M1, as the data samples obtained from before and four days after fertilization clustered together, but likely influenced the development of individual populations later on, like Defluviicoccus-related bacteria and UCYN-C type diazotrophic cyanobacteria. Growth of UCYN-C led to among the highest N2 fixation rates ever measured in this region and enhanced growth of nearly all abundant heterotrophic groups in M1. We further show that different Rhodobacteraceae were the most efficient heterotrophs in the investigated system and we observed niche partitioning within the SAR86 clade. Whereas the location in- or outside the mesocosm had a significant effect on community composition, the temporal effect was significantly stronger and similar in both locations, suggesting that overarching abiotic factors were more influential than the enclosure. While temporal community changes were evident, prokaryotic diversity (Shannon Index) only declined slightly from ~ 6.5 to 5.7 or 6.05 in the lagoon and M1, respectively, throughout the experiment, highlighting the importance of multiple and varying sources of organic matter maintaining competition.

Description: N2 fixation fuels ~ 50 % of new primary production in the oligotrophic South Pacific Ocean. The VAHINE mesocosm experiment designed to track the fate of diazotroph derived nitrogen (DDN) in the New Caledonia lagoon. Here, we examined the temporal dynamics of heterotrophic bacterial production during this experiment. Three replicate large-volume (~ 50 m3) mesocosms were deployed and were intentionally fertilized with dissolved inorganic phosphorus (DIP) to stimulate N2 fixation. We specifically examined relationships between N2 fixation rates and primary production, determined bacterial growth efficiency and established carbon budgets of the system from the DIP fertilization to the end of the experiment (days 5–23). Heterotrophic bacterioplankton production (BP) and alkaline phosphatase activity (APA) were statistically higher during the second phase of the experiment (P2: days 15–23), when chlorophyll biomass started to increase compared to the first phase (P1: days 5–14). Among autotrophs, Synechococcus abundances increased during P2, possibly related to its capacity to assimilate leucine and to produce alkaline phosphatase. Bacterial growth efficiency based on the carbon budget was notably higher than generally cited for oligotrophic environments (27–43 %), possibly due to a high representation of proteorhodopsin-containing organisms within the picoplanctonic community. The carbon budget showed that the main fate of gross primary production (particulate + dissolved) was respiration (67 %), and export through sedimentation (17 %). BP was highly correlated with particulate primary production and chlorophyll biomass during both phases of the experiment but slightly correlated, and only during P2 phase, with N2 fixation rates. Our results suggest that most of the DDN reached the heterotrophic bacterial community through indirect processes, like mortality, lysis and grazing.

Description: The globally important marine diazotrophic cyanobacterium Trichodesmium blooms regularly in the New Caledonian lagoons (Sowthwestern Pacific). We exploited the development of a Trichodesmium bloom in the lagoon waters outside the enclosed VAHINE mesocosms to specifically investigate the cellular processes mediating its decline. Trichodesmium cells (and associated microbiota) were sampled from the time of surface accumulation to biomass demise using a series of enclosed incubations to elucidate the stressors and subcellular underpinning of rapid (~ 24 h) biomass demise and disappearance. The development and decline of Trichodesmium populations was rapid with extensive surface accumulations (blooms) appearing within 24 h on the surface waters of the lagoon. Rapid decline of 〉 90 % biomass after 24 h of peak accumulation was observed in p opulations that were collected and incubated under ambient conditions. Metatranscriptomic profiling of Trichodesmium biomass 8 h and 22 h after bottle incubation of surface accumulations revealed evidence for phosphorus (P) and iron (Fe) stress, with upreg ulation of genes required to increase their availability and transport. In contrast, genes responsible for nutrient storage were downregulated. Total viral abundance, assessed by SYBR-green staining and analytical flow cytometry, oscillated throughout the experiment and showed no significant relationship with Trichodesmium bloom development or demise. Enhanced caspase-specific activity and upregulation of a suite of metacaspase genes during bloom demise implicated autocatalytic programmed cell death (PCD) as the mechanistic cause. At the same time, genes associated with buoyancy and gas-vesicle production were strongly downregulated concomitant with high concentrations of transparent exopolymeric particles (TEP), greatly aiding aggregation and expediting vertical flux to depth. Our results demonstrate that the rapid demise of this high-density, Trichodesmium surface bloom over 24 h was not caused by specific lytic infection but was rather induced by PCD in response to combined nutrient and oxidative stressors.