Acclimation conditions modify physiological response of the diatom Thalassiosira pseudonana to elevated CO2 concentrations in a nitrate-limited chemostat.

Diatoms are responsible for a large proportion of global carbon fixation, with the possibility that they may fix more carbon under future levels of high CO2 . To determine how increased CO2 concentrations impact the physiology of the diatom Thalassiosira pseudonana Hasle et Heimdal, nitrate-limited chemostats were used to acclimate cells to a recent past (333 ± 6 µatm) and two projected future concentrations (476 ± 18 µatm, 816 ± 35 µatm) of CO2 . Samples were harvested under steady-state growth conditions after either an abrupt (15-16 generations) or a longer acclimation process (33-57 generations) to increased CO2 concentrations. The use of un-bubbled chemostat cultures allowed us to calculate the uptake ratio of dissolved inorganic carbon relative to dissolved inorganic nitrogen (DIC:DIN), which was strongly correlated with fCO2 in the shorter acclimations but not in the longer acclimations. Both CO2 treatment and acclimation time significantly affected the DIC:DIN uptake ratio. Chlorophyll a per cell decreased under elevated CO2 and the rates of photosynthesis and respiration decreased significantly under higher levels of CO2 . These results suggest that T. pseudonana shifts carbon and energy fluxes in response to high CO2 and that acclimation time has a strong effect on the physiological response.

Flavobacterial exudates disrupt cell cycle progression and metabolism of the diatom Thalassiosira pseudonana.

Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effects of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive morphology of enlarged cells with increased chloroplast content and enlarged nuclei, similar to what was previously observed when the diatom was co-cultured with live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.

Complex marine microbial communities partition metabolism of scarce resources over the diel cycle.

Complex assemblages of microbes in the surface ocean are responsible for approximately half of global carbon fixation. The persistence of high taxonomic diversity despite competition for a small suite of relatively homogeneously distributed nutrients, that is, 'the paradox of the plankton', represents a long-standing challenge for ecological theory. Here we find evidence consistent with temporal niche partitioning of nitrogen assimilation processes over a diel cycle in the North Pacific Subtropical Gyre. We jointly analysed transcript abundances, lipids and metabolites and discovered that a small number of diel archetypes can explain pervasive periodic dynamics. Metabolic pathway analysis of identified diel signals revealed asynchronous timing in the transcription of nitrogen uptake and assimilation genes among different microbial groups-cyanobacteria, heterotrophic bacteria and eukaryotes. This temporal niche partitioning of nitrogen uptake emerged despite synchronous transcription of photosynthesis and central carbon metabolism genes and associated macromolecular abundances. Temporal niche partitioning may be a mechanism by which microorganisms in the open ocean mitigate competition for scarce resources, supporting community coexistence.

Alternative strategies of nutrient acquisition and energy conservation map to the biogeography of marine ammonia-oxidizing archaea.

Ammonia-oxidizing archaea (AOA) are among the most abundant and ubiquitous microorganisms in the ocean, exerting primary control on nitrification and nitrogen oxides emission. Although united by a common physiology of chemoautotrophic growth on ammonia, a corresponding high genomic and habitat variability suggests tremendous adaptive capacity. Here, we compared 44 diverse AOA genomes, 37 from species cultivated from samples collected across diverse geographic locations and seven assembled from metagenomic sequences from the mesopelagic to hadopelagic zones of the deep ocean. Comparative analysis identified seven major marine AOA genotypic groups having gene content correlated with their distinctive biogeographies. Phosphorus and ammonia availabilities as well as hydrostatic pressure were identified as selective forces driving marine AOA genotypic and gene content variability in different oceanic regions. Notably, AOA methylphosphonate biosynthetic genes span diverse oceanic provinces, reinforcing their importance for methane production in the ocean. Together, our combined comparative physiological, genomic, and metagenomic analyses provide a comprehensive view of the biogeography of globally abundant AOA and their adaptive radiation into a vast range of marine and terrestrial habitats.

Influence of oxygen availability on the activities of ammonia-oxidizing archaea.

Recent studies point to the importance of oxygen (O2 ) in controlling the distribution and activity of marine ammonia-oxidizing archaea (AOA), one of the most abundant prokaryotes in the ocean. The AOA are associated with regions of low O2 tension in oceanic oxygen minimum zones (OMZs), and O2 availability is suggested to influence their production of the ozone-depleting greenhouse gas nitrous oxide (N2 O). We show that marine AOA available in pure culture sustain high ammonia oxidation activity at low µM O2 concentrations, characteristic of suboxic regions of OMZs (<10 µM O2 ), and that atmospheric concentrations of O2 may inhibit the growth of some environmental populations. We quantify the increasing N2 O production by marine AOA with decreasing O2 tensions, consistent with the plausibility of an AOA contribution to the accumulation of N2 O at the oxic-anoxic redox boundaries of OMZs. Variable sensitivity to peroxide also suggests that endogenous or exogenous reactive oxygen species are of importance in determining the environmental distribution of some populations.

INTER- AND INTRASPECIFIC RELATIONSHIPS BETWEEN NUCLEAR DNA CONTENT AND CELL SIZE IN SELECTED MEMBERS OF THE CENTRIC DIATOM GENUS THALASSIOSIRA (BACILLARIOPHYCEAE)(1).

The enormous species diversity of diatoms correlates with the remarkable range of cell sizes in this group. Nuclear DNA content relates fundamentally to cell volume in other eukaryotic cells. The relationship of cell volume to G1 DNA content was determined among selected members of the genus Thalassiosira, one of the most species-rich and well-studied centric diatom genera. Both minimum and maximum species-specific cell volume correlated positively with G1 DNA content. Phylogeny based on 5.8 S and ITS rDNA sequences indicated that multiple changes in G1 DNA content and cell volume occurred in Thalassiosira evolution, leading to a 1,000-fold range in both parameters in the group. Within the Thalassiosira weissflogii (Grunow) G. A. Fryxell et Grunow species complex, G1 DNA content varied 3-fold: differences related to geographic origin and time since isolation; doubling and tripling of G1 DNA content occurred since isolation in certain T. weissflogii isolates; and subcultures of T. weissflogii CCMP 1336 diverged in DNA content by 50% within 7 years of separation. Actin, ß-tubulin, and Spo11/TopVIA genes were selected for quantitative PCR estimation of haploid genome size in subclones of selected T. weissflogii isolates because they occur only once in the T. pseudonana Hasle et Heimdal genome. Comparison of haploid genome size estimates with G1 DNA content suggested that the most recent T. weissflogii isolate was diploid, whereas other T. weissflogii isolates appeared to be polyploid and/or aneuploid. Aberrant meiotic and mitotic cell divisions were observed, which might relate to polyploidization. The structural flexibility of diatom genomes has important implications for their evolutionary diversification and stability during laboratory maintenance.

IDENTIFICATION AND ASSESSMENT OF DOMOIC ACID PRODUCTION IN OCEANIC PSEUDO-NITZSCHIA (BACILLARIOPHYCEAE) FROM IRON-LIMITED WATERS IN THE NORTHEAST SUBARCTIC PACIFIC(1).

We identified and investigated the potential toxicity of oceanic Pseudo-nitzschia species from Ocean Station Papa (OSP), located in a high-nitrate, low-chlorophyll (HNLC) region of the northeast (NE) subarctic Pacific Ocean. Despite their relatively low abundances in the indigenous phytoplankton assemblage, Pseudo-nitzschia species richness is high. The morphometric characteristics of five oceanic Pseudo-nitzschia isolates from at least four species are described using SEM and TEM. The species identified are Pseudo-nitzschia dolorosa Lundholm et Moestrup, P. granii Hasle, P. heimii Manguin, and P. cf. turgidula (Hust.) Hasle. Additional support for the taxonomic classifications based on frustule morphology is provided through the sequencing of the internal transcribed spacer 1 (ITS1) rDNA. Pseudo-nitzschia species identification was also assessed by the construction of ITS1 clone libraries and using automated ribosomal intergenic spacer analysis (ARISA) for environmental samples collected during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), conducted in close proximity to OSP in July of 2002. Based on ITS1 sequences, the presence of P. granii, P. heimii, P. cf. turgidula, and at least five other putative, unidentified Pseudo-nitzschia ITS1 variants was confirmed within iron-enriched phytoplankton assemblages at OSP. None of the oceanic isolates produced detectable levels of particulate domoic acid (DA) when in prolonged stationary phase due to silicic acid starvation. The lack of detectable concentrations of DA suggests that either these strains produce very little or no toxin, or that the physiological conditions required to promote particulate DA production were not met and thus differ from their coastal, toxigenic congeners.