Abstract The genus Nostoc has long been known to be polyphyletic, and recent taxonomic work has segregated several Nostoc ‐like lineages into new genera, including Compactonostoc . Until now, C. shennongjiaensis was the only described species in the genus. In a survey of biological soil crusts from Santa Catalina Island (California, United States), we isolated two Nostoc ‐like strains that phylogenetically affiliate with Compactonostoc . A polyphasic approach based on morphology, 16S rRNA gene phylogeny, 16S‐23S ITS rRNA secondary structures, and cloned ribosomal operons revealed that each strain harbored two highly divergent ribosomal operon types. One operon (Type 2) is closely related to C. shennongjiaensis and contains the two tRNA genes in the ITS rRNA region, whereas the second (Type 1) lacks tRNA genes and is strongly allied to the horizontally acquired Type 1 operon previously reported in Kalymmatonema (as Scytonema hyalinum ), indicating an ancient horizontal gene transfer event from an unsampled Nostocaceaen lineage. In one of the isolated strains, we additionally detected two reciprocal chimeric operons (Type 2/1 and Type 1/2), with crossovers in a conserved region of the 16S rRNA gene, demonstrating ongoing recombination between operon types. The ITS rRNA region percent dissimilarity, ITS rRNA region domain structures, and 16S rRNA gene percent similarity, together with consistent morphological differences, support recognition of the two strains as new species of Compactonostoc, C. mediiheterocytium and C. chimera , both clearly distinct from C. shennongjiaensis . Our results document rare ribosomal‐operon horizontal gene transfer and reciprocal chimerism in Compactonostoc and highlight the need to examine multiple clones and operons when describing cyanobacterial taxa based on rRNA genes.
An improvement in the (H2O)-O-18 technique for phytoplankton gross primary productivity (GPP), net primary productivity (NPP), and gross respiration (GR) is described that reduces cost, labor, and time over the traditional isotope ratio mass spectrometer (IRMS) method. Membrane inlet mass spectrometry (MIMS) was optimized to measure small (3 mL) samples with low isotope enrichment (1% O-18) and at high analytical precision (similar to 1 per mil). A photosynthetron was used to measure up to 10-point light curves of GPP, NPP, and GR using a total of 50 mL of sample water with an isotope cost of <$15 per experiment. A new incubation device is described, a Coronatron, that allows up to 12 samples with nutrient manipulation under uniform irradiance. Example light curves from highly contrasting natural water samples are presented that show elevated GPP relative to NPP and substantial light-stimulated GR. To highlight the improved method, examples nutrient manipulation curves are also presented showing GPP, NPP, and GR as a function of changes in ambient nitrate concentrations and different temperatures under constant light.
Paralytic shellfish toxins (PSTs) are produced by certain species of cyanobacteria and dinoflagellates. Part of the PST biosynthetic pathway has been elucidated in cyanobacteria, and the implication of some sxt genes has been confirmed by experimental studies. Contrary to cyanobacteria, knowledge about PST biosynthesis in dinoflagellates is more limited and generally restricted to comparative studies with the cyanobacterial pathway. To investigate the specificity of the PST pathway in dinoflagellates, 16 toxigenic and nontoxigenic Alexandrium minutum strains from a recombinant cross were compared, without prior assumption about genes or metabolites involved in PST synthesis, using an integrative approach combining untargeted metabolomic and transcriptomic data. Among the 60 most distinguishing transcripts between toxigenic and non-toxigenic strains, only three sxt genes were present: sxtA4, sxtG, and sxtI. In contrast, non-sxt homologs were detected as highly discriminant between these two phenotypes. More specifically, a phyH homolog may act as the analog of the sxtS gene observed in cyanobacteria. Moreover, we identified four putative synthetic PST intermediates. Among these, Int-C'2 correlated with the toxic phenotype, whereas three others were detected in both toxigenic and nontoxigenic strains, suggesting that these strains may share some parts of the biosynthetic pathway. Finally, our analyses identified both horizontally gained sxt genes and endogenous genes, including the candidate gene phyH, as candidate components of the PST biosynthetic pathway in A. minutum.
Abstract An improvement in the H 2 18 O technique for phytoplankton gross primary productivity (GPP), net primary productivity (NPP), and gross respiration (GR) is described that reduces cost, labor, and time over the traditional isotope ratio mass spectrometer (IRMS) method. Membrane inlet mass spectrometry (MIMS) was optimized to measure small (3 mL) samples with low isotope enrichment (1% 18 O) and at high analytical precision (~1 per mil). A photosynthetron was used to measure up to 10‐point light curves of GPP, NPP, and GR using a total of 50 mL of sample water with an isotope cost of <$15 per experiment. A new incubation device is described, a Coronatron, that allows up to 12 samples with nutrient manipulation under uniform irradiance. Example light curves from highly contrasting natural water samples are presented that show elevated GPP relative to NPP and substantial light‐stimulated GR. To highlight the improved method, examples nutrient manipulation curves are also presented showing GPP, NPP, and GR as a function of changes in ambient nitrate concentrations and different temperatures under constant light.
Giant kelp (Macrocystis pyrifera) forests have declined by 95% in Australia since the 1970s due to climate change. Despite expanding restoration efforts, long-term recovery is still lacking. Factors influencing recruitment by the microscopic gametophyte phase may represent a key mechanism underpinning the successful recovery and restoration of M. pyrifera. In the northern hemisphere, kelp gametophytes grow epiphytically and endophytically in red macroalgae and may represent an important "propagule bank" for kelp gametophyte recruitment. Our goal was to determine if M. pyrifera recruitment might depend on endo- or epiphytic gametophyte growth with understory red macroalgae on reefs in Tasmania, Australia. We first examined the composition and biomass of understory macroalgae living under kelp forests in Tasmania. We then tested if M. pyrifera gametophytes were located epiphytically or endophytically within specific red macroalgal hosts of 13 common species. Digital polymerase chain reaction (PCR) with M. pyrifera primers found M. pyrifera gametophytes in eight species of red macroalgae (Ballia callitricha, Gloiocladia halymenioides, Halopeltis australis, Phacelocarpus pepperocarpos, Plocamium angustum, Plocamium dilatatumi, Ptilonia australasica, and Ptilonia subulifera), with Ballia callitricha being the most common host (66.7% positive detections). By sterilizing the surface of host tissue, the detection of M. pyrifera DNA was consistent with endophytic occurrence of kelp gametophytes within the red macroalgal hosts but did not exclude the possibility that they also live epiphytically. These results suggest that red macroalgae may provide an important propagule bank for M. pyrifera gametophytes, and maintaining healthy understory assemblages may facilitate recovery and restoration efforts of M. pyrifera.
