Mutagenesis of Ile184 in the cd-loop of the photosystem II D1 protein modifies acceptor-side function via spontaneous mutation of D1-His252 in Synechocystis sp. PCC 6803.

The Photosystem II water-plastoquinone oxidoreductase is a multi-subunit complex which catalyses the light-driven oxidation of water to molecular oxygen in oxygenic photosynthesis. The D1 reaction centre protein exists in multiple forms in cyanobacteria, including D1FR which is expressed under far-red light. We investigated the role of Phe184 that is found in the lumenal cd-loop of D1FR but is typically an isoleucine in other D1 isoforms. The I184F mutant in Synechocystis sp. PCC 6803 was similar to the control strain but accumulated a spontaneous mutation that introduced a Gln residue in place of His252 located on the opposite side of the thylakoid membrane. His252 participates in the protonation of the secondary plastoquinone electron acceptor QB. The I184F:H252Q double mutant exhibited reduced high-light-induced photodamage and an altered QB-binding site that impaired herbicide binding. Additionally, the H252Q mutant had a large increase in the variable fluorescence yield although the number of photochemically active PS II centres was unchanged. In the I184F:H252Q mutant the extent of the increased fluorescence yield decreased. Our data indicates substitution of Ile184 to Phe modulates PS II-specific variable fluorescence in cells with the His252 to Gln substitution by modifying the QB-binding site.

Phylogenomics and morphology of Celmisiinae (Asteraceae: Astereae): Taxonomic and evolutionary implications.

The tribe Astereae (Asteraceae) includes 36 subtribes and 252 genera, and is distributed worldwide in temperate and tropical regions. One of the subtribes, Celmisiinae Saldivia, has been recently circumscribed to include six genera and ca. 160 species, and is restricted to eastern Australia, New Zealand, and New Guinea. The species show an impressive range of growth habit, from small herbs and ericoid subshrubs to medium-sized trees. They live in a wide range of habitats and are often dominant in subalpine and alpine vegetation. Despite the well-supported circumscription of Celmisiinae, uncertainties have remained about their internal relationships and classification at genus and species levels. This study exploited recent advances in high-throughput sequencing to build a robust multi-gene phylogeny for the subtribe Celmisiinae. The target enrichment Angiosperms353 bait set and the hybpiper-nf and paragone-nf pipelines were used to retrieve, infer, and assemble orthologous loci from 75 taxa representing all the main putative clades within the subtribe. Because of the diploidised ploidy level in Celmisiinae, as well as missing data in the assemblies, uncertainty remains surrounding the inference of orthology detection. However, based on a variety of gene-family sets, coalescent and concatenation-based phylogenetic reconstructions recovered similar topologies. Paralogy and missing data in the gene-families caused some problems, but the estimated phylogenies were well-supported and well-resolved. The phylogenomic evidence supported Celmisiinae and three main clades: the Pleurophyllum clade (Pleurophyllum, Macrolearia and Damnamenia), mostly in the New Zealand Subantarctic Islands, Celmisia of mainland New Zealand and Australia, and Shawia (including 'Olearia pro parte' and Pachystegia) of New Zealand, Australia and New Guinea. The results presented here add to the accumulating support for the Angiosperms353 bait set as an efficient method for documenting plant diversity.

Zeta diversity differentiates factors driving community assembly of rare and common ectomycorrhizal fungi.

A fundamental goal in community ecology is to understand what factors drive community assembly processes. The factors affecting ectomycorrhizal fungal communities are unknown in many regions, particularly in the southern hemisphere. We investigate community assembly using ITS2 metabarcoding of ectomycorrhizal fungi sampled from 3943 hyphal ingrowth bags buried in 81 Nothofagus forests across New Zealand's South Island. By applying zeta diversity analysis and multisite generalized dissimilarity modelling (MS-GDM) we quantify the effects of 43 biotic and environmental variables on community turnover. Unlike traditional beta diversity analyses that are heavily influenced by rare species, the zeta diversity framework differentiates between factors driving turnover of rare and common species, providing a more complete picture of community dynamics. We found that community assembly was dominated by deterministic rather than stochastic processes and identified ecological factors affecting all taxa, as well as others that were specifically important to rare or common taxa. Soil variables were important drivers of turnover for all species, whereas ground cover variables, forest patch size, precipitation and host tree identity had greater effects on rarer species, and tree size and temperature effects were specific to more common species. Interestingly, the effect of temperature on common species is in line with recent evidence from other Kingdoms, pointing to possible generality, and highlighting the importance of considering common species. Applying these methods to fungi has allowed us to identify the distinct ecological processes that structure rare and common taxa during community assembly. This has important implications for understanding the functional effects of community responses to environmental change.

Expression of the far-red D1 protein or introduction of conserved far-red D1 residues into Synechocystis sp. PCC 6803 impairs Photosystem II.

The wavelengths of light harvested in oxygenic photosynthesis are ~400-700 nm. Some cyanobacteria respond to far-red light exposure via a process called far-red light photoacclimation which enables absorption of light at wavelengths >700 nm and its use to support photosynthesis. Far-red-light-induced changes include up-regulation of alternative copies of multiple proteins of Photosystem II (PS II). This includes an alternative copy of the D1 protein, D1FR . Here, we show that D1FR introduced into Synechocystis sp. PCC 6803 (hereafter Synechocystis 6803) can be incorporated into PS II centres that evolve oxygen at low rates but cannot support photoautotrophic growth. Using mutagenesis to modify the psbA2 gene of Synechocystis 6803, we modified residues in helices A, B, and C to be characteristic of D1FR residues. Modification of the Synechocystis 6803 helix A to resemble the D1FR helix A, with modifications in the region of the bound ß-carotene (CarD1 ) and the accessory chlorophyll, ChlZD1 , produced a strain with a similar phenotype to the D1FR strain. In contrast, the D1FR changes in helices B and C had minor impacts on photoautotrophy but impacted the function of PS II, possibly through a change in the equilibrium for electron sharing between the primary and secondary plastoquinone electron acceptors QA and QB in favour of QA - . The addition of combinations of residue changes in helix C indicates compensating effects may occur and highlight the need to experimentally determine the impact of multiple residue changes.

The PsbJ protein is required for photosystem II activity in centers lacking the PsbO and PsbV lumenal subunits.

Photosystem II (PS II) of oxygenic photosynthesis is found in the thylakoid membranes of plastids and cyanobacteria. The mature PS II complex comprises a central core of four membrane proteins that bind the majority of the redox-active cofactors. In cyanobacteria the central core is surrounded by 13 low-molecular-weight (LMW) subunits which each consist of one or two transmembrane helices. Three additional hydrophilic subunits known as PsbO, PsbU and PsbV are found associated with hydrophilic loops belonging to the core proteins protruding into the thylakoid lumen. During biogenesis the majority of the LMW subunits are known to initially associate with individual pre-assembly complexes consisting of one or more of the core proteins; however, the point at which the PsbJ LMW subunit binds to PS II is not known. The majority of models for PS II biogenesis propose that the three extrinsic proteins and PsbJ bind in the final stages of PS II assembly. We have investigated the impact of creating the double mutants ∆PsbJ:∆PsbO, ∆PsbJ:∆PsbU and ∆PsbJ:∆PsbV to investigate potential cooperation between these subunits in the final stages of biogenesis. Our results indicate that PsbJ can bind to PS II in the absence of any one of the extrinsic proteins. However, unlike their respective single mutants, the ∆PsbJ:∆PsbO and ∆PsbJ:∆PsbV strains were not photoautotrophic and were unable to support oxygen evolution suggesting a functional oxygen-evolving complex could not assemble in these strains. In contrast, the PS II centers formed in the ∆PsbJ:∆PsbU strain were capable of photoautotrophic growth and could support oxygen evolution when whole-chain electron transport was supported by the addition of bicarbonate.

Homogeneous environmental selection dominates microbial community assembly in the oligotrophic South Pacific Gyre.

Oligotrophic subtropical gyres are the largest continuous biomes on Earth and play a key role in global biogeochemical cycles. Microbial communities govern primary production and carbon cycling in the oligotrophic South Pacific Gyre, yet the ecological processes which underpin microbial biogeography in the region remain understudied. We investigated microbial biogeography and community assembly processes at three depths over a ~2,000-km the transect was longitudinal, so ran from 32°S, 170°W to 32°S, 152°W). Thus the latitude (32°S) was constant. Microbial communities in the surface waters (15 and 50 m) were remarkably similar across the transect, whilst communities at the deep chlorophyll maximum were distinct from the surface waters and displayed greater compositional heterogeneity. An ecological null model approach indicated that homogeneous selection was the dominant community assembly process in both the surface waters (100%) and at the deep chlorophyll maximum (91.81%), although variable selection (2.34%) and stochastic processes (5.85%) had a minor influence at the deep chlorophyll maximum. Homogeneous selection (76.69%77.90%), dispersal limitation (15.00%-20.05%) and variable selection (3.01%-7.11%) influenced community assembly between the surface waters and the deep chlorophyll maximum. Seawater density and temperature, which were correlated, were the most important environmental modulators of the balance between stochastic and deterministic assembly processes. Our findings demonstrate remarkable similarity in microbial community composition across longitudinal scales in the oligotrophic South Pacific Gyre, underpinned by strong environmental selection which overwhelms the influence of ecological drift. These data significantly advance our understanding of microbial community dynamics in the oligotrophic subtropical gyres which dominate the Earth's surface.

The diversity and distribution of D1 proteins in cyanobacteria.

The psbA gene family in cyanobacteria encodes different forms of the D1 protein that is part of the Photosystem II reaction centre. We have identified a phylogenetically distinct D1 group that is intermediate between previously identified G3-D1 and G4-D1 proteins (Cardona et al. Mol Biol Evol 32:1310-1328, 2015). This new group contained two subgroups: D1INT, which was frequently in the genomes of heterocystous cyanobacteria and D1FR that was part of the far-red light photoacclimation gene cluster of cyanobacteria. In addition, we have identified subgroups within G3, the micro-aerobically expressed D1 protein. There are amino acid changes associated with each of the subgroups that might affect the function of Photosystem II. We show a phylogenetically broad range of cyanobacteria have these D1 types, as well as the genes encoding the G2 protein and chlorophyll f synthase. We suggest identification of additional D1 isoforms and the presence of multiple D1 isoforms in phylogenetically diverse cyanobacteria supports the role of these proteins in conferring a selective advantage under specific conditions.

Effects of multiple drivers of ocean global change on the physiology and functional gene expression of the coccolithophore Emiliania huxleyi.

Ongoing ocean global change due to anthropogenic activities is causing multiple chemical and physical seawater properties to change simultaneously, which may affect the physiology of marine phytoplankton. The coccolithophore Emiliania huxleyi is a model species often employed in the study of the marine carbon cycle. The effect of ocean acidification (OA) on coccolithophore calcification has been extensively studied; however, physiological responses to multiple environmental drivers are still largely unknown. Here we examined two-way and multiple driver effects of OA and other key environmental drivers-nitrate, phosphate, irradiance, and temperature-on the growth, photosynthetic, and calcification rates, and the elemental composition of E. huxleyi. In addition, changes in functional gene expression were examined to understand the molecular mechanisms underpinning the physiological responses. The single driver manipulation experiments suggest decreased nitrate supply being the most important driver regulating E. huxleyi physiology, by significantly reducing the growth, photosynthetic, and calcification rates. In addition, the interaction of OA and decreased nitrate supply (projected for year 2100) had more negative synergistic effects on E. huxleyi physiology than all other two-way factorial manipulations, suggesting a linkage between the single dominant driver (nitrate) effects and interactive effects with other drivers. Simultaneous manipulation of all five environmental drivers to the conditions of the projected year 2100 had the largest negative effects on most of the physiological metrics. Furthermore, functional genes associated with inorganic carbon acquisition (RubisCO, AEL1, and δCA) and calcification (CAX3, AEL1, PATP, and NhaA2) were most downregulated by the multiple driver manipulation, revealing linkages between responses of functional gene expression and associated physiological metrics. These findings together indicate that for more holistic projections of coccolithophore responses to future ocean global change, it is necessary to understand the relative importance of environmental drivers both individually (i.e., mechanistic understanding) and interactively (i.e., cumulative effect) on coccolithophore physiology.

Comparison of D1´- and D1-containing PS II reaction centre complexes under different environmental conditions in Synechocystis sp. PCC 6803.

In oxygenic photosynthesis, the D1 protein of Photosystem II is the primary target of photodamage and environmental stress can accelerate this process. The cyanobacterial response to stress includes transcriptional regulation of genes encoding D1, including low-oxygen-induction of psbA1 encoding the D1´ protein in Synechocystis sp. PCC 6803. The psbA1 gene is also transiently up-regulated in high light, and its deletion has been reported to increase ammonium-induced photoinhibition. Therefore we investigated the role of D1´-containing PS II centres under different environmental conditions. A strain containing only D1´-PS II centres under aerobic conditions exhibited increased sensitivity to ammonium chloride and high light compared to a D1-containing strain. Additionally a D1´-PS II strain was outperformed by a D1-PS II strain under normal conditions; however, a strain containing low-oxygen-induced D1´-PS II centres was more resilient under high light than an equivalent D1 strain. These D1´-containing centres had chlorophyll a fluorescence characteristics indicative of altered forward electron transport and back charge recombination with the donor side of PS II. Our results indicate D1´-PS II centres are important in the reconfiguration of thylakoid electron transport in response to high light and low oxygen.

Characterization of the cyanobacteria and associated bacterial community from an ephemeral wetland in New Zealand.

New Zealand ephemeral wetlands are ecologically important, containing up to 12% of threatened native plant species and frequently exhibiting conspicuous cyanobacterial growth. In such environments, cyanobacteria and associated heterotrophs can influence primary production and nutrient cycling. Wetland communities, including bacteria, can be altered by increased nitrate and phosphate due to agricultural practices. We have characterized cyanobacteria from the Wairepo Kettleholes Conservation Area and their associated bacteria. Use of 16S rRNA amplicon sequencing identified several operational taxonomic units (OTUs) representing filamentous heterocystous and non-heterocystous cyanobacterial taxa. One Nostoc OTU that formed macroscopic colonies dominated the cyanobacterial community. A diverse bacterial community was associated with the Nostoc colonies, including a core microbiome of 39 OTUs. Identity of the core microbiome associated with macroscopic Nostoc colonies was not changed by the addition of nutrients. One OTU was highly represented in all Nostoc colonies (27.6%-42.6% of reads) and phylogenetic analyses identified this OTU as belonging to the genus Sphingomonas. Scanning electron microscopy showed the absence of heterotrophic bacteria within the Nostoc colony but revealed a diverse community associated with the colonies on the external surface.

The importance of the hydrophilic region of PsbL for the plastoquinone electron acceptor complex of Photosystem II.

The PsbL protein is a 4.5kDa subunit at the monomer-monomer interface of Photosystem II (PS II) consisting of a single membrane-spanning domain and a hydrophilic stretch of ~15 residues facing the cytosolic (or stromal) side of the photosystem. Deletion of conserved residues in the N-terminal region has been used to investigate the importance of this hydrophilic extension. Using Synechocystis sp. PCC 6803, three deletion strains: ∆(N6-N8), ∆(P11-V12) and ∆(E13-N15), have been created. The ∆(N6-N8) and ∆(P11-V12) strains remained photoautotrophic but were more susceptible to photodamage than the wild type; however, the ∆(E13-N15) cells had the most severe phenotype. The Δ(E13-N15) mutant showed decreased photoautotrophic growth, a reduced number of PS II centers, impaired oxygen evolution in the presence of PS II-specific electron acceptors, and was highly susceptible to photodamage. The decay kinetics of chlorophyll a variable fluorescence after a single turnover saturating flash and the sensitivity to low concentrations of PS II-directed herbicides in the Δ(E13-N15) strain indicate that the binding of plastoquinone to the QB-binding site had been altered such that the affinity of QB is reduced. In addition, the PS II-specific electron acceptor 2,5-dimethyl-p-benzoquinone was found to inhibit electron transfer through the quinone-acceptor complex of the ∆(E13-N15) strain. The PsbL Y20A mutant was also investigated and it exhibited increased susceptibility to photodamage and increased herbicide sensitivity. Our data suggest that the N-terminal hydrophilic region of PsbL influences forward electron transfer from QA through indirect interactions with the D-E loop of the D1 reaction center protein. Our results further indicate that disruption of interactions between the N-terminal region of PsbL and other PS II subunits or lipids destabilizes PS II dimer formation. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Environmental pH affects photoautotrophic growth of Synechocystis sp. PCC 6803 strains carrying mutations in the lumenal proteins of PSII.

Synechocystis sp. strain PCC 6803 grows photoautotrophically across a broad pH range, but wild-type cultures reach a higher density at elevated pH; however, photoheterotrophic growth is similar at high and neutral pH. A number of PSII mutants each lacking at least one lumenal extrinsic protein, and carrying a second PSII lumenal mutation, are able to grow photoautotrophically in BG-11 medium at pH 10.0, but not pH 7.5. We investigated the basis of this pH effect and observed no pH-specific change in variable fluorescence yield from PSII centers of the wild type or the pH-dependent ΔPsbO:ΔPsbU and ΔPsbV:ΔCyanoQ strains; however, 77 K fluorescence emission spectra indicated increased coupling of the phycobilisome (PBS) antenna at pH 10.0 in all mutants. DNA microarray data showed a cell-wide response to transfer from pH 10.0 to pH 7.5, including decreased mRNA levels of a number of oxidative stress-responsive transcripts. We hypothesize that this transcriptional response led to increased tolerance against reactive oxygen species and in particular singlet oxygen. This response enabled photoautotrophic growth of the PSII mutants at pH 10.0. This hypothesis was supported by increased resistance of all strains to rose bengal at pH 10.0 compared with pH 7.5.

Gene expression indicates a zone of heterocyst differentiation within the thallus of the cyanolichen Pseudocyphellaria crocata.

Development of the symbiotic association in the bipartite lichen Pseudocyphellaria crocata was investigated by characterizing two regions of the thallus. Thallus organization was examined using microscopy. A HIP1-based differential display technique was modified for use on Nostoc strains, including lichenized strains. Northern hybridization and quantitative real-time polymerase chain reaction were used to confirm differential display results, and determine expression levels of key cyanobacterial genes. Photosystem II yield across the thallus was measured using pulse-amplitude modulated fluorescence. Microscopy revealed structural differences in the thallus margins compared with the centre and identified putative heterocysts in both regions. Differential display identified altered transcript levels in both Nostoc punctiforme and a lichenized Nostoc strain. Transcript abundance of cox2, atpA, and ribA was increased in the thallus margin compared with the centre. Expression of cox2 is heterocyst specific and expression of other heterocyst-specific genes (hetR and nifK) was elevated in the margin, whereas, expression of psbB and PSII yield were not. Structural organization of the thallus margin differed from the centre. Both regions contained putative heterocysts but gene expression data indicated increased heterocyst differentiation in the margins where photosystem II yield was decreased. This is consistent with a zone of heterocyst differentiation within the thallus margin.

Gene expression under low-oxygen conditions in the cyanobacterium Synechocystis sp. PCC 6803 demonstrates Hik31-dependent and -independent responses.

We have investigated the response of the cyanobacterium Synechocystis sp. PCC 6803 during growth at very low O2 concentration (bubbled with 99.9 % N(2)/0.1 % CO2). Significant transcriptional changes upon low-O2 incubation included upregulation of a cluster of genes that contained psbA1 and an operon that includes a gene encoding the two-component regulatory histidine kinase, Hik31. This regulatory cluster is of particular interest, since there are virtually identical copies on both the chromosome and plasmid pSYSX. We used a knockout mutant lacking the chromosomal copy of hik31 and studied differential transcription during the aerobic-low-O2 transition in this ΔHik31 strain and the wild-type. We observed two distinct responses to this transition, one Hik31 dependent, the other Hik31 independent. The Hik31-independent responses included the psbA1 induction and genes involved in chlorophyll biosynthesis. In addition, there were changes in a number of genes that may be involved in assembling or stabilizing photosystem (PS)II, and the hox operon and the LexA-like protein (Sll1626) were upregulated during low-O2 growth. This family of responses mostly focused on PSII and overall redox control. There was also a large set of genes that responded differently in the absence of the chromosomal Hik31. In the vast majority of these cases, Hik31 functioned as a repressor and transcription was enhanced when Hik31 was deleted. Genes in this category encoded both core and peripheral proteins for PSI and PSII, the main phycobilisome proteins, chaperones, the ATP synthase cluster and virtually all of the ribosomal proteins. These findings, coupled with the fact that ΔHik31 grew better than the wild-type under low-O2 conditions, suggested that Hik31 helps to regulate growth and overall cellular homeostasis. We detected changes in the transcription of other regulatory genes that may compensate for the loss of Hik31. We conclude that Hik31 regulates an important series of genes that relate to energy production and growth and that help to determine how Synechocystis responds to changes in O2 conditions.

Feedback mechanisms stabilise degraded turf algal systems at a CO2 seep site.

Human activities are rapidly changing the structure and function of coastal marine ecosystems. Large-scale replacement of kelp forests and coral reefs with turf algal mats is resulting in homogenous habitats that have less ecological and human value. Ocean acidification has strong potential to substantially favour turf algae growth, which led us to examine the mechanisms that stabilise turf algal states. Here we show that ocean acidification promotes turf algae over corals and macroalgae, mediating new habitat conditions that create stabilising feedback loops (altered physicochemical environment and microbial community, and an inhibition of recruitment) capable of locking turf systems in place. Such feedbacks help explain why degraded coastal habitats persist after being initially pushed past the tipping point by global and local anthropogenic stressors. An understanding of the mechanisms that stabilise degraded coastal habitats can be incorporated into adaptive management to better protect the contribution of coastal systems to human wellbeing.

Subtle bacterioplankton community responses to elevated CO2 and warming in the oligotrophic South Pacific gyre.

Bacterioplankton play a critical role in primary production, carbon cycling, and nutrient cycling in the oligotrophic ocean. To investigate the effect of elevated CO2 and warming on the composition and function of bacterioplankton communities in oligotrophic waters, we performed two trace-metal clean deck board incubation experiments during the New Zealand GEOTRACES transect of the South Pacific gyre (SPG). High-throughput amplicon sequencing of the 16S rRNA gene revealed that bacterioplankton community composition was distinct between the fringe and ultra-oligotrophic centre of the SPG and changed consistently in response to elevated CO2 at the ultra-oligotrophic centre but not at the mesotrophic fringe of the SPG. The combined effects of elevated CO2 and warming resulted in a high degree of heterogeneity between replicate communities. Community-level protein synthesis rates (3 H-Leucine incorporation) and bacterioplankton abundance were not affected by elevated CO2 alone or in combination with warming at the fringe or ultra-oligotrophic centre of the SPG. These data suggest bacterioplankton community responses to elevated CO2 may be modulated by nutrient regimes in open ocean ecosystems and highlight the need for further investigation in expanding oligotrophic subtropical gyres.

Fungal diversity in canopy soil of silver beech, Nothofagus menziesii (Nothofagaceae).

Adventitious roots in canopy soils associated with silver beech (Nothofagus menziesii Hook.f. (Nothofagaceae)) form ectomycorrhizal associations. We investigated the extent to which canopy ectomycorrhizal communities contribute to overall diversity of ectomycorrhizal fungi associated with silver beech. Hyphal ingrowth bags were buried for 12 months in canopy and terrestrial soils of five trees at one site. We used amplicon sequencing of the nuclear ribosomal internal transcribed spacer 2 region (ITS2) to assess diversity of both ectomycorrhizal and non-ectomycorrhizal OTUs in hyphal ingrowth bags. There was a significant difference in ectomycorrhizal fungal community diversity between the terrestrial and canopy hyphal ingrowth bag communities. Ectomycorrhizal community composition of the terrestrial and canopy environments was also significantly different. Some ectomycorrhizal taxa were significantly differentially represented in either the terrestrial or canopy environment. The hyphal ingrowth bags also accumulated non-ectomycorrhizal species. The non-ectomycorrhizal fungi also had significantly different diversity and community composition between the canopy and terrestrial environments. Like the ectomycorrhizal community, some non-ectomycorrhizal taxa were significantly differentially represented in either the terrestrial or canopy environment. The canopy soil microhabitat provides a novel environment for growth of ectomycorrhizal adventitious roots and enables the spatial partitioning of ectomycorrhizal and non-ectomycorrhizal fungal diversity in the forest.

Stabilization of Photosystem II by the PsbT protein impacts photodamage, repair and biogenesis.

Photosystem II (PS II) catalyzes the light-driven process of water splitting in oxygenic photosynthesis. Four core membrane-spanning proteins, including D1 that binds the majority of the redox-active co-factors, are surrounded by 13 low-molecular-weight (LMW) proteins. We previously observed that deletion of the LMW PsbT protein in the cyanobacterium Synechocystis sp. PCC 6803 slowed electron transfer between the primary and secondary plastoquinone electron acceptors QA and QB and increased the susceptibility of PS II to photodamage. Here we show that photodamaged ∆PsbT cells exhibit unimpaired rates of oxygen evolution if electron transport is supported by HCO3- even though the cells exhibit negligible variable fluorescence. We find that the protein environment in the vicinity of QA and QB is altered upon removal of PsbT resulting in inhibition of QA- oxidation in the presence of 2,5-dimethyl-1,4-benzoquinone, an artificial PS II-specific electron acceptor. Thermoluminescence measurements revealed an increase in charge recombination between the S2 oxidation state of the water-oxidizing complex and QA- by the indirect radiative pathway in ∆PsbT cells and this is accompanied by increased 1O2 production. At the protein level, both D1 removal and replacement, as well as PS II biogenesis, were accelerated in the ∆PsbT strain. Our results demonstrate that PsbT plays a key role in optimizing the electron acceptor complex of the acceptor side of PS II and support the view that repair and biogenesis of PS II share an assembly pathway that incorporates both de novo synthesis and recycling of the assembly modules associated with the core membrane-spanning proteins.

Purple haze: Cryptic purple sequestrate Cortinarius in New Zealand.

CORTINARIUS: is a species-rich ectomycorrhizal genus containing taxa that exhibit agaricoid or sequestrate basidiome morphologies. In New Zealand, one of the most recognizable and common Cortinarius species is the purple sequestrate fungus, C. porphyroideus. We used genome skimming of the almost 100-y-old type specimen from C. porphyroideus to obtain the nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 (ITS barcode) and partial nuc rDNA 28S (28S) sequences. The phylogenetic position of C. porphyroideus was established, and we found that it represents a rarely collected species. Purple sequestrate Cortinarius comprise multiple cryptic species in several lineages. We describe four new species of Cortinarius with strong morphological similarity to C. porphyroideus: Cortinarius diaphorus, C. minorisporus, C. purpureocapitatus, and C. violaceocystidiatus. Based on molecular evidence, Thaxterogaster viola is recognized as Cortinarius violaceovolvatus var. viola. These species are associated with Nothofagus (southern beech) and have very similar morphology to C. porphyroideus but are all phylogenetically distinct based on molecular data.

Multiple-stressor effects of dicyandiamide (DCD) and agricultural stressors on trait-based responses of stream benthic algal communities.

Agricultural practices often result in multiple stressors affecting stream ecosystems, and interacting stressors complicate environmental assessment and management of impacted streams. The nitrification inhibitor dicyandiamide (DCD) is used for nitrogen management on farmland. Effects of leached DCD on stream ecosystems are still largely unstudied, even though it could be relevant as a stressor on its own or in combination with other agricultural stressors. We conducted two experiments in 128 outdoor stream-fed mesocosms to assess stressor effects on biomass, cell density, taxon richness, evenness and functional trait composition of benthic algal communities. First, we examined responses to a wide DCD gradient (eight concentrations, 0-31 mg L-1) and two additional stressors, deposited fine sediment (none, high) and nutrient enrichment (ambient, enriched). Second, we determined algal responses to four stressors: DCD, sediment, nutrients, and reduced flow velocity. Here DCD treatments included controls, constant application (1.4 mg L-1) and two pulsed treatments mimicking concentration patterns in real streams (peaks 3.5 mg L-1, 2.2 mg L-1). Sediment and nutrient enrichment were influential stressors in both experiments, with fine sediment having the most pervasive effects. In Experiment 2, reduced flow velocity had pervasive effects and stressor interactions were mainly restricted to two-way interactions. DCD had few, weak stressor main effects, especially at field-realistic concentrations (Experiment 2). At the highest concentrations in Experiment 1 (above levels observed in real streams), DCD effects were still rare but some significant stressor interactions occurred. Analyses of functional traits were helpful in identifying potential mechanisms driving changes in densities and community composition. These findings suggest that, while DCD on its own may be a minor stressor, it could have adverse effects on algal communities already exposed to other stressors, a scenario common in agricultural streams.