The relationship between cnidarians and their micro-algal symbionts is crucial for normal animal function and the formation of coral reefs. We used the sea anemone Exaiptasia pallida (Aiptasia) as a model cnidarian-dinoflagellate system to determine the effects of white, blue and red light on photo-movement. In white light, phototropism and phototaxis of Aiptasia were dependent on the presence of symbionts; anemones with symbionts bent and moved toward the light, whereas aposymbiotic anemones (lacking algal symbionts) moved, but without strong directionality. Phototaxis and phototropism also occurred in blue light, but to a lesser extent than in white light, with no apparent response to red light. Phototactic behavior was also sensitive to the specific anemone-symbiont pairing. The ability to sense and move in response to light would presumably allow for selection of favorable habitats. Overall, this study demonstrates that the algal symbiont is required for photo-movement of the host and that the extent of movement is influenced by the different anemone-symbiont associations.

CRISPR arrays and associated cas genes are widespread in bacteria and archaea and confer acquired resistance to viruses. To examine viral immunity in the context of naturally evolving microbial populations we analyzed genomic data from two thermophilic Synechococcus isolates (Syn OS-A and Syn OS-B9) as well as a prokaryotic metagenome and viral metagenome derived from microbial mats in hotsprings at Yellowstone National Park. Two distinct CRISPR types, distinguished by the repeat sequence, are found in both the Syn OS-A and Syn OS-B9 genomes. The genome of Syn OS-A contains a third CRISPR type with a distinct repeat sequence, which is not found in Syn OS-B9, but appears to be shared with other microorganisms that inhabit the mat. The CRISPR repeats identified in the microbial metagenome are highly conserved, while the spacer sequences (hereafter referred to as "viritopes'' to emphasize their critical role in viral immunity) were mostly unique and had no high identity matches when searched against GenBank. Searching the viritopes against the viral metagenome, however, yielded several matches with high similarity some of which were within a gene identified as a likely viral lysozyme/lysin protein. Analysis of viral metagenome sequences corresponding to this lysozyme/lysin protein revealed several mutations all of which translate into silent or conservative mutations which are unlikely to affect protein function, but may help the virus evade the host CRISPR resistance mechanism. These results demonstrate the varied challenges presented by a natural virus population, and support the notion that the CRISPR/viritope system must be able to adapt quickly to provide host immunity. The ability of metagenomics to track population-level variation in viritope sequences allows for a culture-independent method for evaluating the fast co-evolution of host and viral genomes and its consequence on the structuring of complex microbial communities.

Polyphosphate, an energy-rich polymer conserved in all kingdoms of life, is integral to many cellular stress responses, including nutrient deprivation, and yet, the mechanisms that underlie its biological roles are not well understood. In this work, we elucidate the physiological function of this polymer in the acclimation of the model alga Chlamydomonas reinhardtii to nutrient deprivation. Our data reveal that polyphosphate synthesis is vital to control cellular adenosine 5'-triphosphate homeostasis and maintain both respiratory and photosynthetic electron transport upon sulfur deprivation. Using both genetic and pharmacological approaches, we show that electron flow in the energy-generating organelles is essential to induce and sustain acclimation to sulfur deprivation at the transcriptional level. These previously unidentified links among polyphosphate synthesis, photosynthetic and respiratory electron flow, and the acclimation of cells to nutrient deprivation could unveil the mechanism by which polyphosphate helps organisms cope with a myriad of stress conditions in a fluctuating environment.

Hosting different symbiont species can affect inter-partner nutritional fluxes within the cnidarian-dinoflagellate symbiosis. Using nanoscale secondary ion mass spectrometry (NanoSIMS), we measured the spatial incorporation of photosynthetically fixed(13)C and heterotrophically derived(15)N into host and symbiont cells of the model symbiotic cnidarian Aiptasia (Exaiptasia pallida) when colonized with its native symbiontBreviolum minutumor the non-nativeDurusdinium trenchii.Breviolum minutumexhibited high photosynthetic carbon assimilationpercell and translocation to host tissue throughout symbiosis establishment, whereasD. trenchiiassimilated significantly less carbon, but obtained more host nitrogen. These findings suggest thatD. trenchiihas less potential to provide photosynthetically fixed carbon to the host despite obtaining considerable amounts of heterotrophically derived nitrogen. These sub-cellular events help explain previous observations that demonstrate differential effects ofD. trenchiicompared toB. minutumon the host transcriptome, proteome, metabolome and host growth and asexual reproduction. Together, these differential effects suggest that the non-native host-symbiont pairing is sub-optimal with respect to the host's nutritional benefits under normal environmental conditions. This contributes to our understanding of the ways in which metabolic integration impacts the benefits of a symbiotic association, and the potential evolution of novel host-symbiont pairings.

Synechococcus sp. represents an ecologically diverse group of cyanobacteria found in numerous environments, including hot-spring microbial mats, where they are spatially distributed along thermal, light and oxygen gradients. These thermophiles engage in photosynthesis and aerobic respiration during the day, but switch to fermentative metabolism and nitrogen fixation at night. The genome of Synechococcus OS-B', isolated from Octopus Spring (Yellowstone National Park) contains a phn gene cluster encoding a phosphonate (Phn) transporter and a C-P lyase. A closely related isolate, Synechococcus OS-A, lacks this cluster, but contains genes encoding putative phosphonatases (Phnases) that appear to be active only in the presence of the Phn substrate. Both isolates grow well on several different Phns as a sole phosphorus (P) source. Interestingly, Synechococcus OS-B' can use the organic carbon backbones of Phns for heterotrophic growth in the dark, whereas in the light this strain releases organic carbon from Phn as ethane or methane (depending on the specific Phn available); Synechococcus OS-A has neither of these capabilities. These differences in metabolic strategies for assimilating the P and C of Phn by two closely related Synechococcus spp. are suggestive of niche-specific constraints in the evolution of nutrient assimilation pathways and syntrophic relationships among the microbial populations of the hotspring mats. Thus, it is critical to evaluate levels of various P sources, including Phn, in thermally active habitats and the potential importance of these compounds in the biogeochemical cycling of P and C (some Phn compounds also contain N) in diverse terrestrial environments. The ISME Journal (2011) 5, 141-149; doi:10.1038/ismej.2010.96; published online 15 July 2010

The uptake and conversion of a free-living cyanobacterium into a photosynthetic organelle by the single-celled Archaeplastida ancestor helped transform the biosphere from low to high oxygen. There are two documented, independent cases of plastid primary endosymbiosis. The first is the well-studied instance in Archaeplastida that occurred ca. 1.6 billion years ago, whereas the second occurred 90-140 million years ago, establishing a permanent photosynthetic compartment (the chromatophore) in amoebae in the genus Paulinella. Here, we briefly summarize knowledge about plastid origin in the Archaeplastida and then focus on Paulinella. In particular, we describe features of the Paulinella chromatophore that make it a model for examining earlier events in the evolution of photosynthetic organelles. Our review stresses recently gained insights into the evolution of chromatophore and nuclear encoded DNA sequences in Paulinella, metabolic connectivity between the endosymbiont and cytoplasm, and systems that target proteins into the chromatophore. We also describe future work with Paulinella, and the potential rewards and challenges associated with developing further this model system.

The relative abundance of transcripts encoding proteins involved in inorganic carbon concentrating mechanisms (CCM), detoxification of reactive oxygen species (ROS) and photosynthesis in the thermophilic cyanobacterium Synechococcus OS-B' was measured in hot spring microbial mats over two diel cycles, and was coupled with in situ determinations of incoming irradiance and microenvironmental dynamics of O-2 and pH. Fluctuations in pH and O-2 in the mats were largely driven by the diel cycle of solar irradiance, with a pH variation from similar to 7.0 to similar to 9.5, and O-2 levels ranging from anoxia to supersaturation during night and day, respectively. Levels of various transcripts from mat cyanobacteria revealed several patterns that correlated with incident irradiance, O-2 and pH within the mat matrix. Transcript abundances for most genes increased during the morning dark-light transition. Some transcripts remained at a near constant level throughout the light period, whereas others showed an additional increase in abundance as the mat underwent transition from low-to-high light (potentially reflecting changes in O-2 concentration and pH), followed by either a decreased abundance in the early afternoon, or a gradual decline during the early afternoon and into the evening. One specific transcipt, psbA1, was the lowest during mid-day under high irradiance and increased when the light levels declined. We discuss these complex in situ transcriptional patterns with respect to environmental and endogenous cues that might impact and regulate transcription over the diel cycle. The ISME Journal (2011) 5, 317-328; doi:10.1038/ismej.2010.131; published online 26 August 2010

Dinoflagellates in the family Symbiodiniaceae can live freely in ocean waters or form a symbiosis with a variety of cnidarians including corals, sea anemones, and jellyfish. Trophic plasticity of Symbiodiniaceae is critical to its ecological success as it moves between environments. However, the molecular mechanisms underlying these trophic shifts in Symbiodiniaceae are still largely unknown. Using Breviolum minutum strain SSB01 (designated SSB01) as a model, we showed that Symbiodiniaceae go through a physiological and transcriptome reprogramming when the alga is grown with the organic nitrogen containing nutrients in hydrolyzed casein, but not with inorganic nutrients. SSB01 grows at a much faster rate and maintains stable photosynthetic efficiency when supplemented with casein amino acids compared to only inorganic nutrients or seawater. These physiological changes are driven by massive transcriptome changes in SSB01 supplemented with casein amino acids. The levels of transcripts encoding proteins involved in altering DNA conformation such as DNA topoisomerases, histones, and chromosome structural components were all significantly changed. Functional enrichment analysis also revealed processes involved in translation, ion transport, generation of second messengers, and phosphorylation. The physiological and molecular changes that underlie in vitro trophic transitions in Symbiodiniaceae can serve as an orthogonal platform to further understand the factors that impact the Symbiodiniaceae lifestyle.