Metabolism of key atmospheric volatile organic compounds by the marine heterotrophic bacterium Pelagibacter HTCC1062 (SAR11).

Plants and phytoplankton are natural sources of the volatile organic compounds (VOCs) acetone and isoprene, which are reactive and can alter atmospheric chemistry. In earlier research we reported that, when co-cultured with a diatom, the marine bacterium Pelagibacter (strain HTCC1062; 'SAR11 clade') reduced the concentration of compounds tentatively identified as acetone and isoprene. In this study, experiments with Pelagibacter monocultures confirmed that these cells are capable of metabolizing acetone and isoprene at rates similar to bacterial communities in seawater and high enough to consume substantial fractions of the total marine acetone and isoprene budgets if extrapolated to global SAR11 populations. Homologues of an acetone/cyclohexanone monooxygenase were identified in the HTCC1062 genome and in the genomes of a wide variety of other abundant marine taxa, and were expressed at substantial levels (c. 10-4 of transcripts) across TARA oceans metatranscriptomes from ocean surface samples. The HTCC1062 genome lacks the canonical isoprene degradation pathway, suggesting an unknown alternative biochemical pathway is used by these cells for isoprene uptake. Fosmidomycin, an inhibitor of bacterial isoprenoid biosynthesis, blocked HTCC1062 growth, but the cells were rescued when isoprene was added to the culture, indicating SAR11 cells may be capable of synthesizing isoprenoid compounds from exogenous isoprene.

Rapid photoacclimation during episodic deep mixing augments the biological carbon pump.

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (C phyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean.

Pelagibacter metabolism of diatom-derived volatile organic compounds imposes an energetic tax on photosynthetic carbon fixation.

Volatile organic compounds (VOCs) produced by phytoplankton are molecules with high vapor pressures that can diffuse across cell membranes into the environment, where they become public goods. VOCs likely comprise a significant component of the marine dissolved organic carbon (DOC) pool utilized by microorganisms, but they are often overlooked as growth substrates because their diffusivity imposes analytical challenges. The roles of VOCs in the growth of the photoautotrophic diatom Thalassiosira pseudonana and heterotrophic bacterium Pelagibacter sp. HTCC1062 (SAR11) were examined using co-cultures and proton-transfer reaction time-of-flight mass spectrometry. VOCs at 82 m/z values were produced in the cultures, and the concentrations of 9 of these m/z values changed in co-culture relative to the diatom monoculture. Several of the m/z values were putatively identified, and their metabolism by HTCC1062 was confirmed by measuring ATP production. Diatom carbon fixation rates in co-culture with HTCC1062 were 20.3% higher than the diatom monoculture. Removal of VOCs from the T. pseudonana monoculture using a hydrocarbon trap caused a similar increase in carbon fixation (18.1%). These results show that a wide range of VOCs are cycled in the environment, and the flux of VOCs from phytoplankton to bacterioplankton imposes a large and unexpected tax on phytoplankton photosynthesis.

Carbon use efficiencies and allocation strategies in Prochlorococcus marinus strain PCC 9511 during nitrogen-limited growth.

We studied cell properties including carbon allocation dynamics in the globally abundant and important cyanobacterium Prochlorococcus marinus strain PCC 9511 grown at three different growth rates in nitrogen-limited continuous cultures. With increasing nitrogen limitation, cellular divinyl chlorophyll a and the functional absorption cross section of Photosystem II decreased, although maximal photosynthetic efficiency of PSII remained unaltered across all N-limited growth rates. Chl-specific gross and net carbon primary production were also invariant with nutrient-limited growth rate, but only 20% of Chl-specific gross carbon primary production was retained in the biomass across all growth rates. In nitrogen-replete cells, 60% of the assimilated carbon was incorporated into the protein pool while only 30% was incorporated into carbohydrates. As N limitation increased, new carbon became evenly distributed between these two pools. While many of these physiological traits are similar to those measured in other algae, there are also distinct differences, particularly the lower overall efficiency of carbon utilization. The latter provides new information needed for understanding and estimating primary production, particularly in the nutrient-limited tropical oceans where P. marinus dominates phytoplankton community composition.

Mechanisms that increase the growth efficiency of diatoms in low light.

Photoacclimation was studied in Thalassiosira pseudonana to help understand mechanisms underlying the success of diatoms in low-light environments, such as coastal and deep mixing ecosystems. Light harvesting and other cell characteristics were combined with oxygen and carbon production measurements to assess the water-splitting reaction at PSII ([Formula: see text]) and intermediate steps leading to net carbon production (NPPC). These measurements revealed that T. pseudonana is remarkably efficient at converting harvested light energy into biomass, with at least 57 % of [Formula: see text] retained as NPPC across all light-limited growth rates examined. Evidence for upregulation of ATP generation pathways that circumvent carbon fixation indicated that high growth efficiency at low light levels was at least partly due to increases in the efficiency of ATP production. Growth rate-dependent demands for ATP and NADPH were reflected in carbon composition and in unexpected shifts in the light-limited slope (α) of photosynthesis-irradiance relationships generated from chlorophyll-specific (14)C-uptake. Overall, these results suggest that pathway gating of carbon and energy flow depends on light availability and is a key factor promoting the efficiency of diatom growth at low light intensities.

A common partitioning strategy for photosynthetic products in evolutionarily distinct phytoplankton species.

· We compare the nutrient-dependent photosynthetic efficiencies of the chlorophyte, Dunaliella tertiolecta, with those of the marine diatom, Thalassiosira weissflogii. Despite considerable evolutionary and physiological differences, these two species appear to use nearly identical growth strategies under a wide range of nutrient limitation. · Using a variety of physiological measurements, we find that, for both species and across all growth rates, 75% of the gross photosynthetic electron flow is invested in carbon fixation and only 30% is retained as net carbon accumulation. A majority of gross photosynthesis (70%) is ultimately used as reductant for biosynthetic pathways and for the generation of ATP. · In both species, newly formed carbon products exhibit much shorter half-lives at slow growth rates than at fast growth rates. We show that this growth rate dependence is a result of increased polysaccharide storage during the S phase of the cell cycle. · We present a model of carbon utilization that incorporates this growth rate-dependent carbon allocation and accurately captures (r(2) = 0.94) the observed time-resolved carbon retention. Together, our findings suggest a common photosynthetic optimization strategy in evolutionarily distinct phytoplankton species and contribute towards a systems-level understanding of carbon flow in photoautotrophs.

The volatilome reveals microcystin concentration, microbial composition, and oxidative stress in a critical Oregon freshwater lake.

IMPORTANCE: Harmful algal blooms are among the most significant threats to drinking water safety. Blooms dominated by cyanobacteria can produce potentially harmful toxins and, despite intensive research, toxin production remains unpredictable. We measured gaseous molecules in Upper Klamath Lake, Oregon, over 2 years and used them to predict the presence and concentration of the cyanotoxin, microcystin, and microbial community composition. Subsets of gaseous compounds were identified that are associated with microcystin production during oxidative stress, pointing to ecosystem-level interactions leading to microcystin contamination. Our approach shows potential for gaseous molecules to be harnessed in monitoring critical waterways.

Altered growth and death in dilution-based viral predation assays.

Viral lysis of phytoplankton is one of the most common forms of death on Earth. Building on an assay used extensively to assess rates of phytoplankton loss to predation by grazers, lysis rates are increasingly quantified through dilution-based techniques. In this approach, dilution of viruses and hosts are expected to reduce infection rates and thus increase host net growth rates (i.e., accumulation rates). The difference between diluted and undiluted host growth rates is interpreted as a measurable proxy for the rate of viral lytic death. These assays are usually conducted in volumes ≥ 1 L. To increase throughput, we implemented a miniaturized, high-throughput, high-replication, flow cytometric microplate dilution assay to measure viral lysis in environmental samples sourced from a suburban pond and the North Atlantic Ocean. The most notable outcome we observed was a decline in phytoplankton densities that was exacerbated by dilution, instead of the increased growth rates expected from lowered virus-phytoplankton encounters. We sought to explain this counterintuitive outcome using theoretical, environmental, and experimental analyses. Our study shows that, while die-offs could be partly explained by a 'plate effect' due to small incubation volumes and cells adhering to walls, the declines in phytoplankton densities are not volume-dependent. Rather, they are driven by many density- and physiology-dependent effects of dilution on predation pressure, nutrient limitation, and growth, all of which violate the original assumptions of dilution assays. As these effects are volume-independent, these processes likely occur in all dilution assays that our analyses show to be remarkably sensitive to dilution-altered phytoplankton growth and insensitive to actual predation pressure. Incorporating altered growth as well as predation, we present a logical framework that categorizes locations by the relative dominance of these mechanisms, with general applicability to dilution-based assays.

Synergistic metabolism of a broad range of C1 compounds in the marine methylotrophic bacterium HTCC2181.

The 1.3 Mbp genome of HTCC2181, a member of the abundant OM43 clade of coastal bacterioplankton, suggested it is an obligate methylotroph. Preliminary experiments demonstrated that methanol and formaldehyde, but not other common C1 compounds such as methylamine, could support growth. Methanol concentrations in seawater are reportedly < 100 nM, suggesting either that the flux of methanol through plankton pools is very rapid, or that methanol may not be the primary growth substrate for HTCC2181. Therefore, we investigated the apparent extreme substrate range restriction of HTCC2181 in greater detail. Growth rate and maximum cell density of HTCC2181 increased with methanol concentration, yielding a K(s) value of 19 µM. In contrast, no growth was observed in the presence of the methylated (C1) compounds, methyl chloride, trimethylamine-oxide (TMAO) or dimethylsulfoniopropionate (DMSP) when they were the sole substrates. However, growth rate, maximum cell density and cellular ATP content were significantly enhanced when any of these methylated compounds were provided in the presence of a limiting concentration of methanol. These observations fit a model in which the metabolic intermediate formaldehyde is required for net carbon assimilation, allowing C1 substrates that do not produce a formaldehyde intermediate to be oxidized for energy, but not assimilated into biomass. Rates of methanol and TMAO oxidation and assimilation were measured with (14)C-radiolabelled compounds in cultures of HTCC2181 and seawater microbial communities collected off the Oregon coast. The results indicated that in nature as well as in culture, C1 substrates are partitioned between those that are mainly oxidized to produce energy and those that are assimilated. These findings indicate that the combined fluxes of C1 compounds in coastal systems are sufficient to support significant populations of obligate methyltrophs by a metabolic strategy that involves the synergistic metabolism of multiple C1 compounds.

Phytoplankton community structuring and succession in a competition-neutral resource landscape.

Phytoplankton community composition and succession affect aquatic food webs and biogeochemistry. Resource competition is commonly viewed as an important governing factor for community structuring and this perception is imbedded in modern ecosystem models. Quantitative consideration of the physical spacing between phytoplankton cells, however, suggests that direct competition for growth-limiting resources is uncommon. Here we describe how phytoplankton size distributions and temporal successions are compatible with a competition-neutral resource landscape. Consideration of phytoplankton-herbivore interactions with proportional feeding size ranges yields small-cell dominated size distributions consistent with observations for stable aquatic environments, whereas predator-prey temporal lags and blooming physiologies shift this distribution to larger mean cell sizes in temporally dynamic environments. We propose a conceptual mandala for understanding phytoplankton community composition where species successional series are initiated by environmental disturbance, guided by the magnitude of these disturbances and nutrient stoichiometry, and terminated with the return toward a 'stable solution'. Our conceptual mandala provides a framework for interpreting and modeling the environmental structuring of natural phytoplankton populations.

In Situ Estimates of Net Primary Production in the Western North Atlantic With Argo Profiling Floats.

The 14C incubation method for net primary production (NPP) has limited spatial/temporal resolution, while satellite approaches cannot provide direct information at depth. With chlorophyll-a and backscatter measurements from BGC-Argo floats, we quantified year-round NPP in the western North Atlantic Ocean using both the Carbon-based Productivity Model (CbPM) and Photoacclimation Productivity Model (PPM). Comparison with NPP profiles from 14C incubation measurements showed advantages and limitations of both models. CbPM reproduced the magnitude of NPP in most cases. However, in the summer the CbPM-based NPP had a large peak in the subsurface, which was an artifact from the subsurface chlorophyll maximum caused by photoacclimation. PPM avoided the artifacts from photoacclimation, but the magnitude of PPM-derived NPP was smaller than the 14C result. Different NPP distribution patterns along a North-South transect in the Western North Atlantic Ocean were observed, including higher winter NPP/lower summer NPP in the south, timing differences in NPP seasonal phenology, and different NPP depth distribution patterns in the summer months. Using a 6-months record of concurrent oxygen and bio-optical measurements from two Argo floats, we also demonstrated the ability of Argo floats to obtain estimates of the net community production to NPP ratio, ranging from 0.3 in July to -1.0 in December 2016. Our results highlight the utility of float bio-optical profiles and indicate that environmental conditions (e.g., light availability, nutrient supply) are major factors controlling the seasonality and spatial (horizontal and vertical) distributions of NPP in the western North Atlantic Ocean.

The Seasonal Flux and Fate of Dissolved Organic Carbon Through Bacterioplankton in the Western North Atlantic.

The oceans teem with heterotrophic bacterioplankton that play an appreciable role in the uptake of dissolved organic carbon (DOC) derived from phytoplankton net primary production (NPP). As such, bacterioplankton carbon demand (BCD), or gross heterotrophic production, represents a major carbon pathway that influences the seasonal accumulation of DOC in the surface ocean and, subsequently, the potential vertical or horizontal export of seasonally accumulated DOC. Here, we examine the contributions of bacterioplankton and DOM to ecological and biogeochemical carbon flow pathways, including those of the microbial loop and the biological carbon pump, in the Western North Atlantic Ocean (∼39-54°N along ∼40°W) over a composite annual phytoplankton bloom cycle. Combining field observations with data collected from corresponding DOC remineralization experiments, we estimate the efficiency at which bacterioplankton utilize DOC, demonstrate seasonality in the fraction of NPP that supports BCD, and provide evidence for shifts in the bioavailability and persistence of the seasonally accumulated DOC. Our results indicate that while the portion of DOC flux through bacterioplankton relative to NPP increased as seasons transitioned from high to low productivity, there was a fraction of the DOM production that accumulated and persisted. This persistent DOM is potentially an important pool of organic carbon available for export to the deep ocean via convective mixing, thus representing an important export term of the biological carbon pump.

Temporal Patterns and Intra- and Inter-Cellular Variability in Carbon and Nitrogen Assimilation by the Unicellular Cyanobacterium Cyanothece sp. ATCC 51142.

Unicellular nitrogen fixing cyanobacteria (UCYN) are abundant members of phytoplankton communities in a wide range of marine environments, including those with rapidly changing nitrogen (N) concentrations. We hypothesized that differences in N availability (N2 vs. combined N) would cause UCYN to shift strategies of intracellular N and C allocation. We used transmission electron microscopy and nanoscale secondary ion mass spectrometry imaging to track assimilation and intracellular allocation of 13C-labeled CO2 and 15N-labeled N2 or NO3 at different periods across a diel cycle in Cyanothece sp. ATCC 51142. We present new ideas on interpreting these imaging data, including the influences of pre-incubation cellular C and N contents and turnover rates of inclusion bodies. Within cultures growing diazotrophically, distinct subpopulations were detected that fixed N2 at night or in the morning. Additional significant within-population heterogeneity was likely caused by differences in the relative amounts of N assimilated into cyanophycin from sources external and internal to the cells. Whether growing on N2 or NO3, cells prioritized cyanophycin synthesis when N assimilation rates were highest. N assimilation in cells growing on NO3 switched from cyanophycin synthesis to protein synthesis, suggesting that once a cyanophycin quota is met, it is bypassed in favor of protein synthesis. Growth on NO3 also revealed that at night, there is a very low level of CO2 assimilation into polysaccharides simultaneous with their catabolism for protein synthesis. This study revealed multiple, detailed mechanisms underlying C and N management in Cyanothece that facilitate its success in dynamic aquatic environments.

Physiological optimization underlies growth rate-independent chlorophyll-specific gross and net primary production.

Characterization of physiological variability in phytoplankton photosynthetic efficiencies is one of the greatest challenges in assessing ocean net primary production (NPP) from remote sensing of surface chlorophyll (Chl). Nutrient limitation strongly influences phytoplankton intracellular pigmentation, but its impact on Chl-specific NPP (NPP(*)) is debated. We monitored six indices of photosynthetic activity in steady-state Dunaliella tertiolecta cultures over a range of nitrate-limited growth rates (µ), including photosynthetic efficiency of PSII (F(v)/F(m)), O(2)-based gross and net production, 20 min and 24 h carbon assimilation, and carbon- and µ-based NPP. Across all growth rates, O(2)-based Chl-specific gross primary production (GPP(*)(O(2))), NPP(*), and F(v)/F(m) were constant. GPP(*)(O(2)) was 3.3 times greater than NPP(*). In stark contrast, Chl-specific short-term C fixation showed clear linear dependence on µ, reflecting differential allocation of photosynthate between short-lived C products and longer-term storage products. Indeed, (14)C incorporation into carbohydrates was five times greater in cells growing at 1.2 day(-1) than 0.12 day(-1). These storage products are catabolized for ATP and reductant generation within the period of a cell cycle. The relationship between Chl-specific gross and net O(2) production, short-term (14)C-uptake, NPP(*), and growth rate reflects cellular-level regulation of fundamental metabolic pathways in response to nutrient limitation. We conclude that growth rate-dependent photosynthate metabolism bridges the gap between gross and net production and resolves a controversial question regarding nutrient limitation effects on primary production measures.

Divergence of photosynthetic strategies amongst marine diatoms.

Marine phytoplankton, and in particular diatoms, are responsible for almost half of all primary production on Earth. Diatom species thrive from polar to tropical waters and across light environments that are highly complex to relatively benign, and so have evolved highly divergent strategies for regulating light capture and utilization. It is increasingly well established that diatoms have achieved such successful ecosystem dominance by regulating excitation energy available for generating photosynthetic energy via highly flexible light harvesting strategies. However, how different light harvesting strategies and downstream pathways for oxygen production and consumption interact to balance excitation pressure remains unknown. We therefore examined the responses of three diatom taxa adapted to inherently different light climates (estuarine Thalassioisira weissflogii, coastal Thalassiosira pseudonana and oceanic Thalassiosira oceanica) during transient shifts from a moderate to high growth irradiance (85 to 1200 µmol photons m-2 s-1). Transient high light exposure caused T. weissflogii to rapidly downregulate PSII with substantial nonphotochemical quenching, protecting PSII from inactivation or damage, and obviating the need for induction of O2 consuming (light-dependent respiration, LDR) pathways. In contrast, T. oceanica retained high excitation pressure on PSII, but with little change in RCII photochemical turnover, thereby requiring moderate repair activity and greater reliance on LDR. T. pseudonana exhibited an intermediate response compared to the other two diatom species, exhibiting some downregulation and inactivation of PSII, but high repair of PSII and induction of reversible PSII nonphotochemical quenching, with some LDR. Together, these data demonstrate a range of strategies for balancing light harvesting and utilization across diatom species, which reflect their adaptation to sustain photosynthesis under environments with inherently different light regimes.

Temperate infection in a virus-host system previously known for virulent dynamics.

The blooming cosmopolitan coccolithophore Emiliania huxleyi and its viruses (EhVs) are a model for density-dependent virulent dynamics. EhVs commonly exhibit rapid viral reproduction and drive host death in high-density laboratory cultures and mesocosms that simulate blooms. Here we show that this system exhibits physiology-dependent temperate dynamics at environmentally relevant E. huxleyi host densities rather than virulent dynamics, with viruses switching from a long-term non-lethal temperate phase in healthy hosts to a lethal lytic stage as host cells become physiologically stressed. Using this system as a model for temperate infection dynamics, we present a template to diagnose temperate infection in other virus-host systems by integrating experimental, theoretical, and environmental approaches. Finding temperate dynamics in such an established virulent host-virus model system indicates that temperateness may be more pervasive than previously considered, and that the role of viruses in bloom formation and decline may be governed by host physiology rather than by host-virus densities.

A Parasitic Arsenic Cycle That Shuttles Energy from Phytoplankton to Heterotrophic Bacterioplankton.

In many regions of the world oceans, phytoplankton face the problem of discriminating between phosphate, an essential nutrient, and arsenate, a toxic analogue. Many phytoplankton, including the most abundant phytoplankton group known, Prochlorococcus, detoxify arsenate (AsV) by reduction to arsenite (AsIII), followed by methylation and excretion of the methylated arsenic products. We synthesized [14C]dimethyl arsenate (DMA) and used it to show that cultured Pelagibacter strain HTCC7211 (SAR11) cells oxidize the methyl group carbons of DMA, producing 14CO2 and ATP. We measured [14C]DMA oxidation rates in the P-depleted surface waters of the Sargasso Sea, a subtropical ocean gyre. [14C]DMA was oxidized to 14CO2 by Sargasso Sea plankton communities at a rate that would cause turnover of the estimated DMA standing stock every 8.1 days. SAR11 strain HTCC7211, which was isolated from the Sargasso Sea, has a pair of arsenate resistance genes and was resistant to arsenate, showing no growth inhibition at As/P ratios of >65:1. Across the global oceans, there was a strong inverse relationship between the frequency of the arsenate reductase (LMWPc_ArsC) in Pelagibacter genomes and phosphate concentrations. We propose that the demethylation of methylated arsenic compounds by Pelagibacter and possibly other bacterioplankton, coupled with arsenate resistance, results in the transfer of energy from phytoplankton to bacteria. We dub this a parasitic cycle because the release of arsenate by Pelagibacter in principle creates a positive-feedback loop that forces phytoplankton to continually regenerate arsenate detoxification products, producing a flow of energy to P-limited ocean regions.IMPORTANCE In vast, warm regions of the oceans, phytoplankton face the problem of arsenic poisoning. Arsenate is toxic because it is chemically similar to phosphate, a scarce nutrient that phytoplankton cells need for growth. Many phytoplankton, including the commonest phytoplankton type in warm oceans, Prochlorococcus, detoxify arsenate by adding methyl groups. Here we show that the most abundant non-photosynthetic plankton in the oceans, SAR11 bacteria, remove the methyl groups, releasing poisonous forms of arsenic back into the water. We postulate that the methylation and demethylation of arsenic compounds creates a cycle in which the phytoplankton can never get ahead and must continually transfer energy to the SAR11 bacteria. We dub this a parasitic process and suggest that it might help explain why SAR11 bacteria are so successful, surpassing all other plankton in their numbers. Field experiments were done in the Sargasso Sea, a subtropical ocean gyre that is sometimes called an ocean desert because, throughout much of the year, there is not enough phosphorous in the water to support large blooms of phytoplankton. Ocean deserts are expanding as the oceans absorb heat and grow warmer.

Anabaena/Dolichospermum as the source of lethal microcystin levels responsible for a large cattle toxicosis event.

Thirty-two 14-month old steers died during a period of four days (19-23 June 2017) after drinking from Junipers Reservoir (southeastern Oregon, USA) during a cyanobacterial bloom. Clinical and histopathological findings were consistent with acute liver disease, and microcystin-LR was present at 3000 µg/L in a reservoir water sample and at 7100 µg/L in the rumen contents of one of the mortalities. Serum biochemistry and histological examination indicated severe liver damage consistent with microcystin toxicosis. Microscopic observation of reservoir water samples, limited to frozen or poorly stored and partially degraded samples, indicated the presence of abundant Anabaena/Dolichospermum, but the presence of other toxic cyanobacteria such as Microcystis could not be excluded. Metagenomic analysis showed the presence in these samples of a single cyanobacterium whose cpcBA, rpoB and rbcL genes indicated membership in the Anabaena/Dolichospermum genus. The sequence of a complete mcy gene cluster with homology to previously identified Anabaena mcy genes was recovered. These results emphasize the capacity for Anabaena/Dolichospermum blooms to produce lethal levels of microcystin, posing a danger to public health and livestock. Further, our findings indicate that such occurrences can occur outside the far-northern latitudes in which microcystin-producing Anabaena have typically been found.

Morphological and transcriptomic evidence for ammonium induction of sexual reproduction in Thalassiosira pseudonana and other centric diatoms.

The reproductive strategy of diatoms includes asexual and sexual phases, but in many species, including the model centric diatom Thalassiosira pseudonana, sexual reproduction has never been observed. Furthermore, the environmental factors that trigger sexual reproduction in diatoms are not understood. Although genome sequences of a few diatoms are available, little is known about the molecular basis for sexual reproduction. Here we show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica. RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium. Some of the induced genes are linked to meiosis or encode flagellar structures of heterokont and cryptophyte algae. The identification of ammonium as an environmental trigger suggests an unexpected link between diatom bloom dynamics and strategies for enhancing population genetic diversity.

The abundant marine bacterium Pelagibacter simultaneously catabolizes dimethylsulfoniopropionate to the gases dimethyl sulfide and methanethiol.

Marine phytoplankton produce ∼10(9) tonnes of dimethylsulfoniopropionate (DMSP) per year(1,2), an estimated 10% of which is catabolized by bacteria through the DMSP cleavage pathway to the climatically active gas dimethyl sulfide(3,4). SAR11 Alphaproteobacteria (order Pelagibacterales), the most abundant chemo-organotrophic bacteria in the oceans, have been shown to assimilate DMSP into biomass, thereby supplying this cell's unusual requirement for reduced sulfur(5,6). Here, we report that Pelagibacter HTCC1062 produces the gas methanethiol, and that a second DMSP catabolic pathway, mediated by a cupin-like DMSP lyase, DddK, simultaneously shunts as much as 59% of DMSP uptake to dimethyl sulfide production. We propose a model in which the allocation of DMSP between these pathways is kinetically controlled to release increasing amounts of dimethyl sulfide as the supply of DMSP exceeds cellular sulfur demands for biosynthesis.