Iron limitation in the marine cyanobacterium Trichodesmium reveals new insights into regulation of photosynthesis and nitrogen fixation

Rapid mode switching facilitates the growth of Trichodesmium: A model analysis

Trichodesmium is one of the dominant dinitrogen (N2) fixers in the ocean, influencing global carbon and nitrogen cycles through biochemical reactions. Although its photosynthetic activity fluctuates rapidly, the physiological or ecological advantage of this fluctuation is unclear. We develop a metabolic model of Trichodesmium that can perform daytime N2 fixation. We examined (1) the effect of the duration of switches between photosynthetic and non-photosynthetic cellular states and (2) the effect of the presence and absence of N2 fixation in photosynthetic states. Results show that a rapid switch between photosynthetic and non-photosynthetic states increases Trichodesmium growth rates by improving metabolic efficiencies due to an improved balance of C and N metabolism. This provides a strategy for previous paradoxical observations that all Trichodesmium cells can contain nitrogenase. This study reveals the importance of fluctuating photosynthetic activity and provides a mechanism for daytime N2 fixation that allows Trichodesmium to fix N2 aerobically in the global ocean.

Marine Colloids Boost Nitrogen Fixation in Trichodesmium erythraeum by Photoelectrophy

Nitrogen fixation by the diazotrophic cyanobacterium Trichodesmium contributes up to 50% of the bioavailable nitrogen in the ocean. N2 fixation by Trichodesmium is limited by the availability of nutrients, such as iron (Fe) and phosphorus (P). Although colloids are ubiquitous in the ocean, the effects of Fe limitation on nitrogen fixation by marine colloids (MC) and the related mechanisms are largely unexplored. In this study, we found that MC exhibit photoelectrochemical properties that boost nitrogen fixation by photoelectrophy in Trichodesmium erythraeum. MC efficiently promote photosynthesis in T. erythraeum, thus enhancing its growth. Photoexcited electrons from MC are directly transferred to the photosynthetic electron transport chain and contribute to nitrogen fixation and ammonia assimilation. Transcriptomic analysis revealed that MC significantly upregulates genes related to the electron transport chain, photosystem, and photosynthesis, which is consistent with elevated photosynthetic capacities (e.g., Fv/Fm and carboxysomes). As a result, MC increase the N2 fixation rate by 67.5-89.3%. Our findings highlight a proof-of-concept electron transfer pathway by which MC boost nitrogen fixation, broadening our knowledge on the role of ubiquitous colloids in marine nitrogen biogeochemistry.

Aerobic methane production by phytoplankton as an important methane source of aquatic ecosystems: Reconsidering the global methane budget

Biological methane, a major source of global methane budget, is traditionally thought to be produced in anaerobic environments. However, the recent reports about methane supersaturation occurring in oxygenated water layer, termed as "methane paradox", have challenged this prevailing paradigm. Significantly, growing evidence has indicated that phytoplankton including prokaryotic cyanobacteria and eukaryotic algae are capable of generating methane under aerobic conditions. In this regard, a systematic review of aerobic methane production by phytoplankton is expected to arouse the public attention, contributing to the understanding of methane paradox. Here, we comprehensively summarize the widespread phenomena of methane supersaturation in oxic layers. The remarkable correlation relationships between methane concentration and several key indicators (depth, chlorophyll a level and organic sulfide concentration) indicate the significance of phytoplankton in in-situ methane accumulation. Subsequently, four mechanisms of aerobic methane production by phytoplankton are illustrated in detail, including photosynthesis-driven metabolism, reactive oxygen species (ROS)-driven demethylation of methyl donors, methanogenesis catalyzed by nitrogenase and demethylation of phosphonates catalyzed by CP lyase. The first two pathways occur in various phytoplankton, while the latter two have been specially discovered in cyanobacteria. Additionally, the effects of four crucial factors on aerobic methane production by phytoplankton are also discussed, including phytoplankton species, light, temperature and crucial nutrients. Finally, the measures to control global methane emissions from phytoplankton, the precise intracellular mechanisms of methane production and a more complete global methane budget model are definitely required in the future research on methane production by phytoplankton. This review would provide guidance for future studies of aerobic methane production by phytoplankton and emphasize the potential contribution of aquatic ecosystems to global methane budget.

Interactions between the nitrogen-fixing cyanobacterium Trichodesmium and siderophore-producing cyanobacterium Synechococcus under iron limitation

As diazotrophic cyanobacteria of tremendous biomass, Trichodesmium continuously provide a nitrogen source for carbon-fixing cyanobacteria and drive the generation of primary productivity in marine environments. However, ocean iron deficiencies limit growth and metabolism of Trichodesmium. Recent studies have shown the co-occurrence of Trichodesmium and siderophore-producing Synechococcus in iron-deficient oceans, but whether siderophores secreted by Synechococcus can be used by Trichodesmium to adapt to iron deficiency is not clear. We constructed a mutant Synechococcus strain unable to produce siderophores to explore this issue. Synechococcus filtrates with or without siderophores were added into a Trichodesmium microbial consortium consisting of Trichodesmium erythraeum IMS 101 as the dominant microbe with chronic iron deficiency. By analyzing the physiological phenotype, metagenome, and metatranscriptome, we investigated the interactions between the nitrogen-fixing cyanobacterium Tricodesmium and siderophore-producing cyanobacterium Synechococcus under conditions of iron deficiency. The results indicated that siderophores secreted by Synechococcus are likely to chelate with free iron in the culture medium of the Trichodesmium consortium, reducing the concentration of bioavailable iron and posing greater challenges to the absorption of iron by Trichodesmium. These findings revealed the characteristics of iron-competitive utilization between diazotrophic cyanobacteria and siderophore-producing cyanobacteria, as well as potential interactions, and provide a scientific basis for understanding the regulatory effects of nutrient limitation on marine primary productivity.

Acclimation to medium-level non-lethal iron limitation: Adjustment of electron flow around the PSII and metalloprotein expression in Trichodesmium erythraeum IMS101

The aim of this study was to investigate how acclimation to medium-level, long-term, non-lethal iron limitation changes the electron flux around the Photosystem II of the oceanic diazotroph Trichodesmium erythraeum IMS101. Fe availability of about 5× and 100× lower than a replete level, i.e. conditions common in the natural environment of this cyanobacterium, were applied in chemostats. The response of the cells was studied not only in terms of growth, but also mechanistically, measuring the chlorophyll fluorescence of dark-adapted filaments via imaging fluorescence kinetic microscopy (FKM) with 0.3 ms time resolution. Combining these measurements with those of metal binding to proteins via online coupling of metal-free HPLC (size exclusion chromatography SEC) to sector-field ICP-MS allowed to track the fate of the photosystems, together with other metalloproteins. General increase of fluorescence has been observed, with the consequent decrease in the quantum yields φ of the PSII, while the efficiency ψ of the electron flux between PSII and the PSI remained surprisingly unchanged. This indicates the ability of Trichodesmium to cope with a situation that makes assembling the many iron clusters in Photosystem I a particular challenge, as shown by decreasing ratios of Fe to Mg in these proteins. The negative effect of Fe limitation on PSII may also be due to its fast turnover. A broader view was obtained from metalloproteomics via HPLC-ICP-MS, revealing a differential protein expression pattern under iron limitation with a drastic down-regulation especially of iron-containing proteins and some increase in low MW metal-binding complexes.

Diurnally dynamic iron allocation promotes N2 fixation in marine dominant diazotroph Trichodesmium

Trichodesmium is the dominant photoautotrophic dinitrogen (N₂) fixer (diazotroph) in the ocean. Iron is an important factor limiting growth of marine diazotrophs including Trichodesmium mainly because of high iron content of its N₂-fixing enzyme, nitrogenase. However, it still lacks a quantitative understanding of how dynamic iron allocation among physiological processes acts to regulate growth and N₂ fixation in Trichodesmium. Here, we constructed a model of Trichodesmium trichome in which intracellular iron could be dynamically re-allocated in photosystems and nitrogenase during the daytime. The results demonstrate that the dynamic iron allocation enhances modeled N₂ fixation and growth rates of Trichodesmium, especially in iron-limited conditions, albeit having a marginal impact under high iron concentrations. Although the reuse of iron during a day is an apparent cause that dynamic iron allocation can benefit Trichodesmium under iron limitation, our model reveals two important mechanisms. First, the release of iron from photosystems downregulates the intracellular oxygen (O₂) production and reduces the demand of respiratory protection, a process that Trichodesmium wastefully respires carbohydrates to create a lower O₂ window for N₂ fixation. Hence, more carbohydrates can be used in growth. Second, lower allocation of iron to nitrogenase during early daytime, a period when photosynthesis is active and intracellular O₂ is high, reduces the amount of iron that is trapped in the inactivated nitrogenase induced by O₂. This mechanism further increases the iron use efficiency in Trichodesmium. Overall, our study provides mechanistic and quantitative insight into the diurnal iron allocation that can alleviate iron limitation to Trichodesmium.

Salmonella Facilitates Iron Acquisition through UMPylation of Ferric Uptake Regulator

Iron limitation is a universal strategy of host immunity during bacterial infection. However, the mechanisms by which pathogens antagonize host nutritional immunity have not been fully elucidated. Here, we identified a requirement for the UMPylator YdiU for this process in Salmonella. The expression of YdiU was dramatically induced by the metal starvation signal. The intracellular iron content was much lower in the ΔydiU strain than in wild-type Salmonella, and the ΔydiU strain exhibited severe growth defect under metal deficiency environments. Genome-wide expression analyses revealed significantly decreased expression of iron uptake genes in ΔydiU strain compared with the wild-type strain. Interestingly, YdiU did not affect the expression level of the major iron uptake regulator Fur but directly UMPylated Fur on its H118 residue in vivo and in vitro. UMPylation destroyed the Fur dimer, promoted Fur aggregation, and eliminated the DNA-binding activity of Fur, thus abolishing the ability of Fur to inhibit iron uptake. Restricting Fur to the deUMPylated state dramatically eliminates Salmonella iron uptake in iron deficiency environments. In parallel, YdiU facilitates Salmonella survival within host cells by regulating the iron uptake pathway.

Assessing the contribution of diazotrophs to microbial Fe uptake using a group specific approach in the Western Tropical South Pacific Ocean

Diazotrophs are often limited by iron (Fe) availability in the oligotrophic ocean. The Western Tropical South Pacific (WTSP) ocean has been suggested as an intense N2 fixation area due to Fe fertilizations through shallow hydrothermal activity. Yet, the Fe demand of diazotrophs in their natural habitat, where they cohabit with other microbial organisms also requiring Fe, remains unknown. Here we develop and apply a method consisting of coupling 55Fe uptake experiments with cell-sorting by flow cytometry, and provide group-specific rates of in situ Fe uptake by the microbial community in the WTSP, in addition to bulk and size fractionation rates. We reveal that the diazotrophs Crocosphaera watsonii and Trichodesmium contribute substantially to the bulk in situ Fe uptake (~33% on average over the studied area), despite being numerically less abundant compared to the rest of the planktonic community. Trichodesmium had the highest cell-specific Fe uptake rates, followed by C. watsonii, picoeukaryotes, Prochlorococcus, Synechococcus and finally heterotrophic bacteria. Calculated Fe:C quotas were higher (by 2 to 52-fold) for both studied diazotrophs compared to those of the non-diazotrophic plankton, reflecting their high intrinsic Fe demand. This translates into a diazotroph biogeographical distribution that appears to be influenced by ambient dissolved Fe concentrations in the WTSP. Despite having low cell-specific uptake rates, Prochlorococcus and heterotrophic bacteria were largely the main contributors to the bulk Fe uptake (~23% and ~12%, respectively). Overall, this group-specific approach increases our ability to examine the ecophysiological role of functional groups, including those of less abundant and/or less active microbes.

Exploring Variability of Trichodesmium Photophysiology Using Multi-Excitation Wavelength Fast Repetition Rate Fluorometry

Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers-potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (F v /F m ), functional absorption cross section of PSII (σ PSII ), and electron transport rates (ETRs) for Trichodesmium assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of F v /F m for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average F v /F m as well as lower σ PSII and non-photochemical quenching (NPQ NSV ) observed in the NWP compared to the NIO-SL (Kruskal-Wallis t-test df = 1, p < 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (α ETR ) and higher maximum electron turnover rate ( E T R P S I I m a x ) observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation E K (= E T R P S I I m a x /α ETR ) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a-1 h-1 across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural Trichodesmium populations.

Mechanistic Model for the Coexistence of Nitrogen Fixation and Photosynthesis in Marine Trichodesmium

Trichodesmium is a major nitrogen-fixing cyanobacterium and exerts a significant influence on the oceanic nitrogen cycle. It is also a widely used model organism in laboratory studies. Since the nitrogen-fixing enzyme nitrogenase is extremely sensitive to oxygen, how these surface-dwelling plankton manage the two conflicting processes of nitrogen fixation and photosynthesis has been a long-standing question. In this study, we developed a simple model of metabolic fluxes of Trichodesmium capturing observed daily cycles of photosynthesis, nitrogen fixation, and boundary layer oxygen concentrations. The model suggests that forming a chain of cells for spatially segregating nitrogen fixation and photosynthesis is essential but not sufficient. It also requires a barrier against oxygen diffusion and high rates of oxygen scavenging by respiration. Finally, the model indicates that the life span of intracellular oxygen is extremely short, thus enabling cells to instantly create a low-oxygen environment upon deactivation of photosynthesis.

The cyanobacterium Trichodesmium is an important contributor of new nitrogen (N) to the surface ocean, but its strategies for protecting the nitrogenase enzyme from inhibition by oxygen (O₂) remain poorly understood. We present a dynamic physiological model to evaluate hypothesized conditions that would allow Trichodesmium to carry out its two conflicting metabolic processes of N₂ fixation and photosynthesis. First, the model indicates that managing cellular O₂ to permit N₂ fixation requires high rates of respiratory O₂ consumption. The energetic cost amounts to ∼80% of daily C fixation, comparable to the observed diminution of the growth rate of Trichodesmium relative to other phytoplankton. Second, by forming a trichome of connected cells, Trichodesmium can segregate N₂ fixation from photosynthesis. The transfer of stored C to N-fixing cells fuels the respiratory O₂ consumption that protects nitrogenase, while the reciprocal transfer of newly fixed N to C-fixing cells supports cellular growth. Third, despite Trichodesmium lacking the structural barrier found in heterocystous species, the model predicts low diffusivity of cell membranes, a function that may be explained by the presence of Gram-negative membrane, production of extracellular polysaccharide substances (EPS), and “buffer cells” that intervene between N₂-fixing and photosynthetic cells. Our results suggest that all three factors—respiratory protection, trichome formation, and diffusion barriers—represent essential strategies that, despite their energetic costs, facilitate the growth of Trichodesmium in the oligotrophic aerobic ocean and permit it to be a major source of new reactive nitrogen.

IMPORTANCETrichodesmium is a major nitrogen-fixing cyanobacterium and exerts a significant influence on the oceanic nitrogen cycle. It is also a widely used model organism in laboratory studies. Since the nitrogen-fixing enzyme nitrogenase is extremely sensitive to oxygen, how these surface-dwelling plankton manage the two conflicting processes of nitrogen fixation and photosynthesis has been a long-standing question. In this study, we developed a simple model of metabolic fluxes of Trichodesmium capturing observed daily cycles of photosynthesis, nitrogen fixation, and boundary layer oxygen concentrations. The model suggests that forming a chain of cells for spatially segregating nitrogen fixation and photosynthesis is essential but not sufficient. It also requires a barrier against oxygen diffusion and high rates of oxygen scavenging by respiration. Finally, the model indicates that the life span of intracellular oxygen is extremely short, thus enabling cells to instantly create a low-oxygen environment upon deactivation of photosynthesis.

Reduced nitrogenase efficiency dominates response of the globally important nitrogen fixer Trichodesmium to ocean acidification

The response of the prominent marine dinitrogen (N2)-fixing cyanobacteria Trichodesmium to ocean acidification (OA) is critical to understanding future oceanic biogeochemical cycles. Recent studies have reported conflicting findings on the effect of OA on growth and N2 fixation of Trichodesmium. Here, we quantitatively analyzed experimental data on how Trichodesmium reallocated intracellular iron and energy among key cellular processes in response to OA, and integrated the findings to construct an optimality-based cellular model. The model results indicate that Trichodesmium growth rate decreases under OA primarily due to reduced nitrogenase efficiency. The downregulation of the carbon dioxide (CO2)-concentrating mechanism under OA has little impact on Trichodesmium, and the energy demand of anti-stress responses to OA has a moderate negative effect. We predict that if anthropogenic CO2 emissions continue to rise, OA could reduce global N2 fixation potential of Trichodesmium by 27% in this century, with the largest decrease in iron-limiting regions.

An Integrated Response of Trichodesmium erythraeum IMS101 Growth and Photo-Physiology to Iron, CO2, and Light Intensity

We have assessed how varying CO₂ (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m^-2 s^-1) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe') concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rP_m). Under iron-limiting concentrations, high-light increased growth rates and rP_m; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO₂ increased growth rates across all iron concentrations, enabled growth to occur at lower Fe' concentrations, increased rP_m and lowered the iron half saturation constants for growth (K_m). We attribute these CO₂ responses to the operation of the CCM and the ATP spent/saved for CO₂ uptake and transport at low and high CO₂, respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO₂, light intensity and iron-limitation. These results are important given predictions of increased dissolved CO₂ and water column stratification (i.e., higher light exposures) over the coming decades.

Iron and phosphorus deprivation induce sociality in the marine bloom-forming cyanobacterium Trichodesmium

Trichodesmium spp. are diazotrophic cyanobacteria that exist as single filaments (trichomes) and as macroscopic colonies of varying shapes formed by aggregating trichomes. The causes and dynamics of colony formation and disassociation are not yet elucidated. we demonstrate that limited availability of dissolved phosphorus (P) or iron (Fe) stimulated trichome mobility and induced colony formation in Trichodesmium erythraeum IMS101 cultures. The specific nutrient limitation differentially affected the rate of colony formation and morphology of the colonies. Fe starvation promoted rapid colony formation (10-48 h from depletion) while 5-7 days were required for colonies to form in P-depleted cultures. Video analyses confirmed that the probability of trichomes to cluster increased from 12 to 35% when transferred from nutrient replete to Fe-depleted conditions. Moreover, the probability for Fe-depleted aggregates to remain colonial increased to 50% from only 10% in nutrient replete cultures. These colonies were also characterized by stronger attachment forces between the trichomes. Enrichment of nutrient-depleted cultures with the limited nutrient-stimulated colony dissociation into single trichomes. We postulate that limited P and Fe availability enhance colony formation of Trichodesmium and primarily control the abundance and distribution of its different morphologies in the nutrient-limited surface ocean.

A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101

Trichodesmium is a globally important marine diazotroph that accounts for approximately 60 - 80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt.

Deficiency and toxicity of nanomolar copper in low irradiance-A physiological and metalloproteomic study in the aquatic plant Ceratophyllum demersum

Essential trace elements (Cu(2+), Zn(2+), etc) lead to toxic effects above a certain threshold, which is a major environmental problem in many areas of the world. Here, environmentally relevant sub-micromolar concentrations of Cu(2+) and simulations of natural light and temperature cycles were applied to the aquatic macrophyte Ceratophyllum demersum a s a model for plant shoots. In this low irradiance study resembling non-summer conditions, growth was optimal in the range 7.5-35nM Cu, while PSII activity (Fv/Fm) was maximal around 7.5nM Cu. Damage to the light harvesting complex of photosystem II (LHCII) was the first target of Cu toxicity (>50nM Cu) where Cu replaced Mg in the LHCII-trimers. This was associated with a subsequent decrease of Chl a as well as heat dissipation (NPQ). The growth rate was decreased from the first week of Cu deficiency. Plastocyanin malfunction due to the lack of Cu that is needed for its active centre was the likely cause of diminished electron flow through PSII (ΦPSII). The pigment decrease added to the damage in the photosynthetic light reactions. These mechanisms ultimately resulted in decrease of starch and oxygen production.

Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport

Iron-containing metalloproteins are the main cornerstones for efficient electron transport in biological systems. The abundance and diversity of iron-dependent proteins in cyanobacteria makes those organisms highly dependent of this micronutrient. To cope with iron imbalance, cyanobacteria have developed a survey of adaptation strategies that are strongly related to the regulation of photosynthesis, nitrogen metabolism and other central electron transfer pathways. Furthermore, either in its ferrous form or as a component of the haem group, iron plays a crucial role as regulatory signalling molecule that directly or indirectly modulates the composition and efficiency of cyanobacterial redox reactions. We present here the major mechanism used by cyanobacteria to couple iron homeostasis to the regulation of electron transport, making special emphasis in processes specific in those organisms.

Quantifying Integrated Proteomic Responses to Iron Stress in the Globally Important Marine Diazotroph Trichodesmium

Trichodesmium is a biogeochemically important marine cyanobacterium, responsible for a significant proportion of the annual 'new' nitrogen introduced into the global ocean. These non-heterocystous filamentous diazotrophs employ a potentially unique strategy of near-concurrent nitrogen fixation and oxygenic photosynthesis, potentially burdening Trichodesmium with a particularly high iron requirement due to the iron-binding proteins involved in these processes. Iron availability may therefore have a significant influence on the biogeography of Trichodesmium. Previous investigations of molecular responses to iron stress in this keystone marine microbe have largely been targeted. Here a holistic approach was taken using a label-free quantitative proteomics technique (MSE) to reveal a sophisticated multi-faceted proteomic response of Trichodesmium erythraeum IMS101 to iron stress. Increased abundances of proteins known to be involved in acclimation to iron stress and proteins known or predicted to be involved in iron uptake were observed, alongside decreases in the abundances of iron-binding proteins involved in photosynthesis and nitrogen fixation. Preferential loss of proteins with a high iron content contributed to overall reductions of 55-60% in estimated proteomic iron requirements. Changes in the abundances of iron-binding proteins also suggested the potential importance of alternate photosynthetic pathways as Trichodesmium reallocates the limiting resource under iron stress. Trichodesmium therefore displays a significant and integrated proteomic response to iron availability that likely contributes to the ecological success of this species in the ocean.

Altered levels of LIL3 isoforms in Arabidopsis lead to disturbed pigment-protein assembly and chlorophyll synthesis, chlorotic phenotype and impaired photosynthetic performance

Light-harvesting complex (LHC)-like (LIL) proteins contain two transmembrane helices of which the first bears a chlorophyll (Chl)-binding motif. They are widespread in photosynthetic organisms, but almost nothing is known about their expression and physiological functions. We show that two LIL3 paralogues (LIL3:1 and LIL3:2) in Arabidopsis thaliana are expressed in photosynthetically active tissues and their expression is differentially influenced by light stress. Localization studies demonstrate that both isoforms are associated with subcomplexes of LHC antenna of photosystem II. Transgenic plants with reduced amounts of LIL3:1 exhibited a slightly impaired growth and have reduced Chl and carotenoid contents as compared to wild-type plants. Ectopic overexpression of either paralogue led to a developmentally regulated switch to co-suppression of both LIL3 isoforms, resulting in a circular chlorosis of the leaf rosettes. Chlorotic sectors show severely diminished levels of LIL3 isoforms and other proteins, and thylakoid morphology was changed. Additionally, the levels of enzymes involved in Chl biosynthesis are altered in lil3 mutant plants. Our data support a role of LIL3 paralogues in the regulation of Chl biosynthesis under light stress and under standard growth conditions as well as in a coordinated ligation of newly synthesized and/or rescued Chl molecules to their target apoproteins.

Uptake mechanisms for inorganic iron and ferric citrate in Trichodesmium erythraeum IMS101

Growth of the prevalent marine organism Trichodesmium can be limited by iron in natural and laboratory settings. This study investigated the iron uptake mechanisms that the model organism T. erythraeum IMS101 uses to acquire iron from inorganic iron and iron associated with the weak ligand complex, ferric citrate. IMS101 was observed to employ two different iron uptake mechanisms: superoxide-mediated reduction of inorganic iron in the surrounding milieu and a superoxide-independent uptake system for ferric citrate complexes. While the detailed pathway of ferric citrate utilization remains to be elucidated, transport of iron from this complex appears to involve reduction and/or exchange of the iron out of the complex prior to uptake, either at the outer membrane of the cell or within the periplasmic space. Various iron uptake strategies may allow Trichodesmium to effectively scavenge iron in oligotrophic ocean environments.

Trichodesmium’s strategies to alleviate phosphorus limitation in the future acidified oceans

Global warming may exacerbate inorganic nutrient limitation, including phosphorus (P), in the surface waters of tropical oceans that are home to extensive blooms of the marine diazotrophic cyanobacterium, Trichodesmium. We examined the combined effects of P limitation and pCO(2), forecast under ocean acidification scenarios, on Trichodesmium erythraeum IMS101 cultures. We measured nitrogen acquisition,glutamine synthetase activity, C uptake rates, intracellular Adenosine Triphosphate (ATP) concentration and the pool sizes of related key proteins. Here, we present data supporting the idea that cellular energy re-allocation enables the higher growth and N(2) fixation rates detected in Trichodesmium cultured under high pCO(2). This is reflected in altered protein abundance and metabolic pools. Also modified are particulate organic carbon and nitrogen production rates,enzymatic activities, and cellular ATP concentrations. We suggest that adjusting these cellular pathways to changing environmental conditions enables Trichodesmium to compensate for low P availability and to thrive in acidified oceans. Moreover, elevated pCO(2) could provide Trichodesmium with a competitive dominance that would extend its niche, particularly in P-limited regions of the tropical and subtropical oceans.

Responses to iron limitation are impacted by light quality and regulated by RcaE in the chromatically acclimating cyanobacterium Fremyella diplosiphon

Photosynthetic organisms adapt to environmental fluctuations of light and nutrient availability. Iron is critical for photosynthetic organismal growth, as many cellular processes depend upon iron cofactors. Whereas low iron levels can have deleterious effects, excess iron can lead to damage, as iron is a reactive metal that can result in the production of damaging radicals. Therefore, organisms regulate cellular iron levels to maintain optimal iron homeostasis. In particular, iron is an essential factor for the function of photosystems associated with photosynthetic light-harvesting complexes. Photosynthetic organisms, including cyanobacteria, generally respond to iron deficiency by reduced growth, degradation of non-essential proteins and in some cases alterations of cellular morphology. In response to fluctuations in ambient light quality, the cyanobacterium Fremyella diplosiphon undergoes complementary chromatic adaptation (CCA). During CCA, phycobiliprotein composition of light-harvesting antennae is altered in response to green light (GL) and red light (RL) for efficient utilization of light energy for photosynthesis. We observed light-regulated responses to iron limitation in F. diplosiphon. RL-grown cells exhibited significant reductions in growth and pigment levels, and alterations in iron-associated proteins, which impact the accumulation of reactive oxygen species under iron-limiting conditions, whereas GL-grown cells exhibited partial resistance to iron limitation. We investigated the roles of known CCA regulators RcaE, RcaF and RcaC in this light-dependent iron-acclimation response. Through comparative analyses of wild-type and CCA mutant strains, we determined that photoreceptor RcaE has a central role in light-induced oxidative stress associated with iron limitation, and impacts light-regulated iron-acclimation responses, physiologically and morphologically.

Response of the unicellular diazotrophic cyanobacterium Crocosphaera watsonii to iron limitation

Iron (Fe) is widely suspected as a key controlling factor of N₂ fixation due to the high Fe content of nitrogenase and photosynthetic enzymes complex, and to its low concentrations in oceanic surface seawaters. The influence of Fe limitation on the recently discovered unicellular diazotrophic cyanobacteria (UCYN) is poorly understood despite their biogeochemical importance in the carbon and nitrogen cycles. To address this knowledge gap, we conducted culture experiments on Crocosphaera watsonii WH8501 growing under a range of dissolved Fe concentrations (from 3.3 to 403 nM). Overall, severe Fe limitation led to significant decreases in growth rate (2.6-fold), C, N and chlorophyll a contents per cell (up to 4.1-fold), N₂ and CO₂ fixation rates per cell (17- and 7-fold) as well as biovolume (2.2-fold). We highlighted a two phased response depending on the degree of limitation: (i) under a moderate Fe limitation, the biovolume of C. watsonii was strongly reduced, allowing the cells to keep sufficient energy to maintain an optimal growth, volume-normalized contents and N₂ and CO₂ fixation rates; (ii) with increasing Fe deprivation, biovolume remained unchanged but the entire cell metabolism was affected, as shown by a strong decrease in the growth rate, volume-normalized contents and N₂ and CO₂ fixation rates. The half-saturation constant for growth of C. watsonii with respect to Fe is twice as low as that of the filamentous Trichodesmium indicating a better adaptation of C. watsonii to poor Fe environments than filamentous diazotrophs. The physiological response of C. watsonii to Fe limitation was different from that previously shown on the UCYN Cyanothece sp, suggesting potential differences in Fe requirements and/or Fe acquisition within the UCYN community. These results contribute to a better understanding of how Fe bioavailability can control the activity of UCYN and explain the biogeography of diverse N₂ fixers in ocean.

Effects of nanomolar copper on water plants--comparison of biochemical and biophysical mechanisms of deficiency and sublethal toxicity under environmentally relevant conditions

Toxicity and deficiency of essential trace elements like Cu are major global problems. Here, environmentally relevant sub-micromolar concentrations of Cu (supplied as CuSO4) and simulations of natural light- and temperature cycles were applied to the aquatic macrophyte Ceratophyllum demersum. Growth was optimal at 10nM Cu, while PSII activity (Fv/Fm) was maximal around 2 nM Cu. Damage to the PSII reaction centre was the first target of Cu toxicity, followed by disturbed regulation of heat dissipation (NPQ). Only after that, electron transport through PSII (ΦPSII) was inhibited, and finally chlorophylls decreased. Copper accumulation in the plants was stable until 10nM Cu in solution, but strongly increased at higher concentrations. The vein was the main storage site for Cu up to physiological concentrations (10nM). At toxic levels it was also sequestered to the epidermis and mesophyll until export from the vein became inhibited, accompanied by inhibition of Zn uptake. Copper deficiency led to a complete stop of growth at "0"nM Cu after 6 weeks. This was accompanied by high starch accumulation although electron flow through PSII (ΦPSII) decreased from 2 weeks, followed by decrease in pigments and increase of non photochemical quenching (NPQ). Release of Cu from the plants below 10nM Cu supply in the nutrient solution indicated lack of high-affinity Cu transporters, and on the tissue level copper deficiency led to a re-distribution of zinc.

Regulation of photosynthesis during heterocyst differentiation in Anabaena sp. strain PCC 7120 investigated in vivo at single-cell level by chlorophyll fluorescence kinetic microscopy

Changes of photosynthetic activity in vivo of individual heterocysts and vegetative cells in the diazotrophic cyanobacterium Anabaena sp. strain PCC 7120 during the course of diazotrophic acclimation were determined using fluorescence kinetic microscopy (FKM). Distinct phases of stress and acclimation following nitrogen step-down were observed. The first was a period of perception, in which the cells used their internally stored nitrogen without detectable loss of PS II activity or pigments. In the second, the stress phase of nitrogen limitation, the cell differentiation occurred and an abrupt decline of fluorescence yield was observed. This decline in fluorescence was not paralleled by a corresponding decline in photosynthetic pigment content and PS II activity. Both maximal quantum yield and sustained electron flow were not altered in vegetative cells, only in the forming heterocysts. The third, acclimation phase started first in the differentiating heterocysts with a recovery of PS II photochemical yields [Formula: see text] Afterwards, the onset of nitrogenase activity was observed, followed by the restoration of antenna pigments in the vegetative cells, but not in the heterocysts. Surprisingly, mature heterocysts were found to have an intact PS II as judged by photochemical yields, but a strongly reduced PS II-associated antenna as judged by decreased F 0. The possible importance of the functional PS II in heterocysts is discussed. Also, the FKM approach allowed to follow in vivo and evaluate the heterogeneity in photosynthetic performance among individual vegetative cells as well as heterocysts in the course of diazotrophic acclimation. Some cells along the filament (so-called "superbright cells") were observed to display transiently increased fluorescence yield, which apparently proceeded by apoptosis.

FurA influences heterocyst differentiation in Anabaena sp. PCC 7120

In Anabaena sp. PCC 7120, FurA is a global transcriptional regulator whose expression is strongly induced by NtcA in proheterocysts and remains stably expressed in mature heterocysts. In the present study, overexpression of furA partially suppressed heterocyst differentiation by impairing morphogenesis at an early stage. Recombinant purified FurA specifically bound in vitro to the promoter regions of ntcA, while quantitative RT-PCR analyses indicated that furA overexpression strongly affected the transient increase of ntcA expression that occurs shortly after nitrogen step-down. Overall, the results suggest a connection between iron homeostasis and heterocyst differentiation via FurA, by modulating the expression of ntcA.

Photophysiological and photosynthetic complex changes during iron starvation in Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942

Iron is an essential component in many protein complexes involved in photosynthesis, but environmental iron availability is often low as oxidized forms of iron are insoluble in water. To adjust to low environmental iron levels, cyanobacteria undergo numerous changes to balance their iron budget and mitigate the physiological effects of iron depletion. We investigated changes in key protein abundances and photophysiological parameters in the model cyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803 over a 120 hour time course of iron deprivation. The iron stress induced protein (IsiA) accumulated to high levels within 48 h of the onset of iron deprivation, reaching a molar ratio of ∼42 IsiA : Photosystem I in Synechococcus PCC 7942 and ∼12 IsiA : Photosystem I in Synechocystis PCC 6803. Concomitantly the iron-rich complexes Cytochrome b₆f and Photosystem I declined in abundance, leading to a decrease in the Photosystem I : Photosystem II ratio. Chlorophyll fluorescence analyses showed a drop in electron transport per Photosystem II in Synechococcus, but not in Synechocystis after iron depletion. We found no evidence that the accumulated IsiA contributes to light capture by Photosystem II complexes.

Trichodesmium--a widespread marine cyanobacterium with unusual nitrogen fixation properties

The last several decades have witnessed dramatic advances in unfolding the diversity and commonality of oceanic diazotrophs and their N₂-fixing potential. More recently, substantial progress in diazotrophic cell biology has provided a wealth of information on processes and mechanisms involved. The substantial contribution by the diazotrophic cyanobacterial genus Trichodesmium to the nitrogen influx of the global marine ecosystem is by now undisputable and of paramount ecological importance, while the underlying cellular and molecular regulatory physiology has only recently started to unfold. Here, we explore and summarize current knowledge, related to the optimization of its diazotrophic capacity, from genomics to ecophysiological processes, via, for example, cellular differentiation (diazocytes) and temporal regulations, and suggest cellular research avenues that now ought to be explored.

Abundances of iron-binding photosynthetic and nitrogen-fixing proteins of Trichodesmium both in culture and in situ from the North Atlantic

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high iron-flux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean.

Molecular evidence of iron limitation and availability in the global diazotroph Trichodesmium

The activity of the N(2)-fixing cyanobacterial genus Trichodesmium is critical to the global nitrogen (N) and carbon (C) cycles. Although iron (Fe) has been shown to be an important element limiting the growth and N(2) fixation of Trichodesmium, there have been no specific data demonstrating the in situ affect of Fe on Trichodesmium. We surveyed Trichodesmium populations from the Atlantic and Pacific Oceans for Fe limitation using a novel quantitative reverse transcriptase-PCR (qRT-PCR) method monitoring the expression of an Fe limitation-induced gene, isiB. Here we report the first molecular evidence of in situ Fe limitation of Trichodesmium N(2) fixation, which was evident in samples from the Pacific Ocean, whereas limitation appeared minimal to nonexistent in Atlantic Ocean samples. As our method is Trichodesmium clade specific, we were also able to determine that representatives from the Trichodesmium tenue clade were the most biologically active group of Trichodesmium in the majority of our samples, which speaks to their dominance in open ocean regimes. Furthermore, comparisons of our field expression and chemical data with laboratory studies suggest that the majority of dissolved Fe in the open ocean is available to Trichodesmium colonies regardless of Fe complexation.

Iron utilization in marine cyanobacteria and eukaryotic algae

Iron is essential for aerobic organisms. Additionally, photosynthetic organisms must maintain the iron-rich photosynthetic electron transport chain, which likely evolved in the iron-replete Proterozoic ocean. The subsequent rise in oxygen since those times has drastically decreased the levels of bioavailable iron, indicating that adaptations have been made to maintain sufficient cellular iron levels in the midst of scarcity. In combination with physiological studies, the recent sequencing of marine microorganism genomes and transcriptomes has begun to reveal the mechanisms of iron acquisition and utilization that allow marine microalgae to persist in iron limited environments.

Analysis of the global ocean sampling (GOS) project for trends in iron uptake by surface ocean microbes

Microbial metagenomes are DNA samples of the most abundant, and therefore most successful organisms at the sampling time and location for a given cell size range. The study of microbial communities via their DNA content has revolutionized our understanding of microbial ecology and evolution. Iron availability is a critical resource that limits microbial communities' growth in many oceanic areas. Here, we built a database of 2319 sequences, corresponding to 140 gene families of iron metabolism with a large phylogenetic spread, to explore the microbial strategies of iron acquisition in the ocean's bacterial community. We estimate iron metabolism strategies from metagenome gene content and investigate whether their prevalence varies with dissolved iron concentrations obtained from a biogeochemical model. We show significant quantitative and qualitative variations in iron metabolism pathways, with a higher proportion of iron metabolism genes in low iron environments. We found a striking difference between coastal and open ocean sites regarding Fe²⁺ versus Fe³⁺ uptake gene prevalence. We also show that non-specific siderophore uptake increases in low iron open ocean environments, suggesting bacteria may acquire iron from natural siderophore-like organic complexes. Despite the lack of knowledge of iron uptake mechanisms in most marine microorganisms, our approach provides insights into how the iron metabolic pathways of microbial communities may vary with seawater iron concentrations.

THE CYANOBACTERIAL CHLOROPHYLL-BINDING-PROTEIN ISIA ACTS TO INCREASE THE IN VIVO EFFECTIVE ABSORPTION CROSS-SECTION OF PSI UNDER IRON LIMITATION(1)

Iron availability limits primary production in >30% of the world's oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron-stress-induced) under iron stress, which can become the most abundant chl-binding protein in the cell. Within iron-limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross-section of the photosynthetic reaction center PSI (σPSI ) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that accumulation of IsiA results in a ∼60% increase in σPSI , in agreement with the theoretical increase in cross-section based on the structure of the biochemically isolated IsiA-PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light-harvesting antenna for PSI. On progressive iron-stress in culture, IsiA continues to accumulate without a concomitant increase in σPSI , suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA-bound chl is directly involved in photosynthetic electron transport.

Vertical distribution of epibenthic freshwater cyanobacterial Synechococcus spp. strains depends on their ability for photoprotection

Background

Epibenthic cyanobacteria often grow in environments where the fluctuation of light intensity and quality is extreme and frequent. Different strategies have been developed to cope with this problem depending on the distribution of cyanobacteria in the water column.

Principal Findings

Here we provide an experimental proof that the light intensity plays an important role in the vertical distribution of seven, closely related, epibenthic Synechococcus spp. strains isolated from various water depths from the littoral zone of Lake Constance in Germany and cultivated under laboratory conditions. Pigment analysis revealed that the amount of chlorophyll a and total carotenoids decreased with the time of light stress exposure in three phycoerythrin-rich strains collected from 7.0 m water depth and remained low during the recovery phase. In contrast, a constant level of chlorophyll a and either constant or enhanced levels of carotenoids were assayed in phycocyanin-rich strains collected from 1.0 and 0.5 m water depths. Protein analysis revealed that while the amount of biliproteins remained constant in all strains during light stress and recovery, the amount of D1 protein from photosystem II reaction centre was strongly reduced under light stress conditions in strains from 7.0 m and 1.0 m water depth, but not in strains collected from 0.5 m depth.

Conclusion

Based on these data we propose that light intensity, in addition to light quality, is an important selective force in the vertical distribution of Synechococcus spp. strains, depending on their genetically fixed mechanisms for photoprotection.

Determining cell shape: adaptive regulation of cyanobacterial cellular differentiation and morphology

Similar to other bacteria, cyanobacteria exist in a wide-ranging diversity of shapes and sizes. However, three general shapes are observed most frequently: spherical, rod and spiral. Bacteria can also grow as filaments of cells. Some filamentous cyanobacteria have differentiated cell types that exhibit distinct morphologies: motile hormogonia, nitrogen-fixing heterocysts, and spore-like akinetes. Cyanobacterial cell shapes, which are largely controlled by the cell wall, can be regulated by developmental and/or environmental cues, although the mechanisms of regulation and the selective advantage(s) of regulating cellular shape are still being elucidated. In this review, recent insights into developmental and environmental regulation of cell shape in cyanobacteria and the relationship(s) of cell shape and differentiation to organismal fitness are discussed.

Cadmium uptake and sequestration kinetics in individual leaf cell protoplasts of the Cd/Zn hyperaccumulator Thlaspi caerulescens

Hyperaccumulators store accumulated metals in the vacuoles of large leaf epidermal cells (storage cells). For investigating cadmium uptake, we incubated protoplasts obtained from leaves of Thlaspi caerulescens (Ganges ecotype) with a Cd-specific fluorescent dye. A fluorescence kinetic microscope was used for selectively measuring Cd-uptake and photosynthesis in different cell types, so that physical separation of cell types was not necessary. Few minutes after its addition, cadmium accumulated in the cytoplasm before its transport into the vacuole. This demonstrated that vacuolar sequestration is the rate-limiting step in cadmium uptake into protoplasts of all leaf cell types. During accumulation in the cytoplasm, Cd-rich vesicle-like structures were observed. Cd uptake rates into epidermal storage cells were higher than into standard-sized epidermal cells and mesophyll cells. This shows that the preferential heavy metal accumulation in epidermal storage cells, previously observed for several metals in intact leaves of various hyperaccumulator species, is due to differences in active metal transport and not differences in passive mechanisms like transpiration stream transport or cell wall adhesion. Combining this with previous studies, it seems likely that the transport steps over the plasma and tonoplast membranes of leaf epidermal storage cells are driving forces behind the hyperaccumulation phenotype.

FdTonB is involved in the photoregulation of cellular morphology during complementary chromatic adaptation in Fremyella diplosiphon

We have characterized a Fremyella diplosiphon TonB protein (FdTonB) and investigated its function during complementary chromatic adaptation. Sequence similarity analysis of FdTonB (571 aa) led to identification of several conserved domains characteristic of TonB proteins, including an N-terminal transmembrane domain, a central proline-rich spacer and a C-terminal TonB-related domain (TBRD). We identified a novel glycine-rich domain containing (Gly-X) n repeats. To assess FdTonB function, we constructed a ΔtonB mutant through homologous recombination based upon truncation of the central proline-rich spacer, glycine-rich domain and TBRD. Our ΔtonB mutant exhibited an aberrant cellular morphology under green light, with expanded cell width compared to the parental wild-type (WT) strain. The cellular morphology of the ΔtonB mutant recovered upon WT tonB expression. Interestingly, tonB expression was found to be independent of RcaE. As ΔtonB and WT strains respond in the same way when grown under iron-replete versus iron-limited conditions, our results suggest that FdTonB is not involved in the classic TonB function of mediating cellular adaptation to iron limitation, but exhibits a novel function related to the photoregulation of cellular morphology in F. diplosiphon.

Acclimation of Trichodesmium erythraeum ISM101 to high and low irradiance analysed on the physiological, biophysical and biochemical level

As the nonheterocystous diazotrophic cyanobacterium Trichodesmium lives both at the ocean surface and deep in the water column, it has to acclimate to vastly different irradiances. Here, we investigate its strategy of light acclimation in several ways. In this study, we used spectrally resolved fluorescence kinetic microscopy to investigate the biophysics of photosynthesis in individual cells, analysed cell extracts for pigment and phycobiliprotein composition, measured nitrogenase activity and the abundance of key proteins, and assayed protein synthesis/degradation by radioactive labelling. After acclimation to high light, Trichodesmium grew faster at 1000 micromol m(-2) s(-1) than at 100 micromol m(-2) s(-1). This acclimation was associated with decreasing cell diameter, faster protein turnover, the down-regulation of light-harvesting pigments and the outer part of the phycobiliprotein antenna, the up-regulation of light-protective carotenoids, changes in the coupling of phycobilisomes to the reaction centres and in the coupling of individual phycobiliproteins to the phycobilisomes. The latter was particularly interesting, as it represents an as yet unreported light acclimation strategy. Only in the low light-acclimated culture and only after the onset of actinic light did phycourobilin and phycoerythrin contribute to photochemical fluorescence quenching, showing that these phycobiliproteins may become quickly (in seconds) very closely coupled to photosystem II. This fast reversible coupling also became visible in the nonphotochemical changes of the fluorescence quantum yield.