Cyanobacterium Trichodesmium Erythraeum Research Articles

Aluminum-induced changes in the net carbon fixation and carbon decomposition of a nitrogen-fixing cyanobacterium Trichodesmium erythraeum

Aluminum-induced changes in the net carbon fixation and carbon decomposition of a nitrogen-fixing cyanobacterium Trichodesmium erythraeum

Trichodesmium erythraeum produces a higher photocurrent than other cyanobacterial species in bio-photo electrochemical cells

The increase in world energy consumption, and the worries from potential future disasters that may derive from climate change have stimulated the development of renewable energy technologies. One promising method is the utilization of whole photosynthetic cyanobacterial cells to produce photocurrent in a bio-photo electrochemical cell (BPEC). The photocurrent can be derived from either the respiratory or photosynthetic pathways, via the redox couple NADP+/NADPH mediating cyclic electron transport between photosystem I inside the cells, and the anode. In the past, most studies have utilized the fresh-water cyanobacterium Synechocystis sp. PCC 6803 (Syn). Here, we show that the globally important marine cyanobacterium Trichodesmium erythraeum flourishing in the subtropical oceans can provide improved currents as compared to Syn. We applied 2D-fluorescence measurements to detect the secretion of NADPH and show that the resulting photocurrent production is enhanced by increasing the electrolyte salinity, Further enhancement of the photocurrent can be obtained by the addition of electron mediators such as NAD+, NADP+, cytochrome C, vitamin B1, or potassium ferricyanide. Finally, we produce photocurrent from additional cyanobacterial species: Synechocystis sp. PCC6803, Synechococcus elongatus PCC7942, Acaryochloris marina MBIC 11017, and Spirulina, using their cultivation media as electrolytes for the BPEC.

Spatial variability in the community structure of microphytoplankton along the southeastern Arabian sea during an unusually warm winter monsoon season

The microphytoplankton community structure and composition were investigated along the South Eastern Arabian Sea (SEAS) during the late winter monsoon season (January 2020). A total of 186 species of phytoplankton were identified, with cyanobacteria Trichodesmium erythraeum (72%) being the most dominant, followed by diatoms (11%) and dinoflagellates (2.7%). Major blooms of T. erythraeum were observed towards the southern region of SEAS characterised by high SST (28.75 ± 0.7 °C), high surface salinity (34.6 ± 1.2 psu) and low wind speed (∼3.6 m s−1). The T. erythraeum bloom areas generally observed high nitrate and low silicate and phosphate concentrations with high chlorophyll a concentration (8.96 ± 2.92 mg m−3). In non-bloom regions, centric diatoms like Chaetoceros spp., Bacteriastrum spp. and Leptocylindrus sp. dominated. The abundance of diatoms observed a decreasing trend towards the southern extend of SEAS. The remote sensing data derived for the area observed sea surface temperatures are about 1.0 °C higher than the climatological mean of January. The higher temperatures (29 °C–32 °C) are clearly visible in the Indian coastal region from Kanyakumari to the Gujarat coast, suggesting unusual warming in the eastern coast of the Arabian Sea in January 2020. This warming which was more prominent towards the southern extend of SEAS, might have augmented the proliferation of T. erythraeum along the surface waters of SEAS.

Massive Blooms of Cyanobacteria Trichodesmium erythraeum and Synechococcus sp. in Mandapam Group of Islands, Southeast Coast of India, Tamil Nadu

Massive Blooms of Cyanobacteria Trichodesmium erythraeum and Synechococcus sp. in Mandapam Group of Islands, Southeast Coast of India, Tamil Nadu

Distinct Responses of the Nitrogen-Fixing Marine Cyanobacterium Trichodesmium to a Thermally Variable Environment as a Function of Phosphorus Availability.

Surface temperature in the ocean is projected to be elevated and more variable in the future, which will interact with other environmental changes like reduced nutrient supplies. To explore these multiple stressor relationships, we tested the influence of thermal variation on the key marine diazotrophic cyanobacterium Trichodesmium erythraeum GBRTRLI101 as a function of the limiting nutrient phosphorus (P). Two constant temperature treatments represented current winter (22°C) and summer (30°C) mean values. Three variable temperature treatments fluctuated around the constant control values: Mean 22°C, either ± 2°C or ± 4°C; and mean 30°C ± 2°C. Each thermal treatment was grown under both P-replete (10 μmol/L) and P-limiting conditions (0.2 μmol/L). Effects of thermal variability on Trichodesmium were mainly found in the two winter variable temperature treatments (22°C ± 2°C or ± 4°C). P availability affected growth and physiology in all treatments and had significant interactions with temperature. P-replete cultures had higher growth and nitrogen and carbon fixation rates in the 22°C constant control, than in the corresponding variable treatments. However, physiological rates were not different in the P-replete constant and variable treatments at 30°C. In contrast, in P-limited cultures an advantage of constant temperature over variable temperature was not apparent. Phosphorus use efficiencies (PUE, mol N or C fixed h-1 mol cellular P-1) for nitrogen and carbon fixation were significantly elevated under P-limited conditions, and increased with temperature from 22 to 30°C, implying a potential advantage in a future warmer, P-limited environment. Taken together, these results imply that future increasing temperature and greater thermal variability could have significant feedback interactions with the projected intensification of P-limitation of marine N2-fixing cyanobacteria.

Transcriptional and Proteomic Choreography Under Phosphorus Deficiency and Re-supply in the N2 Fixing Cyanobacterium Trichodesmium erythraeum.

The N2 fixing cyanobacterium Trichodesmium is a critically important organism in oligotrophic marine ecosystems, supplying “new” nitrogen (N) to the otherwise N-poor tropical and subtropical regions where it occurs. Low concentrations of phosphorus (P) in these regions can constrain Trichodesmium distribution and N2 fixation rates. Physiological characterization of a single species in a mixed community can be challenging, and ‘omic approaches are increasingly important tools for tracking nutritional physiology in a taxon-specific manner. As such, studies examining the dynamics of gene and protein markers of physiology (e.g., nutrient stress) are critical for the application and interpretation of such ‘omic data in situ. Here we leveraged combined transcriptomics, proteomics, and enzyme activity assays to track the physiological response of Trichodesmium erythraeum IMS101 to P deficiency and subsequent P re-supply over 72 h of sampling. P deficiency resulted in differential gene expression, protein abundance, and enzyme activity that highlighted a synchronous shift in P physiology with increases in the transcripts and corresponding proteins for hydrolyzing organic phosphorus, taking up phosphate with higher affinity, and modulating intracellular P demand. After P deficiency was alleviated, gene expression of these biomarkers was reduced to replete levels within 4 h of P amendment. A number of these gene biomarkers were adjacent to putative pho boxes and their expression patterns were similar to a sphR response regulator. Protein products of the P deficiency biomarkers were slow to decline, with 84% of the original P deficient protein set still significantly differentially expressed after 72 h. Alkaline phosphatase activity tracked with proteins for this enzyme. With the rapid turnover time of transcripts, they appear to be good biomarkers of a P stress phenotype, whereas proteins, with a slower turnover time, may better reflect cellular activities. These results highlight the importance of validating and pairing transcriptome and proteome data that can be applied to physiological studies of key species in situ.

Seasonal variation of the phytoplankton community in tuna farms in Bahia de La Paz, southern Gulf of California, Mexico

The phytoplankton community of tuna farms in Bahia de La Paz, southern Baja California, Mexico was studied. Samples were taken from tuna farms near San Juan de la Costa, Rancheros del Mar and from a station in El Mogote, away from the influence of these farms. Water samples were collected monthly (June 2006-May 2007) with a van Dorn bottle at different depths in San Juan de la Costa (0, 25 and 50 m), Rancheros del Mar (0, 15 and 30 m) and El Mogote (0 m) to measure temperature, dissolved oxygen, inorganic nutrients, and chlorophyll-a and to determine phytoplankton composition and abundance. The index of simple dominance, Shannon-Weaver diversity, and Pielou evenness were estimated. The water column was stratified from June to September (~27.5°C) and mixed from October to May (~18.5°C). Nutrient concentrations at the three sampling sites did not increase significantly. Nanophytoplankton dominated, followed by microplanktonic diatoms, and the highest diatom abundance occurred during the warm period. The phytoplankton community was more diverse in San Juan de la Costa than in Rancheros del Mar. In the latter, a superficial bloom of the cyanobacterium Trichodesmium erythraeum (1630×103 cel L -1 ) occurred in September, and in San Juan de la Costa the diatom Pseudo-nitzschia fraudulenta reached the highest abundance (183.8×103 cel L -1 ) in July.

Cysteine-Free Intramolecular Ligation of N-Sulfanylethylanilide Peptide Using 4-Mercaptobenzylphosphonic Acid: Synthesis of Cyclic Peptide Trichamide

An N-sulfanylethylanilide (SEAlide)-based ligation was developed for the preparation of trichamide, a thiazole-containing cyclic peptide isolated from bloom-forming cyanobacterium Trichodesmium erythraeum. In this cysteine-free ligation, 4-mercaptobenzylphosphonic acid (MBPA) functions as a dual promoter both for the N–S acyl-transfer-mediated activation of the SEAlide unit and for subsequent ligation. Furthermore, we established a high-yielding route to enantiomerically pure thiazole amino acids using a one-pot Hantzsch process.

Diversity and Activity of Diazotrophs in Great Barrier Reef Surface Waters.

Discrepancies between bioavailable nitrogen (N) concentrations and phytoplankton growth rates in the oligotrophic waters of the Great Barrier Reef (GBR) suggest that undetermined N sources must play a significant role in supporting primary productivity. One such source could be biological dinitrogen (N2) fixation through the activity of “diazotrophic” bacterioplankton. Here, we investigated N2 fixation and diazotroph community composition over 10° S of latitude within GBR surface waters. Qualitative N2 fixation rates were found to be variable across the GBR but were relatively high in coastal, inner and outer GBR waters, reaching 68 nmol L-1 d-1. Diazotroph assemblages, identified by amplicon sequencing of the nifH gene, were dominated by the cyanobacterium Trichodesmium erythraeum, γ-proteobacteria from the Gamma A clade, and δ-proteobacterial phylotypes related to sulfate-reducing genera. However, diazotroph communities exhibited significant spatial heterogeneity, correlated with shifts in dissolved inorganic nutrient concentrations. Specifically, heterotrophic diazotrophs generally increased in relative abundance with increasing concentrations of phosphate and N, while Trichodesmium was proportionally more abundant when concentrations of these nutrients were low. This study provides the first in-depth characterization of diazotroph community composition and N2 fixation dynamics within the oligotrophic, N-limited surface waters of the GBR. Our observations highlight the need to re-evaluate N cycling dynamics within oligotrophic coral reef systems, to include diverse N2 fixing assemblages as a potentially significant source of dissolved N within the water column.

Management and Treatment of Dengue and Chikungunya - Natural Products to the Rescue.

Neglected tropical diseases (NTDs) flourish mostly in impoverished developing nations of the world. It is estimated that NTDs plague up to 1 billion people every year thereby inducing a massive economic and health burden worldwide. Following explosive outbreaks mostly in Asia, Latin America, Europe and the Indian Ocean, two common NTDs namely, Chikungunya and Dengue both transmitted by an infected mosquito vector principally Aedes aegypti have emerged as a major public health threat. Given the limitations of conventional medicine in specifically targeting the Chikungunya and Dengue virus (CHIKV and DENV), natural products present an interesting avenue to explore in the quest of developing novel anti; mosquito, CHIKV and DENV agents. In this endeavor, a number of plant extracts, isolated phytochemicals, essential oils and seaweeds have shown promising larvicidal and insecticidal activity against some mosquito vectors as well as anti CHIKV and DENV activity invitro. Other natural products that have depicted good potential against these diseases include; the symbiotic bacterial genus Wolbachia which can largely reduce the life span and infectivity of mosquito vectors and the marine Cyanobacterium Trichodesmium erythraeum which has shown anti- CHIKV activity at minimal cytotoxic level. The impetus of modern drug discovery approaches such as high throughput screening, drug repositioning, synthesis and computer-aided drug design will undeniably enhance the process of developing more stable lead molecules from natural products which have shown promising antiviral activity in-vitro.

Floraciones algales nocivas durante el periodo 2008-2010 en el Golfo de Nicoya, Costa Rica

En este estudio se analizaron los eventos de floraciones algales nocivas (FAN) en cuatro estaciones de muestreo fijas en el Golfo de Nicoya durante el periodo de enero de 2008 a diciembre de 2010. Se recolectaron muestras de agua a 1 y 5 m de profundidad durante cada evento de FAN. Además, se midieron in situ los parámetros fisicoquímicos: profundidad del disco de Secchi, temperatura, salinidad, oxígeno disuelto y el porcentaje de saturación de oxígeno del agua. Los datos de radiación solar y precipitación fueron suministrados por el Instituto Meteorológico Nacional de Costa Rica. Se determinaron las concentraciones de las microalgas responsables y las especies asociadas. Se registraron 11 eventos, de los cuales el 72.7% se presentaron en la época lluviosa y un 27.3% en la época seca, la mayoría de éstos, durante el fenómeno atmosférico de La Niña. Las especies responsables fueron los dinoflagelados Alexandrium monilatum, Levanderina fissa, Cochlodinium polykrikoides y Prorocentrum minimum, la diatomea Skeletonema costatum y la cianobacteria Trichodesmium erythraeum. Las especies asociadas que se registraron con mayor frecuencia fueron los dinoflagelados Dinophysis caudata, Prorocentrum micans, Tripos fusus, Tripos furca y Protoperidinium conicum. Las FAN en el Golfo de Nicoya fueron más frecuentes durante la época lluviosa, probablemente debido a los aportes fluviales al sistema estuarino de los ríos Tempisque, Barranca y Grande de Tárcoles. Esa estacionalidad, en conjunto con las alteraciones provocadas por eventos atmosféricos globales como La Niña y El Niño, determinó la formación de FAN en el golfo durante el periodo de estudio.

Levels of daily light doses under changed day-night cycles regulate temporal segregation of photosynthesis and N2 Fixation in the cyanobacterium Trichodesmium erythraeum IMS101.

While the diazotrophic cyanobacterium Trichodesmium is known to display inverse diurnal performances of photosynthesis and N2 fixation, such a phenomenon has not been well documented under different day-night (L-D) cycles and different levels of light dose exposed to the cells. Here, we show differences in growth, N2 fixation and photosynthetic carbon fixation as well as photochemical performances of Trichodesmium IMS101 grown under 12L:12D, 8L:16D and 16L:8D L-D cycles at 70 μmol photons m-2 s-1 PAR (LL) and 350 μmol photons m-2 s-1 PAR (HL). The specific growth rate was the highest under LL and the lowest under HL under 16L:8D, and it increased under LL and decreased under HL with increased levels of daytime light doses exposed under the different light regimes, respectively. N2 fixation and photosynthetic carbon fixation were affected differentially by changes in the day-night regimes, with the former increasing directly under LL with increased daytime light doses and decreased under HL over growth-saturating light levels. Temporal segregation of N2 fixation from photosynthetic carbon fixation was evidenced under all day-night regimes, showing a time lag between the peak in N2 fixation and dip in carbon fixation. Elongation of light period led to higher N2 fixation rate under LL than under HL, while shortening the light exposure to 8 h delayed the N2 fixation peaking time (at the end of light period) and extended it to night period. Photosynthetic carbon fixation rates and transfer of light photons were always higher under HL than LL, regardless of the day-night cycles. Conclusively, diel performance of N2 fixation possesses functional plasticity, which was regulated by levels of light energy supplies either via changing light levels or length of light exposure.

Cellular inorganic carbon fluxes in Trichodesmium: a combined approach using measurements and modelling.

To predict effects of climate change on phytoplankton, it is crucial to understand how their mechanisms for carbon acquisition respond to environmental conditions. Aiming to shed light on the responses of extra- and intracellular inorganic C (Ci) fluxes, the cyanobacterium Trichodesmium erythraeum IMS101 was grown with different nitrogen sources (N2 vs NO3 (-)) and pCO2 levels (380 vs 1400 µatm). Cellular Ci fluxes were assessed by combining membrane inlet mass spectrometry (MIMS), (13)C fractionation measurements, and modelling. Aside from a significant decrease in Ci affinity at elevated pCO2 and changes in CO2 efflux with different N sources, extracellular Ci fluxes estimated by MIMS were largely unaffected by the treatments. (13)C fractionation during biomass production, however, increased with pCO2, irrespective of the N source. Strong discrepancies were observed in CO2 leakage estimates obtained by MIMS and a (13)C-based approach, which further increased under elevated pCO2. These offsets could be explained by applying a model that comprises extracellular CO2 and HCO3 (-) fluxes as well as internal Ci cycling around the carboxysome via the CO2 uptake facilitator NDH-14. Assuming unidirectional, kinetic fractionation between CO2 and HCO3 (-) in the cytosol or enzymatic fractionation by NDH-14, both significantly improved the comparability of leakage estimates. Our results highlight the importance of internal Ci cycling for (13)C composition as well as cellular energy budgets of Trichodesmium, which ought to be considered in process studies on climate change effects.

Erratum: A decadal decline in relative abundance and a shift in microphytoplankton composition at a long‐term coastal station off southeast Australia

In this study, we examined 11 yr (1998–2009) of water samples collected from Port Hacking coastal monitoring station 8 km offshore from Sydney, Australia, to assess changes in the microphytoplankton in relation to climate-related trends in environmental variables. A total of 152 taxa (85 genera) were identified, with the small diatom Thalassiosira cf. partheneia and the tropical cyanobacterium Trichodesmium erythraeum being the dominant species over the past decade. Taxon richness showed a distinct seasonal pattern, peaking in the austral winter. Ordination analyses revealed significant seasonal and interannual trends in species composition, including a decadal decline in dinoflagellates relative to diatoms toward the present. This decadal shift in taxonomic composition was, in turn, significantly associated with declines in water temperatures over this time period. Total abundance varied by 12 orders of magnitude (8.5 × 10−8 to 7.4 × 104 cells L−1), with values separable into bloom and nonbloom regimes at an abundance threshold of 7.1 × 10−5 cells L−1. Significant temporal declines in abundance were observed during both bloom and nonbloom regimes. Blooms occurred most consistently in March, September, and December.

The TrpA protein of Trichodesmium erythraeum IMS101 is a non-fibril-forming collagen and a component of the outer sheath.

Collagen molecules are structural in nature and primarily found in eukaryotic, multicellular organisms. Recently, a collagen-like protein, TrpA, was identified and characterized in the marine cyanobacterium Trichodesmium erythraeum IMS 101, and it was shown to be involved in maintaining the structural integrity of the trichomes. The TrpA protein contains one glycine interruption in the otherwise perfectly uninterrupted collagenous domain. In this study, we used phylogenetic analysis to determine that the TrpA protein sequence is most closely associated with non-fibril-forming collagen proteins. Structural modelling and circular dichroism data suggest that the glycine insertion decreases the stability of TrpA compared to uninterrupted collagen sequences. Additionally, scanning electron microscopy revealed that TrpA is expressed entirely on the surface of the trichomes, with no specific pattern of localization. These data indicate that the TrpA protein is part of the outer sheath of this organism. As such, this protein may function to promote adhesion between individual T. erythraeum trichomes, and between this organism and heterotrophic bacteria found in the same environment.

A decadal decline in relative abundance and a shift in microphytoplankton composition at a long‐term coastal station off southeast Australia

In this study, we examined 11 yr (1998–2009) of water samples collected from Port Hacking coastal monitoring station 8 km offshore from Sydney, Australia, to assess changes in the microphytoplankton in relation to climate‐related trends in environmental variables. A total of 152 taxa (85 genera) were identified, with the small diatom Thalassiosira cf. partheneia and the tropical cyanobacterium Trichodesmium erythraeum being the dominant species over the past decade. Taxon richness showed a distinct seasonal pattern, peaking in the austral winter. Ordination analyses revealed significant seasonal and interannual trends in species composition, including a decadal decline in dinoflagellates relative to diatoms toward the present. This decadal shift in taxonomic composition was, in turn, significantly associated with declines in water temperatures over this time period. Total abundance varied by 12 orders of magnitude (8.5 × 10−8 to 7.4 × 104 cells L−1), with values separable into bloom and nonbloom regimes at an abundance threshold of 7.1 × 10−5 cells L−1. Significant temporal declines in abundance were observed during both bloom and nonbloom regimes. Blooms occurred most consistently in March, September, and December.

Anti-Chikungunya Viral Activities of Aplysiatoxin-Related Compounds from the Marine Cyanobacterium Trichodesmium erythraeum

Tropical filamentous marine cyanobacteria have emerged as a viable source of novel bioactive natural products for drug discovery and development. In the present study, aplysiatoxin (1), debromoaplysiatoxin (2) and anhydrodebromoaplysiatoxin (3), as well as two new analogues, 3-methoxyaplysiatoxin (4) and 3-methoxydebromoaplysiatoxin (5), are reported for the first time from the marine cyanobacterium Trichodesmium erythraeum. The identification of the bloom-forming cyanobacterial strain was confirmed based on phylogenetic analysis of its 16S rRNA sequences. Structural determination of the new analogues was achieved by extensive NMR spectroscopic analysis and comparison with NMR spectral data of known compounds. In addition, the antiviral activities of these marine toxins were assessed using Chikungunya virus (CHIKV)-infected cells. Post-treatment experiments using the debrominated analogues, namely compounds 2, 3 and 5, displayed dose-dependent inhibition of CHIKV when tested at concentrations ranging from 0.1 µM to 10.0 µM. Furthermore, debromoaplysiatoxin (2) and 3-methoxydebromoaplysiatoxin (5) exhibited significant anti-CHIKV activities with EC50 values of 1.3 μM and 2.7 μM, respectively, and selectivity indices of 10.9 and 9.2, respectively.

Increased intracellular concentrations of DMSP and DMSO in iron‐limited oceanic phytoplankton Thalassiosira oceanica and Trichodesmium erythraeum

We investigated the link between iron (Fe) limitation and intracellular dimethylsulfoniopropionate (DMSP) concentration in two oceanic phytoplankton species, the diatom Thalassiosira oceanica and the diazotrophic cyanobacterium Trichodesmium erythraeum. Dimethylsulfoxide (DMSO) concentrations were also measured in Fe‐replete and Fe‐limited T. oceanica. Fe limitation decreased the growth rates of T. oceanica and T. erythraeum by 33‐fold and 3.5‐fold, respectively and increased intracellular DMSP (DMSPp) concentrations by 12‐fold (from 2.8 to 33.7 mmol Lcell−1) and by 45‐fold (from 0.05 to 2.27 mmol Ltrichome−1), respectively. Intracellular dimethylsulfoxide (DMSOp) concentrations in T. oceanica increased by 5‐fold under severe Fe limitation, from 0.78 mmol Lcell−1 in Fe‐replete cells to 3.86 mmol Lcell−1. The increase in DMSPp and DMSOp under Fe limitation provides support for the role of these sulfur compounds as antioxidants. Under severe Fe limitation, the large increase in DMSPp : C and DMSP : chlorophyll a (Chl a) ratios for both T. oceanica (by 16‐ and 40‐fold, respectively) and T. erythraeum (by 18‐ and 145‐fold, respectively) places these species above the range of values generally attributed to diatoms and cyanophytes. Comparison of these values with in situ results, such as those from Fe fertilization experiments, suggests that the decrease in DMSPp : Chl a and DMSOp : Chl a that is generally observed with alleviation of Fe limitation may be partly related to decreases in DMSPp and DMSOp in individual species. The role of diatoms and diazotrophic cyanobacteria in the biogeochemical cycle of dimethylsulfide and associated sulfur compounds in Fe‐limited oceanic environments should not be overlooked.

Occurrence and Distribution of a Diatom-Diazotrophic Cyanobacteria Association during a Trichodesmium Bloom in the Southeastern Arabian Sea

Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata and Rhizosolenia formosa with endosymbiotic cyanobacteria Richelia intracellularis was noticed and documented for the first time during a bloom of the cyanobacterium Trichodesmium erythraeum in the oligotrophic shelf waters along Kochi and Mangalore transects, southeastern Arabian Sea (SEAS), during spring intermonsoon (April 2012). Although the host is frequently seen, the symbiont is rarely reported in the Indian EEZ. The presence of nitrogen-fixing symbiotic association of Rhizosolenia-Richelia along with Trichodesmium erythraeum highlights the significance of DDAs on the nutrient and energy budgets of phytoplankton in the oligotrophic environments of the Arabian Sea during spring intermonsoon.

NITROGEN FIXATION, HYDROGEN CYCLING, AND ELECTRON TRANSPORT KINETICS IN TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA) STRAIN IMS101(1).

This study describes the relationships between dinitrogen (N2 ) fixation, dihydrogen (H2 ) production, and electron transport associated with photosynthesis and respiration in the marine cyanobacterium Trichodesmium erythraeum Ehrenb. strain IMS101. The ratio of H2 produced:N2 fixed (H2 :N2 ) was controlled by the light intensity and by the light spectral composition and was affected by the growth irradiance level. For Trichodesmium cells grown at 50 μmol photons · m(-2) · s(-1) , the rate of N2 fixation, as measured by acetylene reduction, saturated at light intensities of 200 μmol photons · m(-2) · s(-1) . In contrast, net H2 production continued to increase with light levels up to 1,000 μmol photons · m(-2) · s(-1) . The H2 :N2 ratios increased monotonically with irradiance, and the variable fluorescence measured using a fast repetition rate fluorometer (FRRF) revealed that this increase was accompanied by a progressive reduction of the plastoquinone (PQ) pool. Additions of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), an inhibitor of electron transport from PQ pool to PSI, diminished both N2 fixation and net H2 production, while the H2 :N2 ratio increased with increasing level of PQ pool reduction. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), nitrogenase activity declined but could be prolonged by increasing the light intensity and by removing the oxygen supply. These results on the coupling of N2 fixation and H2 cycling in Trichodesmium indicate how light intensity and light spectral quality of the open ocean can influence the H2 :N2 ratio and modulate net H2 production.