Regulation of nitrogen-fixation by different nitrogen sources in the marine non-heterocystous cyanobacterium Trichodesmium sp. NIBB1067

Sunscreen exposure interferes with physiological processes while inducing oxidative stress in seagrass Posidonia oceanica (L.) Delile

The effects of a commercial sunscreen mixture on the Mediterranean seagrass Posidonia oceanica were investigated, evaluating its response in physiological processes and biochemical indicators of oxidative stress. Short-term laboratory experiments were conducted recreating summer conditions, and two sunscreen concentrations were tested in whole P. oceanica plants placed inside aquaria. Although primary productivity of leaf segments seemed to benefit from sunscreen addition, probably due to inorganic nutrients released, the rest of the biological parameters reflected possible impairments in the overall functioning of P. oceanica as a result of oxidative damages. Chlorophyll production and nitrogen fixation associated with old leaves were inhibited under high sunscreen concentrations, which concurred with elevated reactive oxygen species production, catalase activity and polyphenols content in the seagrass leaves. These results emphasize the importance of directing future investigations on determining which specific components of sunscreen products are likely threatening the wellbeing of critical species, such as P. oceanica.

Independence of a Marine Unicellular Diazotroph to the Presence of NO3. Microorganisms 2021;9:microorganisms9102073. [PMID: 34683393 PMCID: PMC8540418 DOI: 10.3390/microorganisms9102073]

Marine nitrogen (N₂) fixation was historically considered to be absent or reduced in nitrate (NO₃⁻) rich environments. This is commonly attributed to the lower energetic cost of NO₃⁻ uptake compared to diazotrophy in oxic environments. This paradigm often contributes to making inferences about diazotroph distribution and activity in the ocean, and is also often used in biogeochemical ocean models. To assess the general validity of this paradigm beyond the traditionally used model organism Trichodesmium spp., we grew cultures of the unicellular cyanobacterium Crocosphaera watsonii WH8501 long term in medium containing replete concentrations of NO₃⁻. NO₃⁻ uptake was measured in comparison to N₂ fixation to assess the cultures’ nitrogen source preferences. We further measured culture growth rate, cell stoichiometry, and carbon fixation rate to determine if the presence of NO₃⁻ had any effect on cell metabolism. We found that uptake of NO₃⁻ by this strain of Crocosphaera was minimal in comparison to other N sources (~2–4% of total uptake). Furthermore, availability of NO₃⁻ did not statistically alter N₂ fixation rate nor any aspect of cell physiology or metabolism measured (cellular growth rate, cell stoichiometry, cell size, nitrogen fixation rate, nitrogenase activity) in comparison to a NO₃⁻ free control culture. These results demonstrate the capability of a marine diazotroph to fix nitrogen and grow independently of NO₃⁻. This lack of sensitivity of diazotrophy to NO₃⁻ suggests that assumptions often made about, and model formulations of, N₂ fixation should be reconsidered.

Unusual marine cyanobacteria/haptophyte symbiosis relies on N2 fixation even in N-rich environments

The microbial fixation of N₂ is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N₂-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N₂ fixation to DIN is unknown. We used stable isotope incubations, catalyzed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry (nanoSIMS), to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N₂ fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO₃⁻) and meet little of their N demands via ammonium (NH₄⁺) uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N₂ fixation to supply large portions of the haptophyte’s N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N₂ fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by NO₃⁻ additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. The results suggest N₂ fixation may be an evolutionarily viable strategy for diazotroph–eukaryote symbioses, even in N-rich coastal or high latitude waters.

The physiological cost of diazotrophy for Trichodesmium erythraeum IMS101

Trichodesmium plays a significant role in the oligotrophic oceans, fixing nitrogen in an area corresponding to half of the Earth's surface, representing up to 50% of new production in some oligotrophic tropical and subtropical oceans. Whilst Trichodesmium blooms at the surface exhibit a strong dependence on diazotrophy, colonies at depth or at the surface after a mixing event could be utilising additional N-sources. We conducted experiments to establish how acclimation to varying N-sources affects the growth, elemental composition, light absorption coefficient, N2 fixation, PSII electron transport rate and the relationship between net and gross photosynthetic O2 exchange in T. erythraeum IMS101. To do this, cultures were acclimated to growth medium containing NH4+ and NO3- (replete concentrations) or N2 only (diazotrophic control). The light dependencies of O2 evolution and O2 uptake were measured using membrane inlet mass spectrometry (MIMS), while PSII electron transport rates were measured from fluorescence light curves (FLCs). We found that at a saturating light intensity, Trichodesmium growth was ~ 10% and 13% lower when grown on N2 than with NH4+ and NO3-, respectively. Oxygen uptake increased linearly with net photosynthesis across all light intensities ranging from darkness to 1100 μmol photons m-2 s-1. The maximum rates and initial slopes of light response curves for C-specific gross and net photosynthesis and the slope of the relationship between gross and net photosynthesis increased significantly under non-diazotrophic conditions. We attribute these observations to a reduced expenditure of reductant and ATP for nitrogenase activity under non-diazotrophic conditions which allows NADPH and ATP to be re-directed to CO2 fixation and/or biosynthesis. The energy and reductant conserved through utilising additional N-sources could enhance Trichodesmium's productivity and growth and have major implications for its role in ocean C and N cycles.

Comment on "The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium"

Hong et al (Reports, 5 May 2017, p. 527) suggested that previous studies of the biogeochemically significant marine cyanobacterium Trichodesmium showing increased growth and nitrogen fixation at projected future high CO₂ levels suffered from ammonia or copper toxicity. They reported that these rates instead decrease at high CO₂ when contamination is alleviated. We present and discuss results of multiple published studies refuting this toxicity hypothesis.

Dissolved organic matter uptake by Trichodesmium in the Southwest Pacific

The globally distributed diazotroph Trichodesmium contributes importantly to nitrogen inputs in the oligotrophic oceans. Sites of dissolved organic matter (DOM) accumulation could promote the mixotrophic nutrition of Trichodesmium when inorganic nutrients are scarce. Nano-scale secondary ion mass spectrometry (nanoSIMS) analyses of individual trichomes sampled in the South Pacific Ocean, showed significant ¹³C-enrichments after incubation with either ¹³C-labeled carbohydrates or amino acids. These results suggest that DOM could be directly taken up by Trichodesmium or primarily consumed by heterotrophic epibiont bacteria that ultimately transfer reduced DOM compounds to their host trichomes. Although the addition of carbohydrates or amino acids did not significantly affect bulk N₂ fixation rates, N₂ fixation was enhanced by amino acids in individual colonies of Trichodesmium. We discuss the ecological advantages of DOM use by Trichodesmium as an alternative to autotrophic nutrition in oligotrophic open ocean waters.

Diazotrophic bacterial community variability in a subtropical deep reservoir is correlated with seasonal changes in nitrogen

Nitrogen-fixing microorganisms (diazotrophs) play important roles in aquatic biogeochemistry and ecosystem functioning. However, little is known about the spatiotemporal variation of diazotrophic microbial communities in deep subtropical reservoirs. In this study, denaturing gradient gel electrophoresis (DGGE), clone libraries, quantitative PCR, and quantitative reverse transcription (RT)-PCR were used together to examine the vertical and seasonal patterns of diazotrophic microbial communities based on nitrogenase (nifH) gene sequences in the Dongzhen Reservoir, China, across time (every 3 months for 1 year) and space (five different water depths). In general, the numbers of DGGE bands increased with water depth during the stratification seasons (spring, summer, and autumn), with the clone-library-based operational taxonomic unit (OTU) number and nifH gene diversity being highest in autumn (6 OTUs at depth 0 m; 15 OTUs at 33 m) and winter (12 OTUs at 0 m, 13 OTUs at 33 m) but decreasing drastically in spring (2 OTUs at 0 m, 3 OTUs at 33 m) and summer (3 OTUs at 0 m, 2 OTUs at 33 m). The nifH gene abundance was lowest in the water mixing season (winter average, 5.17 × 10(7) copies/L) but increased in the three other seasons (9.03 × 10(9) copies/L). Cyanobacteria (dominated by filamentous thermophilic cyanobacteria and Cylindrospermopsis raciborskii) were the most dominant diazotrophic group at all depths and seasons, while both alphaproteobacteria and gammaproteobacteria were co-dominant in the bottom waters in autumn and winter. The distinct seasonal and spatial patterns in diazotrophic communities were significantly related to total nitrogen (TN) and ammonium nitrogen (NH4-N) in the reservoir (P < 0.01). Further, TN showed a significant positive correlation with nifH RNA copy number (P < 0.05) and DGGE band number (P < 0.01), whereas the NH4-N was negatively correlated with nifH DNA copy number (P < 0.01) and positively with both RNA/DNA ratio (P < 0.01) and DGGE band number (P < 0.01). Our data indicated that water stratification, mixing, and nitrogen might drive the diazotrophic community structure and activity in complex ways, thereby influencing the aquatic nitrogen cycle. Therefore, adaptive reservoir management strategies should carefully consider the effects of water stratification for protecting drinking water quality and for controlling the potential for diazotrophic cyanobacteria blooms.

Light-limited growth rate modulates nitrate inhibition of dinitrogen fixation in the marine unicellular cyanobacterium Crocosphaera watsonii

Biological N₂ fixation is the dominant supply of new nitrogen (N) to the oceans, but is often inhibited in the presence of fixed N sources such as nitrate (NO₃⁻). Anthropogenic fixed N inputs to the ocean are increasing, but their effect on marine N₂ fixation is uncertain. Thus, global estimates of new oceanic N depend on a fundamental understanding of factors that modulate N source preferences by N₂-fixing cyanobacteria. We examined the unicellular diazotroph Crocosphaera watsonii (strain WH0003) to determine how the light-limited growth rate influences the inhibitory effects of fixed N on N₂ fixation. When growth (µ) was limited by low light (µ = 0.23 d⁻¹), short-term experiments indicated that 0.4 µM NH₄⁺ reduced N₂-fixation by ∼90% relative to controls without added NH₄⁺. In fast-growing, high-light-acclimated cultures (µ = 0.68 d⁻¹), 2.0 µM NH₄⁺ was needed to achieve the same effect. In long-term exposures to NO₃⁻, inhibition of N₂ fixation also varied with growth rate. In high-light-acclimated, fast-growing cultures, NO₃⁻ did not inhibit N₂-fixation rates in comparison with cultures growing on N₂ alone. Instead NO₃⁻ supported even faster growth, indicating that the cellular assimilation rate of N₂ alone (i.e. dinitrogen reduction) could not support the light-specific maximum growth rate of Crocosphaera. When growth was severely light-limited, NO₃⁻ did not support faster growth rates but instead inhibited N₂-fixation rates by 55% relative to controls. These data rest on the basic tenet that light energy is the driver of photoautotrophic growth while various nutrient substrates serve as supports. Our findings provide a novel conceptual framework to examine interactions between N source preferences and predict degrees of inhibition of N₂ fixation by fixed N sources based on the growth rate as controlled by light.

Combined effects of different CO2 levels and N sources on the diazotrophic cyanobacterium Trichodesmium

To predict effects of climate change and possible feedbacks, it is crucial to understand the mechanisms behind CO2 responses of biogeochemically relevant phytoplankton species. Previous experiments on the abundant N2 fixers Trichodesmium demonstrated strong CO2 responses, which were attributed to an energy reallocation between its carbon (C) and nitrogen (N) acquisition. Pursuing this hypothesis, we manipulated the cellular energy budget by growing Trichodesmium erythraeum IMS101 under different CO2 partial pressure (pCO2 ) levels (180, 380, 980 and 1400 µatm) and N sources (N2 and NO3 (-) ). Subsequently, biomass production and the main energy-generating processes (photosynthesis and respiration) and energy-consuming processes (N2 fixation and C acquisition) were measured. While oxygen fluxes and chlorophyll fluorescence indicated that energy generation and its diurnal cycle was neither affected by pCO2 nor N source, cells differed in production rates and composition. Elevated pCO2 increased N2 fixation and organic C and N contents. The degree of stimulation was higher for nitrogenase activity than for cell contents, indicating a pCO2 effect on the transfer efficiency from N2 to biomass. pCO2 -dependent changes in the diurnal cycle of N2 fixation correlated well with C affinities, confirming the interactions between N and C acquisition. Regarding effects of the N source, production rates were enhanced in NO3 (-) grown cells, which we attribute to the higher N retention and lower ATP demand compared with N2 fixation. pCO2 effects on C affinity were less pronounced in NO3 (-) users than N2 fixers. Our study illustrates the necessity to understand energy budgets and fluxes under different environmental conditions for explaining indirect effects of rising pCO2 .

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.

Decoupling of ammonium regulation and ntcA transcription in the diazotrophic marine cyanobacterium Trichodesmium sp. IMS101

Nitrogen (N) physiology in the marine cyanobacterium Trichodesmium IMS101 was studied along with transcript accumulation of the N-regulatory gene ntcA and of two of its target genes: napA (nitrate assimilation) and nifH (N(2) fixation). N(2) fixation was impaired in the presence of nitrite, nitrate and urea. Strain IMS101 was capable of growth on these combined N sources at <2 μM but growth rates declined at elevated concentrations. Assimilation of nitrate and urea was impaired in the presence of ammonium. Whereas ecologically relevant N concentrations (2-20 μM) suppressed growth and assimilation, much higher concentrations were required to affect transcript levels. Transcripts of nifH accumulated under nitrogen-fixing conditions; these transcript levels were maintained in the presence of nitrate (100 μM) and ammonium (20 μM). However, nifH transcript levels were below detection at ammonium concentrations >20 μM. napA mRNA was found at low levels in both N(2)-fixing and ammonium-utilizing filaments, and it accumulated in filaments grown with nitrate. The positive effect of nitrate on napA transcription was abolished by ammonium additions of >200 μM. This effect was restored upon addition of the glutamine synthetase inhibitor L-methionin-DL-sulfoximine. Surprisingly, ntcA transcript levels remained high in the presence of ammonium, even at elevated concentrations. These findings indicate that ammonium repression is decoupled from transcriptional activation of ntcA in Trichodesmium IMS101.

Diazocyte development in the marine diazotrophic cyanobacterium Trichodesmium

The establishment of non-diazotrophic cultures of the filamentous marine cyanobacterium Trichodesmium erythraeum IMS101 enabled the first detailed investigation of the process leading to the development of its unique nitrogen-fixing cell type, the diazocyte. Trichome heterogeneity was apparent already within 3-8 h, while the differentiation of mature diazocytes, containing the nitrogenase enzyme, required 27 h after the removal of combined nitrogen. The distribution of 'pro-diazocytes' within the trichomes correlates with the localization of mature diazocytes, which suggests that pattern regulation is an early event during diazocyte development. The development was initially identified as changes in the subcellular ultrastructure, most notably the degradation of glycogen granules and gas vacuoles. These changes were preceded by the induced expression of the global nitrogen regulator ntcA at an early stage of combined nitrogen deprivation, followed by elevated expression of genes related to nitrogen metabolism and their corresponding proteins. The strongest induction (10-fold) was related to the transcription of the respiratory gene coxB2, apparent already at an early stage, which suggests an important role for respiration and the subsequent energy generation in the subcellular changes found, and in the creation of the reducing environment required for nitrogen fixation in diazocytes.

NifH expression by five groups of phototrophs compared with nitrogenase activity in coastal microbial mats

Diazotrophic (nitrogen-fixing) Cyanobacteria are often structurally dominant in coastal microbial mats but diazotrophs from other bacterial lineages are also present and active. The expression of nifH by four nonheterocystous Cyanobacteria and one member of the Gammaproteobacteria was followed over a 24-h cycle using quantitative reverse transcriptase-PCR. Daily nifH expression patterns were compared with the actual nitrogenase activity (NA) of the entire mat community. Lyngbya sp. was identified as the dominant cyanobacterium but, although recognized as a diazotroph, its cell-specific and abundance-related nifH expression was low. Unexpectedly, the other three cyanobacterial phylotypes dominated community nifH expression at all stations. Also, the gammaproteobacterium showed high levels of cell-specific nifH expression but its nifH copy number was low. Its contribution to the whole community nifH expression was therefore low. These results indicate that there were varying levels of cell-specific expression of nifH in the different mat types and more so, varying contributions to the overall nifH expression by the different diazotrophs. Furthermore, NA did not follow nifH expression patterns.

Nitrogenase gene expression in the Chesapeake Bay Estuary

Like many estuaries, the Chesapeake Bay has pronounced gradients in salinity and nutrients. Previous studies have shown that there is a high diversity of nitrogenase (nifH) genes in the estuary, and that there are specific distributions of individual nifH phylotypes. In contrast to previous work that revealed the remarkable diversity of nifH phylotypes in the Chesapeake estuary, in this study of nifH expression we only detected two phylotypes, and both were phylogenetically related to cyanobacterial nifH genes. One of the phylotypes was closely related to a nifH sequence from the filamentous, heterocystous cyanobacterium Anabaena cylindrica, and was found at the head of the estuary. The other phylotype was found in a sample collected near the mouth of the estuary and was closely related to nifH sequences from Group A unicellular cyanobacteria, which has previously been reported in oceanic waters only. These nifH phylotypes had distinct patterns of expression that were restricted to different regions of the Chesapeake Bay. This study provides the first evidence of nifH expression in the Chesapeake Bay, and suggests that diazotrophic unicellular cyanobacteria have a broader distribution and activity than previously recognized.

The nitrogen physiology of the marine N2-fixing cyanobacteria Trichodesmium spp

Trichodesmium spp. have proved to be enigmatic organisms, and their ecology and physiology are unusual among diazotrophs. Recent research shows that they can simultaneously fix N2 and take up combined nitrogen. The co-occurrence of these two processes is thought to be incompatible, but they could be obligatory in Trichodesmium spp. if only a small fraction of cells within a colony or along a filament are capable of N2 fixation. Combined nitrogen is released from cells during periods of active growth and N2 fixation, and concomitantly taken up by Trichodesmium spp. or cells living in association with colonies. Although the nitrogenase of Trichodesmium spp. is affected by high concentrations of combined nitrogen, it might be relatively less sensitive to low concentrations of combined nitrogen typical of the oligotrophic ocean and culture conditions. Nitrogenase activity and synthesis exhibits an endogenous rhythm in Trichodesmium spp. cultures, which is affected by the addition of nitrogen.

Nitrate assimilation genes of the marine diazotrophic, filamentous cyanobacterium Trichodesmium sp. strain WH9601

A 4.0-kb DNA fragment of Trichodesmium sp. strain WH9601 contained gene sequences encoding the nitrate reduction enzymes, nirA and narB. A third gene positioned between nirA and narB encodes a putative membrane protein with similarity to the nitrate permeases of Bacillus subtilis (NasA) and Emericella nidulans (CrnA). The gene was shown to functionally complement a DeltanasA mutant of B. subtilis and was assigned the name napA (nitrate permease). NapA was involved in both nitrate and nitrite uptake by the complemented B. subtilis cells. napA is distinct from the nrt genes that encode the nitrate transporter of freshwater cyanobacteria.

Whole-cell immunolocalization of nitrogenase in marine diazotrophic cyanobacteria, trichodesmium spp

The mechanism by which planktonic marine cyanobacteria of the genus Trichodesmium fix N2 aerobically during photosynthesis without heterocysts is unknown. As an aid in understanding how these species protect nitrogenase, we have developed an immunofluorescence technique coupled to light microscopy (IF-LM) with which intact cyanobacteria can be immunolabeled and the distribution patterns of nitrogenase and other proteins can be described and semiquantified. Chilled ethanol was used to fix the cells, which were subsequently made permeable to antibodies by using dimethyl sulfoxide. Use of this technique demonstrated that about 3 to 20 cells (mean +/- standard deviation, 9 +/- 4) consecutively arranged in a Trichodesmium trichome were labeled with the nitrogenase antibody. The nitrogenase-containing cells were distributed more frequently around the center of the trichome and were rarely found at the ends. On average 15% of over 300 randomly encountered cells examined contained nitrogenase. The percentage of nitrogenase-containing cells (nitrogenase index [NI]) in an exponential culture was higher early in the light period than during the rest of the light-dark cycle, while that for a stationary culture was somewhat constant at a lower level throughout the light-dark cycle. The NI was not affected by treatment of the cultures with the photosynthetic inhibitor dichloro 1,3'-dimethyl urea or with low concentrations of ammonium (NH4Cl). However, incubation of cultures with 0.5 &mgr;M NH4Cl over 2 days reduced the NI. The IF technique combined with 14C autoradiography showed that the CO2 fixation rate was lower in nitrogenase-containing cells. The results of the present study suggest that (i) the IF-LM technique may be a useful tool for in situ protein localization in cyanobacteria, (ii) cell differentiation occurs in Trichodesmium and only a small fraction of cells in a colony have the potential to fix nitrogen, (iii) the photosynthetic activity (CO2 uptake) is reduced if not absent in N2-fixing cells, and (iv) variation in the NI may be a modulator of nitrogen-fixing activity.

Regulation of nitrogen fixation in Azospirillum brasilense

The regulation of nitrogen fixation in Azospirillum brasilense is very complicated, and it responds to exogenous fixed nitrogen or a change of oxygen concentration. This regulation occurs at both transcriptional and posttranslational levels. Unlike regulation seen in Klebsiella pneumoniae, transcription of nifA does not require NTRB/NTRC in A. brasilense and the expression of nifHDK is controlled by posttranslational regulation of NIFA activity. Addition of NH4+ or a shift from microaerobic to anaerobic conditions also causes a rapid loss of nitrogenase activity in A. brasilense. This posttranslational regulation of nitrogenase activity involves the DRAT/DRAG regulatory system, which is similar to that of Rhodospirillum rubrum. Both DRAT and DRAG activities are regulated in vivo, but the mechanisms for their regulation are unknown.

Reciprocal light-dark transcriptional control of nif and rbc expression and light-dependent posttranslational control of nitrogenase activity in Synechococcus sp. strain RF-1

Synechococcus sp. strain RF-1 exhibits a circadian rhythm of N2 fixation when cells are grown under a light-dark cycle, with nitrogenase activity observed only during the dark period. This dark-dependent activity correlated with nif gene transcription in strain RF-1. By using antibodies against dinitrogenase reductase (the Fe protein of the nitrogenase complex), it was found that there was a distinct shift in the mobility of this protein on sodium dodecyl sulfate gels during the light-dark cycle. The Fe protein was present only when cells were incubated in the dark. Upon illumination, there was a conversion of all Fe protein to a modified form, after which it rapidly disappeared from extracts. These studies indicated that all nitrogenase activity present during the dark cycle resulted from de novo synthesis of nitrogenase. Upon entering the light phase, cells appeared to quickly degrade the modified form of Fe protein, perhaps as a result of activating or inducing a protease. By contrast, transcription of the rbcL gene, which encodes the catalytic subunit of the key enzyme of CO2 fixation (a light-dependent process), was enhanced in the light.

Modification of the Fe protein of the nitrogenase of Gloeothece (Nägeli) sp. ATCC 27152 during growth under alternating light and darkness

The Fe protein of the nitrogenase of the unicellular Cyanobacterium Gloeothece (Nägeli) sp. ATCC 27152 can be resolved by SDS-PAGE into two antigenically detectable components of approximate M_r 38500 and 40000 respectively. The larger form of this protein may be produced by modification of the smaller form. Modification of the Fe protein is promoted under conditions where O₂ (but not O₂ ^- or H₂ O₂ ) has increased access to the enzyme, but does not allow nitrogenase to function under conditions of O₂ stress. During growth of Gloeothece under alternating light and darkness, antigenically detectable Fe protein is absent throughout most of the light period. The restriction of nitrogenase activity to the period of darkness is better explained in terms of regulation of nitrogenase synthesis and degradation than by reversible modification of a constant intracellular concentration of Fe protein. However, newly synthesized Fe protein always appeared initially as its larger form, which may be catalytically inactive in aerobic cultures of Gloeothece. Conversion of this form to the smaller, assumed active, form of the Fe protein may be an additional factor in explaining the increase in nitrogenase activity that occurs during the first few hours of each dark phase.

Modification of the Fe Protein of Nitrogenase in Natural Populations of Trichodesmium thiebautii

The Fe protein of nitrogenase in the marine nonheterocystous cyanobacterium Trichodesmium thiebautii is interconverted between two forms, which is reminiscent of the ADP-ribosylation described in the purple bacterium Rhodospirillum rubrum. In natural populations of T. thiebautii during the day, when nitrogenase activity (NA) is present and while photosynthetic rates are high, a low-molecular-mass form of the Fe protein is present. In the late afternoon, the low-molecular-mass form is partially converted to a higher-molecular-mass form (approximately equal distribution of high- and low-molecular-mass forms of the Fe protein subunits), concurrent with cessation of NA. Some of the higher-molecular-mass form persists through the night until the very early morning, when the lower-molecular-mass form appears. New synthesis of both the Fe and MoFe proteins of nitrogenase appears to occur at this time. The higher-molecular-mass form of the Fe protein is also produced rapidly in response to artificially elevated external O 2 levels (40%) during the day. T. thiebautii is capable of recovery of NA in less than 1 h following exposure to 40% O 2 , which is correlated with the return of the Fe protein to the lower-molecular-mass form. Recovery from exposure to O 2 is not dependent upon protein synthesis. The modification of the Fe protein is clearly involved in regulation of NA during the diel cycle of NA in T. thiebautii but may also be involved in protecting the Fe protein during transient O 2 concentration increases.