THE EFFECT OF IRON NUTRITION ON PHOTOSYNTHESIS AND NITROGEN FIXATION IN CULTURES OF TRICHODESMIUM (CYANOPHYCEAE)1

Cultures of Trichodesmium NIBB 1067 were grown in the synthetic medium AQUIL with a range of iron added from none to 5 × 10^?7 M Fe for 15 days. Chlorophyll-a, cell counts, and total cell volume were two or three times higher in medium with 10^?7 M Fe than with no added Fe. Oxygen production rate per chlorophyll-a was over 60% higher with higher iron. Increased iron stimulated photosynthesis at all irradiances from about 12–250 μE · m^?2· s^?1. Nitrogen fixation rate, estimated from acetylene reduction, for 10^?7 and 10^?8 M Fe cultures was approximately twice that of the cultures with no added Fe. The range of rates of O₂ production and N₂ fixation in cultures at the iron concentrations we used were similar to the rates from natural samples of Trichodesmium from both the Atlantic, and the Pacific oceans. This similarity may allow this clone to be used, with some caution, for future physiological ecology studies. This study demonstrates the importance of iron to photosynthesis and nitrogen fixation and suggests that Trichodesmium plays a central role in the biogeochemical cycles of iron, carbon and nitrogen.     

RESPONSE OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) CULTURES TO IRON NUTRITION1

Three clones of marine Synechococcus (WH6501, WH7803, and WH8018) were grown through at least three transfers, at 6-day intervals, in synthetic medium with total iron concentrations from 10^?9 to 10^?6 M. After 6 days of exponential growth, these cultures were harvested, and the cell density and protein and pigment concentrations were measured. Aliquots of the culture were assayed for their carbon fixation rates at two light intensities. Cell density and protein concentration increased by up to 7.8 times over a range of iron from the lowest (10^?9 M) to the highest concentrations (10^?6 M). The concentration of chlorophyll-a and phycobiliproteins showed a wider range of response, increasing by up to 48 times. The carbon fixation rate (per mL of culture) also increased approximately 40 times over the total range of iron concentration. The ranges of these biochemical and physiological responses were much lower than the range of total available iron, which was 1000-fold, and the range of total cellular iron, which was estimated to be about 160-fold. This “less-than-linear” relationship indicates that the cells are adapting to make more efficient use of iron under limiting conditions. Our results demonstrate characteristics of iron-limited Synechococcus that may be important in understanding the relationships between primary productivity and iron availability in the oceans.     

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.     

A PERSPECTIVE ON PHOTOSYNTHESIS IN THE OLIGOTROPHIC OCEANS: HYPOTHESES CONCERNING ALTERNATE ROUTES OF ELECTRON FLOW1

Many regions of the open, oligotrophic oceans are depleted of nutrients, especially nitrogen and iron. The biogenesis and the functioning of the photosynthetic apparatus may be specialized and tailored to the various marine habitats. In this minireview, we discuss some new findings with respect to photosynthetic processes in the oceans. We focus on findings that suggest that some cyanobacteria may route electrons derived from the splitting of H₂O to the reduction of O₂ and H⁺ in a water‐to‐water cycle, and that other cyanobacteria that fix nitrogen during the day are likely missing PSII and enzymes involved in the fixation of inorganic carbon. Both of these proposed “variant” forms of photosynthetic electron flow provide new insights into ways in which marine phytoplankton satisfy their energetic and nutritive requirements.     

THE EFFECTS OF LIGHT AND NITROGEN ON GROWTH,PIGMENT CONTENT,AND BIOCHEMICAL COMPOSITION OF GRACILARIA FOLIIFERA V. ANGUSTISSIMA (GIGARTINALES,RHODOPHYTA)1

The combined effects of light intensity and nitrogen (NO₃^?) on growth rate, pigment content, and biochemical composition of Gracilaria foliifera v. angustissima (Harvey) Taylor was investigated using outdoor continuous cultures. Growth of Gracilaria increased linearly with increasing light to 0.43 doublings d^?1 at high light levels (383 ly d^?1 of in situ light), suggesting that light may often limit growth of this plant in nature. Chlorophyll a and phycoerythrin contents were inversely proportional to light level and growth rate. However, pigment content did not affect the growth capacity of Gracilaria. There was no increase in growth or pigment content with increasing additions of nitrogen. The low nitrogen treatment was unenriched seawater that had higher NO₃^? levels than most coastal waters (influent = 8.61 μM; residual = 0.94 μM). When growing near its maximum rate under high light intensities, Gracilaria had a significantly (P < 0.001) lower phycoerythrin: chlorophyll a ratio (phyco: Chl a) than did Gracilaria growing more slowly under lower light (Phyco:Chl a of 2.8 ± 0.2 vs. 3.8 ± 0.3). Faster growing plants also had C:N ratios above 10, indicating N- limitation. In addition to harvesting light the phycobiliproteins of Gracilaria may store nitrogen. Growth rates of Gracilaria correlated negatively with ash (r =–0.85) and positively with the carbon: phycoerythrin ratio (r = 0.85), suggesting that these two indices can be used to estimate growth in the field.     

NUTRIENT CONTROLS ON NITROGEN UPTAKE AND METABOLISM BY NATURAL POPULATIONS AND CULTURES OF TRICHODESMIUM (CYANOBACTERIA)

The effects of inorganic nutrient (ammonium [NH₄ ⁺ ] and nitrate [NO₃ ⁻ ]) and amino acid (glutamate [glu] and glutamine [gln]) additions on rates of N₂ fixation, N uptake, glutamine synthetase (GS) activity, and concentrations of intracellular pools of gln and glu were examined in natural and cultured populations of Trichodesmium. Additions of 1 μM glu, gln, NO₃ ⁻ , or NH₄ ⁺ did not affect short-term rates of N₂ fixation. This may be an important factor that allows for continued N₂ fixation in oligotrophic areas where recycling processes are active. N₂ fixation rates decreased when nutrients were supplied at higher concentrations (e.g. 10 μM). Uptake of combined N (NH₄ ⁺ , NO₃ ⁻ , and amino acids) by Trichodesmium was stimulated by increased concentrations. For NO₃ ⁻ , proportional increases in NO₃ ⁻ uptake and decreases in N₂ fixation were observed when additions were made to cultures before the onset of the light period. GS activity did not change much in response to the addition of NH₄ ⁺ , NO₃ ⁻ , glu, or gln. GS is necessary for N metabolism, and the bulk of this enzyme pool may be conserved. Intracellular pools of glu and gln varied in response to 10 μM additions of NH₄ ⁺ , glu, or gln. Cells incubated with NH₄ ⁺ became depleted in intracellular glu and enriched with intracellular gln. The increase in the gln/glu ratio corresponded to a decrease in the rate of N₂ fixation. Although the gln/glu ratio decreased in cells exposed to the amino acids, there was only a corresponding decrease in N₂ fixation after the gln addition. The results presented here suggest that combined N concentrations on the order of 1 μM do not affect rates of N₂ fixation and metabolism, although higher concentrations (e.g. 10 μM) can. Moreover, these effects are exerted through products of NH₄ ⁺ assimilation rather than exogenous N, as has been suggested for other species. These results may help explain how cultures of Trichodesmium are able to simultaneously fix N₂ and take up NH₄ ⁺ and how natural populations continue to fix N₂ once combined N concentrations increase within a bloom.     

A molecular assessment of the iron stress response in the two phylogenetic clades of Trichodesmium

Trichodesmium spp. play key roles in global carbon and nitrogen budgets and thus defining what controls their productivity is important for understanding climate change. While iron availability has been shown to be an important chemical factor for controlling both growth and nitrogen fixation rates in Trichodesmium , all culture experiments to date have focused solely on representatives from one clade of Trichodesmium . Genomic sequence analysis determined that the Trichodesmium erythraeum (IMS101) genome contains many of the archetypical genes involved in the prokaryotic iron stress response. Focusing on three of these genes, isiB , idiA and feoB , we found that all three showed an iron stress response in axenic T. erythraeum (IMS101), and their sequences were well conserved across four species in our Trichodesmium culture collection [consisting of two T. erythraeum strains (IMS101 and GBRTRLI101), two Trichodesmium tenue strains (Z-1 and H9-4), Trichodesmium thiebautii and Trichodesmium spiralis ]. With clade-specific quantitative PCR (qPCR) primers for one of these genes, isiB , we found that high isiB expression at low Fe levels corresponded to specific reductions in N₂ fixation rates in both major phylogenetic clades of Trichodesmium (the T. erythraeum clade and T. tenue clade). With regard to the two clades, the most significant difference determined was temperature optima, while more subtle differences in growth, N₂ fixation rate and gene expression responses to Fe stress were also observed. However the apparent conservation of the Fe stress response in the Trichodesmium genus suggests that it is an important adaptation for their niche in the oligotrophic ocean.     

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 (MS^E) 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.     

Carbon and nitrogen dynamics in a maritime Antarctic stream

• 1 The carbon and nitrogen dynamics in a maritime Antarctic lake outflow stream were investigated. The stream and the algal communities could be split into two zones: a semi-aquatic margin consisting of a perennial cyanobacteria/diatom mat and a flowing channel with a similar perennial mat that was overgrown by annual filamentous chlorophytes during the course of the summer.
• 2 Neither algal community was limited by nutrient availability. Major nutrients were always available in the stream water. There were slight differences in the atomic ratios of the mats, the N:P ratios in the channel mat being lower than those in the marginal mat. However, both these and the total dissolved N:P ratio in the stream water were all close to those that indicate a balanced supply.
• 3 There was no net carbon or nitrogen accumulation by the marginal mat suggesting that uptake processes were balanced by loss processes.
• 4 Maximum rates of carbon fixation (0.1–0.5mgCg^?1 dry weight h^?1) were similar to those of other perennial Antarctic algal mats. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated.
• 5 There were no heterocystous cyanobacteria in the mat communities and rates of atmospheric nitrogen fixation were very low (0–10ngNmg^?1 mat Nh^?1). Fixation accounted for only 0.3% of the nitrogen accumulation of the channel mats, but was higher in the marginal mat where uptake of other sources of nitrogen was also low.
• 6 Nitrogen accumulation by the channel mat averaged 0.34gNm^?2 day^?1. Only 0.05gNm^?2 day^?1 was accounted for by the uptake of dissolved inorganic nitrogen (nitrate plus ammonium). The major (80%) source of nitrogen appeared to be dissolved organic nitrogen. Recycling of nitrogen within the stream ecosystem may also be important.

     

BUOYANCY REGULATION IN THE COLONIAL DIAZOTROPHIC CYANOBACTERIUM TRICHODESMIUM TENUE: ULTRASTRUCTURE AND STORAGE OF CARBOHYDRATE,POLYPHOSPHATE, AND NITROGEN1

Trichodesmium tenue Wille (1904) was examined using transmission electron microscopy to determine the role of carbohydrate, phosphorus, and nitrogen storage in buoyancy regulation. Carbohydrate storage area (mean = 2.06 ± 0.61 [SE] μm²; 6.62% of total cell area) in negatively buoyant colonies (NBCs) was significantly higher (P < 0.001) than in positively buoyant colonies (PBCs) (mean = 0.38 ± 0.06 μm²; 0.73%). Distinct diel periodicity of carbohydrate content was found in NBCs demonstrated by an increase from darkness to afternoon. Polyphosphate content was significantly higher (P < 0.001) in NBCs, with a mean of 0.44± 0.10 μm² (1.54%), as compared to PBCs, with a mean of 0.14 ± 0.05 μm² (0.24%). Polyphosphate content increased in NBCs from morning to evening, and PBCs had a 10% decrease from morning to afternoon. Calculations indicated that averaged effects of polyphosphate on increased cell density is approximately 20% of that from carbohydrate accumulation. Density contribution due to ballast weight of carbohydrate and polyphosphate indicated that NBCs were 12 times more dense than PBCs. Mean area of cyanophycin granules (N storage) was not significantly different between PBCs and NBCs. In conclusion, Trichodesmium tenue can regulate buoyancy by carbohydrate ballasting similar to that noted in limnetic cyanobacteria. Polyphosphate storage and possibly nitrogen storage products play a significant role in buoyancy regulation.     

Comparative proteomic profiles of the marine cyanobacterium Trichodesmium erythraeum IMS101 under different nitrogen regimes

Trichodesmium is a marine filamentous diazotrophic cyanobacterium and an important contributor of "new" nitrogen in the oligotrophic surface waters of the tropical and sub-tropical oceans. It is unique in that it exclusively fixes N(2) at daytime, although it belongs to the non-heterocystous filamentous segment of the cyanobacterial radiation. Here we present the first quantitative proteomic analysis of Trichodesmium erythraeum IMS101 when grown under different nitrogen regimes using 2-DE/MALDI-TOF-MS. Addition of combined nitrogen (NO3-) prevented development of the morphological characteristics of the N(2)-fixing cell type (diazocytes), inhibited expression of the nitrogenase enzyme subunits and consequently N(2) fixation activity. The diazotrophic regime (N(2) versus NO3- cultures) elicited the differential expression of more than 100 proteins, which represented 13.5% of the separated proteins. Besides proteins directly related to N(2) fixation, proteins involved in the synthesis of reducing equivalents and the generation of a micro-oxic environment were strongly up-regulated, as was in particular Dps, a protein related to iron acquisition and potentially other vital cellular processes. In contrast, proteins involved in the S-adenosylmethionine (SAM) cycle, synthesis of amino acids and production of carbon skeletons for storage and synthesis of amino acids were suppressed. The data are discussed in the context of Trichodesmium's unusual N(2)-fixing physiology.     

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton1

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)‐replete growth rate increased monotonically with light until it reached a threshold intensity (~200 μmol photons · m^?2 · s^?1). For Chlorella sp., the nitrogen quota (pg · μm^?3) was greatest below this threshold, beyond which it was reduced by the effect of N‐stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons^?1) over the range of light and N‐gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g^?1) increased as a function of light and N‐stress, while C:N (mol · mol^?1) and relative neutral lipid:C (rel. lipid · g^?1) were most strongly influenced by N‐stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N‐availability) and energy gradients influenced C‐allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.     

Temporal expression of effects of varying nitrogen supply on canopy growth, photosynthesis and fruit production for Actinidia deliciosa vines in the field

The effects of varying nitrogen supply on canopy leaf area, response of leaf net photosynthesis (A_n) to quantum flux density (Q), and fruit yields of kiwifruit vines (Actinidia deliciosa var. deliciosa) were examined in a two-year field experiment. Vines were grown with 0, 250 or 750 kg N ha^?1 year^?1. The responses to nitrogen supply were compared with responses to shade, to examine the impact of reduced carbon assimilation on canopy leaf area and fruit yields. Nitrogen supply did not affect significantly any of the measured variables during the first season of the experiment. In the second season, canopy leaf area was reduced significantly where nitrogen supply was limited. The quantum efficiency of photosynthesis (φ_q) increased from 0. 03 mol CO₂ mol^?1 Q soon after leaf emergence to more than 0. 05 mol CO₂ mol^?1 Q during the middle of the growing season. The quantum saturated rate of A_n (A_sat) also increased during the season, from 7–10 μmol CO₂ m^?2 s^?1 soon after leaf emergence, to 15–20 (μmol CO₂ m^?2 s^?1 during the middle of the growing season. φ_q and A_sat increased significantly with nitrogen supply at all measurement times during the second season. For vines with high nitrogen, fruit yields in both seasons were similar, averaging 3. 05 kg m^?2. Fruit yields in the second season were reduced significantly where nitrogen supply was limited, due to reduced fruit numbers. The relative effects of reduced leaf area and reduced leaf photosynthesis for carbon assimilation by nitrogen deficient vines were examined using a mathematical model of canopy photosynthesis for kiwifruit vines. Simulations of canopy photosynthesis indicated that effects on leaf area and on leaf photosynthesis were of similar importance in the overall effects of nitrogen deficiency on carbon assimilation. The effects of nitrogen supply on fruit numbers (i. e. flower development) preceded the measured effects on carbon assimilation, indicating that the nitrogen supply affected carbon partitioning to reserves in the first season.     

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N₂) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N₂ fluxes (157.5 μmol N m^?2 h^?1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (?74.5 μmol N m^?2 h^?1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N₂, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N₂ fluxes during early delta evolution.     

Molecular analysis of the phosphorus starvation response in Trichodesmium spp.

The marine diazotroph Trichodesmium is a major contributor to primary production and nitrogen fixation in the tropical and subtropical oceans. These regions are often characterized by low phosphorus (P) concentrations, and P starvation of Trichodesmium could limit growth, and potentially constrain nitrogen fixation. To better understand how this genus responds to P starvation we examined four genes involved in P acquisition: two copies of a high-affinity phosphate binding protein ( pstS and sphX ) and two putative alkaline phosphatases ( phoA and phoX ). Sequence analysis of these genes among cultured species of Trichodesmium ( T. tenue, T. erythraeum, T. thiebautii and T. spiralis ) showed that they all are present and conserved within the genus. In T. erythraeum IMS101, the expression of sphX , phoA and phoX were sensitive to P supply whereas pstS was not. The induction of alkaline phosphatase activity corresponded with phoA and phoX expression, but enzyme activity persisted after the expression of these genes returned to basal levels. Additionally, nifH (nitrogenase reductase; involved in nitrogen fixation) expression was downregulated under P starvation conditions. These data highlight molecular level responses to low P and lay a foundation for better understanding the dynamics of Trichodesmium P physiology in low-P environments.     

OLISTHODISCUS LUTEUS (CHRYSOPHYCEAE). III. UPTAKE AND UTILIZATION OF NITROGEN AND PHOSPHORUS1

Uptake and assimilation of nitrogen and phosphorus were studied in Olisthodiscus luteus Carter. A diel periodicity in nitrate reductase activity was observed in log and stationary phase cultures; there was a 10-fold difference in magnitude between maximum and minimum rates, but other cellular features such as chlorophyll a, carbon, nitrogen, C:N ratio (atoms) · cell^?1 were less variable. K_s values (~2 μM) for uptake of nitrate-N and ammonium-N were observed. Phosphorus assimilated · cell^?1· day^?1 varied with declining external phosphorus concentrations; growth rates <0.5 divisions · day^?1 were common at <0.5 μM PO_4-P. Phosphate uptake rates (K_s= 1.0–1.98 μM) varied with culture age and showed multiphasic kinetic features. Alkaline phosphatase activity was not detected. Comparisons of the nutrient dynamics of O. luteus to other phytoplankton species and the ecological implications as related to the phytoplankton community of Narragansett Bay (Rhode Island) are discussed.     

Pelagic Sargassum community metabolism: Carbon and nitrogen

The production, nitrogen fixation, and release rates and fate of dissolved organic matter of a pelagic Sargassum community have been investigated at eight stations in the Gulf Stream and the Sargasso Sea. Net production and gross nitrogen fixation rates of Sargassum and epiphytes varied significantly between stations, 328 ± 114μg C (g dry wt)^?1h^?1 and 18 ± 7.4μg N g^?1h^?1, respectively. The net release rates of dissolved organic carbon (287 ± 150μg DOC g^?1h^?1) also showed the same variability between stations. On the other hand, the community carbon and nitrogen content, 268 ± 4.8 and 16.9 ± 2.4 mg g dry wt^?1, respectively, remained constant at all stations. The results of chemical measurements indicate that ≈ 0–50 % of the gross production was lost as a result of photosynthate release. From ¹⁴C-tracer experiments it was found that the planktonic and epiphytic heterotrophs mineralized 50–70 % of the photosynthate released by Sargassum and epiphytic algae. Based on the community gross production and fixation rates, carbon and nitrogen content, the amount of nitrogen required for the observed production rates, the Sargassum community appears to obtain a substantial part (40%) of its nitrogen from nitrogen fixation.     

Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium

The increases in atmospheric pCO₂ over the last century are accompanied by higher concentrations of CO₂(aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)‐fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO₂(aq). We examined the influence of pCO₂ [preindustrial∼250 ppmv (low), ambient∼400, future∼900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom‐forming cyanobacterium contributing substantial inputs of ‘new N’ to the oligotrophic subtropical and tropical oceans. High pCO₂ enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO₂ cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO₂ cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO₂. Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO₂ to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.     

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.     

INTERACTIVE EFFECTS OF IRRADIANCE AND CO2 ON CO2 FIXATION AND N2 FIXATION IN THE DIAZOTROPH TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA)1

The diazotrophic cyanobacteria Trichodesmium spp. contribute approximately half of the known marine dinitrogen (N₂) fixation. Rapidly changing environmental factors such as the rising atmospheric partial pressure of carbon dioxide (pCO₂) and shallower mixed layers (higher light intensities) are likely to affect N₂‐fixation rates in the future ocean. Several studies have documented that N₂ fixation in laboratory cultures of T. erythraeum increased when pCO₂ was doubled from present‐day atmospheric concentrations (～380 ppm) to projected future levels (～750 ppm). We examined the interactive effects of light and pCO₂ on two strains of T. erythraeum Ehrenb. (GBRTRLI101 and IMS101) in laboratory semicontinuous cultures. Elevated pCO₂ stimulated gross N₂‐fixation rates in cultures growing at 38 μmol quanta · m^?2 · s^?1 (GBRTRLI101 and IMS101) and 100 μmol quanta · m^?2 · s^?1 (IMS101), but this effect was reduced in both strains growing at 220 μmol quanta · m^?2 · s^?1. Conversely, CO₂‐fixation rates increased significantly (P < 0.05) in response to high pCO₂ under mid‐ and high irradiances only. These data imply that the stimulatory effect of elevated pCO₂ on CO₂ fixation and N₂ fixation by T. erythraeum is correlated with light. The ratio of gross:net N₂ fixation was also correlated with light and trichome length in IMS101. Our study suggests that elevated pCO₂ may have a strong positive effect on Trichodesmium gross N₂ fixation in intermediate and bottom layers of the euphotic zone, but perhaps not in light‐saturated surface layers. Climate change models must consider the interactive effects of multiple environmental variables on phytoplankton and the biogeochemical cycles they mediate.