Posts Tagged 'nitrogen fixation'

Effects of seawater acidification and alkalization on the farmed seaweed, Pyropia haitanensis (Bangiales, Rhodophyta), grown under different irradiance conditions


• Either seawater acidification and alkalization or reduced light inhibited nitrogen metabolism of Pyropia haitanensis
• Reduced irradiance alleviate negative effects of seawater alkalization on the algal growth and photosynthesis
• Lowered irradiance aggravated adverse impacts of seawater acidification on the growth and photosynthesis of P. haitanensis


The thalli of Pyropia haitanensis were cultured under different pH levels (7.8, 8.2, and 9.0) and under decreased (60 μmol photons mm−2 s−1) and ambient (300 μmol photons m−2 s−1) levels of light irradiance conditions, aiming to examine the influence of different pH and decreased light irradiance on this farmed seaweed species in Southern China. Either the decreased (7.8) or increased (9.0) pH values in seawater inhibited nitrogen uptake rates and nitrate reductase activity of P. haitanensis. The capacity of nitrogen uptake and maximum inorganic carbon (Ci)-saturated photosynthetic rate (Vmax) were reduced in P. haitanensis grown at decreased irradiance compared with the algae grown at ambient irradiance. Decreased pH had no significant effect on the algal growth and photosynthesis under ambient light conditions, but it significantly inhibited growth and photosynthesis under decreased light conditions. Increased seawater pH resulted in decreased relative growth rate (RGR), maximal quantum yield of photosystem II ((Fv/Fm), and non-photochemical quenching (NPQ) of P. haitanensis when the algae were grown under ambient light conditions. However, a slight decrease was observed with decreasing growth irradiances. Collectively, our results indicated that either the changed pH (acidification and alkalization) or reduced irradiance displayed a disadvantageous influence on nitrogen metabolism of P. haitanensis. We suggested that, during P. haitanensis mariculture, the decreased light irradiance resulting from increasing algal mats density alleviates the negative effects of seawater alkalization, but it aggravates the adverse effects of seawater acidification on the growth and photosynthesis of the algae.

Continue reading ‘Effects of seawater acidification and alkalization on the farmed seaweed, Pyropia haitanensis (Bangiales, Rhodophyta), grown under different irradiance conditions’

Marine iron biogeochemistry under a changing climate: impact on the phytoplankton and the diazotroph communities

Diatoms constitute a major group of phytoplankton, accounting for ~20% of the world’s primary production. Biological dinitrogen (N2) fixation by diazotrophic cyanobacteria has great biogeochemical implications in nitrogen (N) cycling, being the major source of new N input to the oceans and thereby contributing significantly to carbon (C) export production. It has been shown that iron (Fe) can be the limiting nutrient for phytoplankton growth, in particular, in the HNLC (High Nutrient Low Chlorophyll) regions. Iron plays thus an essential role in governing the marine primary productivity and the efficiency of biological carbon pump. Oceanic systems are undergoing continuous modifications at varying rates and magnitudes as a result of changing climate. The objective of our research is to evaluate the effects of global climate change processes (changing dust deposition, ocean acidification and sea-surface warming) on phytoplankton growth, biological N2 fixation, biogeochemical cycles, and the controlling role of Fe within these impacts. Laboratory culture experiments using a marine diatom Chaetoceros socialis were conducted at two temperatures (13 ℃ and 18 ℃) and two carbon dioxide partial pressures (pCO2, 400 µatm and 800 µatm). The present study clearly highlights the effect of ocean acidification on enhancing the release of Fe upon dust deposition. Our results also confirm that being a potential source of Fe, mineral dust provides in addition a readily utilizable source of macronutrients such as phosphorus (P) and silicon (Si). However, elevated atmospheric CO2 concentrations and ocean acidification may also have an adverse impact on diatom growth, causing a decrease in cell size and possible further changes in phytoplankton composition. Meanwhile, increasing temperature and ocean warming may lead to the reduction of diatom production as well as cell size, inducing poleward shifts in the biogeographic distribution of diatoms. Numerous factors can affect the extent of N2 fixation. A better understanding of the major environmental and nutrient controls governing this process is highly required. Iron and/or phosphorus are thought to be limiting factors in most oceanic regions. Special attention has been given to studying the effects of mineral dust deposition which is believed to promote N2 fixation as it increases the availability of both Fe and P. Three laboratory bioassays (+Fe, +P, +Dust) via incubation experiments were performed on Trichodesmium IMS101, an important contributor to marine N2 fixation. Each addition of Fe, P or desert dust was found to stimulate the growth and the N2 fixation activity of Trichodesmium IMS101. Several adaptive nutrient utilization strategies were observed, such as a Fe luxury uptake mechanism, a P-sparing effect and colony formation. In addition, during a field study in the temperate Northeast Atlantic Ocean using natural phytoplankton assemblages, N2 fixation was remarkably stimulated through the addition of dissolved Fe under low temperature and depleted P conditions, highlighting the critical role of Fe. At the time of this study, no Trichodesmium filaments were found in the region of investigation. The diazotrophic community was dominated by the unicellular cyanobacteria symbiont (prymnesiophyte-UCYN-A1) and heterotrophic diazotrophs, therefore suggesting that Fe could be the ultimate factor limiting N2 fixation of these smaller diazotrophs as well. Recently, the effects of ongoing climate change (ocean warming and acidification) on N2 fixation drew much attention, but various studies led to controversial conclusions. Semi-continuous dilution growth experiments were conducted on Trichodesmium IMS101 under future high pCO2 and warming seawater conditions (800 µatm and 28 °C) and compared to the present-day situations (400 µatm and 24 °C). The results indicate that higher pCO2 and therefore ocean acidification may be beneficial for Trichodesmium growth and N2 fixation. However, the present study suggests that Fe or P limitation in oligotrophic seawaters may offset the stimulation induced on Trichodesmium IMS101 due to ocean acidification. In contrast, ocean warming may not play an important role in Trichodesmium growth and N2 fixation with a 4 °C increase from 24 °C to 28 °C. Nevertheless, ocean warming was previously predicted to cause a shift in the geographical distribution of Trichodesmium toward higher latitudes, extending its niche to subtropical regions and potentially reducing its range in tropical ocean basins.Overall, the biological responses of the marine diatom Chaetoceros socialis and the N2-fixing cyanobacteria Trichodesmium IMS101 to several key climate change processes were presented and discussed in this study. These processes included dust deposition, and ocean acidification and warming, which were shown to have a significant impact on oceanic phytoplankton growth, cell size and primary productivity, biological N2 fixation, phytoplankton distribution and community composition. They would thus affect the C, N, P, Si and Fe biogeochemical cycles in various ways. Iron, as one of the most crucial micronutrients for marine phytoplankton, has in particular strong links to climate change and biogeochemical feedback mechanisms.

Continue reading ‘Marine iron biogeochemistry under a changing climate: impact on the phytoplankton and the diazotroph communities’

Nutrient co-limited Trichodesmium as nitrogen source or sink in a future ocean

Nitrogen-fixing (N2) cyanobacteria provide bioavailable nitrogen to vast ocean regions but are in turn limited by iron (Fe) and/or phosphorus (P), which may force them to employ alternative nitrogen acquisition strategies. The adaptive responses of nitrogen-fixers to global-change drivers under nutrient-limited conditions could profoundly alter the current ocean nitrogen and carbon cycles. Here, we show that the globally-important N2-fixer Trichodesmium fundamentally shifts nitrogen metabolism towards organic-nitrogen scavenging following long-term high-CO2 adaptation under iron and/or phosphorus (co)-limitation. Global shifts in transcripts and proteins under high CO2/Fe-limited and/or P-limited conditions include decreases in the N2-fixing nitrogenase enzyme, coupled with major increases in enzymes that oxidize trimethylamine (TMA). TMA is an abundant, biogeochemically-important organic nitrogen compound that supports rapid Trichodesmium growth while inhibiting N2 fixation. In a future high-CO2 ocean, this whole-cell energetic reallocation towards organic nitrogen scavenging and away from N2-fixation may reduce new-nitrogen inputs by Trichodesmium, while simultaneously depleting the scarce fixed-nitrogen supplies of nitrogen-limited open ocean ecosystems.

Continue reading ‘Nutrient co-limited Trichodesmium as nitrogen source or sink in a future ocean’

Response to Comment on “The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium”

Hutchins et al. question the validity of our results showing that under fast growth conditions, the beneficial effect of high CO2 on Trichodesmium is overwhelmed by the deleterious effect of the concomitant decrease in ambient and cellular pH. The positive effect of acidification reported by Hutchins and co-workers is likely caused by culture conditions that support suboptimal growth rates.

Continue reading ‘Response to Comment on “The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium”’

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.

Continue reading ‘Comment on “The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium”’

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

Acidification of seawater caused by anthropogenic carbon dioxide (CO2) is anticipated to influence the growth of dinitrogen (N2)–fixing phytoplankton, which contribute a large fraction of primary production in the tropical and subtropical ocean. We found that growth and N2-fixation of the ubiquitous cyanobacterium Trichodesmium decreased under acidified conditions, notwithstanding a beneficial effect of high CO2. Acidification resulted in low cytosolic pH and reduced N2-fixation rates despite elevated nitrogenase concentrations. Low cytosolic pH required increased proton pumping across the thylakoid membrane and elevated adenosine triphosphate production. These requirements were not satisfied under field or experimental iron-limiting conditions, which greatly amplified the negative effect of acidification.

Continue reading ‘The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium’

Influence of ocean acidification and deep water upwelling on oligotrophic plankton communities in the subtropical North Atlantic: Insights from an in situ mesocosm study

Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. Increasing evidence indicates that these changes – summarized by the term ocean acidification (OA) – can significantly affect marine food webs and biogeochemical cycles. However, current scientific knowledge is largely based on laboratory experiments with single species and artificial boundary conditions, whereas studies of natural plankton communities are still relatively rare. Moreover, the few existing community-level studies were mostly conducted in rather eutrophic environments, while less attention has been paid to oligotrophic systems such as the subtropical ocean gyres.

Here we report from a recent in situ mesocosm experiment off the coast of Gran Canaria in the eastern subtropical North Atlantic, where we investigated the influence of OA on the ecology and biogeochemistry of plankton communities in oligotrophic waters under close-to-natural conditions. This paper is the first in this Research Topic of Frontiers in Marine Biogeochemistry and provides (1) a detailed overview of the experimental design and important events during our mesocosm campaign, and (2) first insights into the ecological responses of plankton communities to simulated OA over the course of the 62-day experiment.

One particular scientific objective of our mesocosm experiment was to investigate how OA impacts might differ between oligotrophic conditions and phases of high biological productivity, which regularly occur in response to upwelling of nutrient-rich deep water in the study region. Therefore, we specifically developed a deep water collection system that allowed us to obtain ~85 m3 of seawater from ~650 m depth. Thereby, we replaced ~20% of each mesocosm’s volume with deep water, and thus successfully simulated a deep water upwelling event that induced a pronounced plankton bloom.

Our study revealed significant effects of OA on the entire food web, leading to a restructuring of plankton communities that emerged during the oligotrophic phase, and was further amplified during the bloom that developed in response to deep water addition. Such CO2-related shifts in plankton community composition could have consequences for ecosystem productivity, biomass transfer to higher trophic levels, and biogeochemical element cycling of oligotrophic ocean regions.

Continue reading ‘Influence of ocean acidification and deep water upwelling on oligotrophic plankton communities in the subtropical North Atlantic: Insights from an in situ mesocosm study’

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Ocean acidification in the IPCC AR5 WG II

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