Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean.
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ABSTRACT:
SUBMITTER: Irion S
PROVIDER: S-EPMC8397732 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Primary production in the Southern Ocean (SO) is limited by iron availability. Hydrothermal vents have been identified as a potentially important source of iron to SO surface waters. Here we identify a recurring phytoplankton bloom in the high-nutrient, low-chlorophyll waters of the Antarctic Circumpolar Current in the Pacific sector of the SO, that we argue is fed by iron of hydrothermal origin. In January 2014 the bloom covered an area of ~266,000 km2 with depth-integrated chlorophyll a > 300 mg m-2, primary production rates >1 g C m-2 d-1, and a mean CO2 flux of -0.38 g C m-2 d-1. The elevated iron supporting this bloom is likely of hydrothermal origin based on the recurrent position of the bloom relative to two active hydrothermal vent fields along the Australian Antarctic Ridge and the association of the elevated iron with a distinct water mass characteristic of a nonbuoyant hydrothermal vent plume.
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Project description:Pseudo-nitzschia blooms often occur in coastal and open ocean environments, sometimes leading to the production of the neurotoxin domoic acid that can cause severe negative impacts to higher trophic levels. Increasing evidence suggests a close relationship between phytoplankton bloom and bacterial assemblages, however, the microbial composition and succession during a bloom process is unknown. Here, we investigate the bacterial assemblages before, during and after toxic and non-toxic Pseudo-nitzschia blooms to determine the patterns of bacterial succession in a natural bloom setting. Opportunistic sampling of bacterial community profiles were determined weekly at Santa Cruz Municipal Wharf by 454 pyrosequencing and analyzed together with domoic acid levels, phytoplankton community and biomass, nutrients and temperature. We asked if the bacterial communities are similar between bloom and non-bloom events and if domoic acid or the presence of toxic algal species acts as a driving force that can significantly structure phytoplankton-associated bacterial communities. We found that bacterial diversity generally increases when Pseudo-nitzschia numbers decline. Furthermore, bacterial diversity is higher when the low-DA producing P. fraudulenta dominates the algal bloom while bacterial diversity is lower when high-DA producing P. australis dominates the algal bloom, suggesting that the presence of algal toxin can structure bacterial community. We also found bloom-related succession patterns among associated bacterial groups; Gamma-proteobacteria, were dominant during low toxic P. fraudulenta blooms comprising mostly of Vibrio spp., which increased in relative abundance (6-65%) as the bloom progresses. On the other hand, Firmicutes bacteria comprising mostly of Planococcus spp. (12-86%) dominate during high toxic P. australis blooms, with the bacterial assemblage showing the same bloom-related successional patterns in three independent bloom events. Other environmental variables such as nitrate and phosphate and temperature appear to influence some low abundant bacterial groups as well. Our results suggest that phytoplankton-associated bacterial communities are strongly affected not just by phytoplankton bloom in general, but also by the type of algal species that dominates in the natural bloom.
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Project description:Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. Variability in utilization alters oceanic CO2 sequestration at glacial-interglacial timescales. The role for insufficient iron has been examined in detail, but manganese also has an essential function in photosynthesis and dissolved concentrations in the Southern Ocean can be strongly depleted. However, clear evidence for or against manganese limitation in this system is lacking. Here we present results from ten experiments distributed across Drake Passage. We found manganese (co-)limited phytoplankton growth and macronutrient consumption in central Drake Passage, whilst iron limitation was widespread nearer the South American and Antarctic continental shelves. Spatial patterns were reconciled with the different rates and timescales for removal of each element from seawater. Our results suggest an important role for manganese in modelling Southern Ocean productivity and understanding major nutrient drawdown in glacial periods.
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Project description:We present improved estimates of air-sea CO2 exchange over three latitude bands of the Southern Ocean using atmospheric CO2 measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (Mθe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO2 measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO2 data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science 374, 1275-1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θe, or Mθe surfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air-sea CO2 exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO2 observations.
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Project description:The diatom Skeletonema costatum is cosmopolitan and forms algal blooms in coastal waters, being exposed to varying levels of solar UV radiation (UVR) and reduced levels of carbon dioxide (CO2). While reduced CO2 availability is known to enhance CO2 concentrating mechanisms (CCMs) in this diatom and others, little is known on the effects of UV on microalgal CCMs, especially when CO2 levels fluctuate in coastal waters. Here, we show that S. costatum upregulated its CCMs in response to UVR (295-395 nm), especially to UVA (320-395 nm) in the presence and absence of photosynthetically active radiation (PAR). The intensity rise of UVA and/or UVR alone resulted in an increase of the activity of extracellular carbonic anhydrase (CAe); and the addition of UVA enhanced the activity of CCMs-related CAe by 23-27% when PAR levels were low. Such UV-stimulated CCMs activity was only significant at the reduced CO2 level (3.4 μmol L-1). In addition, UVA alone drove active HCO3 - uptake although it was not as obvious as CAe activity, another evidence for its role in enhancing CCMs activity. In parallel, the addition of UVA enhanced photosynthetic carbon fixation only at the lower CO2 level compared to PAR alone. In the absence of PAR, carbon fixation increased linearly with increased intensities of UVA or UVR regardless of the CO2 levels. These findings imply that during S. costatum blooming period when CO2 and PAR availability becomes lower, solar UVR (mainly UVA) helps to upregulate its CCMs and thus carbon fixation, enabling its success of frequent algal blooms.
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Project description:The potential interactive effects of iron (Fe) limitation and Ocean Acidification in the Southern Ocean (SO) are largely unknown. Here we present results of a long-term incubation experiment investigating the combined effects of CO2 and Fe availability on natural phytoplankton assemblages from the Weddell Sea, Antarctica. Active Chl a fluorescence measurements revealed that we successfully cultured phytoplankton under both Fe-depleted and Fe-enriched conditions. Fe treatments had significant effects on photosynthetic efficiency (Fv/Fm; 0.3 for Fe-depleted and 0.5 for Fe-enriched conditions), non-photochemical quenching (NPQ), and relative electron transport rates (rETR). pCO2 treatments significantly affected NPQ and rETR, but had no effect on Fv/Fm. Under Fe limitation, increased pCO2 had no influence on C fixation whereas under Fe enrichment, primary production increased with increasing pCO2 levels. These CO2-dependent changes in productivity under Fe-enriched conditions were accompanied by a pronounced taxonomic shift from weakly to heavily silicified diatoms (i.e. from Pseudo-nitzschia sp. to Fragilariopsis sp.). Under Fe-depleted conditions, this functional shift was absent and thinly silicified species dominated all pCO2 treatments (Pseudo-nitzschia sp. and Synedropsis sp. for low and high pCO2, respectively). Our results suggest that Ocean Acidification could increase primary productivity and the abundance of heavily silicified, fast sinking diatoms in Fe-enriched areas, both potentially leading to a stimulation of the biological pump. Over much of the SO, however, Fe limitation could restrict this possible CO2 fertilization effect.
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Project description:Iron limits primary productivity in vast regions of the ocean. Given that marine phytoplankton contribute up to 40% of global biological carbon fixation, it is important to understand what parameters control the availability of iron (iron bioavailability) to these organisms. Most studies on iron bioavailability have focused on the role of siderophores; however, eukaryotic phytoplankton do not produce or release siderophores. Here, we report on the pivotal role of saccharides--which may act like an organic ligand--in enhancing iron bioavailability to a Southern Ocean cultured diatom, a prymnesiophyte, as well as to natural populations of eukaryotic phytoplankton. Addition of a monosaccharide (>2 nM of glucuronic acid, GLU) to natural planktonic assemblages from both the polar front and subantarctic zones resulted in an increase in iron bioavailability for eukaryotic phytoplankton, relative to bacterioplankton. The enhanced iron bioavailability observed for several groups of eukaryotic phytoplankton (i.e., cultured and natural populations) using three saccharides, suggests it is a common phenomenon. Increased iron bioavailability resulted from the combination of saccharides forming highly bioavailable organic associations with iron and increasing iron solubility, mainly as colloidal iron. As saccharides are ubiquitous, present at nanomolar to micromolar concentrations, and produced by biota in surface waters, they also satisfy the prerequisites to be important constituents of the poorly defined "ligand soup," known to weakly bind iron. Our findings point to an additional type of organic ligand, controlling iron bioavailability to eukaryotic phytoplankton--a key unknown in iron biogeochemistry.
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Project description:Over the last ten years, satellite and geographically constrained in situ observations largely focused on the northern hemisphere have suggested that annual phytoplankton biomass cycles cannot be fully understood from environmental properties controlling phytoplankton division rates (e.g., nutrients and light), as they omit the role of ecological and environmental loss processes (e.g., grazing, viruses, sinking). Here, we use multi-year observations from a very large array of robotic drifting floats in the Southern Ocean to determine key factors governing phytoplankton biomass dynamics over the annual cycle. Our analysis reveals seasonal phytoplankton accumulation ('blooming') events occurring during periods of declining modeled division rates, an observation that highlights the importance of loss processes in dictating the evolution of the seasonal cycle in biomass. In the open Southern Ocean, the spring bloom magnitude is found to be greatest in areas with high dissolved iron concentrations, consistent with iron being a well-established primary limiting nutrient in this region. Under ice observations show that biomass starts increasing in early winter, well before sea ice begins to retreat. The average theoretical sensitivity of the Southern Ocean to potential changes in seasonal nutrient and light availability suggests that a 10% change in phytoplankton division rate may be associated with a 50% reduction in mean bloom magnitude and annual primary productivity, assuming simple changes in the seasonal magnitude of phytoplankton division rates. Overall, our results highlight the importance of quantifying and accounting for both division and loss processes when modeling future changes in phytoplankton biomass cycles.
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