Posts Tagged 'nutrients'

Phosphate limitation and ocean acidification co-shape phytoplankton physiology and community structure

Published 16 May 2023 Science Closed
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A new study reports synergistic inhibitory effects of ocean acidification and phosphate limitation on the nitrogen-fixing capacity of a globally important cyanobacterium species. Inspired by the report, this Comment presents the complexity of how ocean acidification and phosphate limitation affect phytoplankton physiologies and species beyond nitrogen fixation and cyanobacteria, and what future research is needed to address the remaining crucial questions.

Increasing CO2 emission and climate change have manifold impacts on ocean primary production and carbon sequestration. One of the direct effects comes from ocean acidification due to the dissolution of ~30% of the increased CO2 into the ocean, whereas indirect impacts mainly stem from warming-driven ocean stratification that impedes upwelling of nutrient-rich deep waters leading to oligotrophication of the vast central ocean basin1. Between nitrogen and phosphate, the two major productivity-limiting nutrients, phosphate is the ‘ultimate’ limiting nutrient as it has no biogenic source, and its growth-limiting condition in the oceans is more prevalent than previously thought2. Nitrogen, in contrast, can be sourced from the atmosphere by diazotrophic bacteria through nitrogen fixation, which is often co-limited by phosphate and iron scarcity2.

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Responses of biogenic dimethylated sulfur compounds to environmental changes in the northwestern Pacific continental sea

Published 11 May 2023 Science Closed
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Continental seas are facing rapid environmental shifts, but how biogenic dimethylated sulfur compounds, including dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethylsulfoxide (DMSO), will respond to these environmental changes remains poorly understood. Here we investigated the effects of nutrient input, ocean acidification, and dust deposition on the phytoplankton community and organic sulfur cycle in the East China Sea. Nutrient input promoted phytoplankton growth and increased the concentrations of DMS, DMSP, and DMSO. With sufficient nutrients, especially nitrate, the dissolved DMSP degradation was inhibited, and the bacterial DMSP-cleavage pathway (inferred by dddP gene abundance) was enhanced, causing increased DMS production. The sensitivity of phytoplankton biomass and DMS to ocean acidification varied with different initial nutrient levels, demonstrating insensitivity under eutrophic conditions and negative responses under nutrient-limited conditions. The ocean acidification promoted the dissolved DMSP degradation and bacterial DMSP-demethylation pathway (inferred by dmdA gene abundance) and weakened the DMS production, causing the decreases of DMS and DMSP. The nutrient from dust deposition (2 mg L−1) was identified as the key factor in enhancing phytoplankton biomass and the organic sulfur compounds concentrations, but trace metals input from dust deposition had no significant effect. This study has identified environmental drivers and suppressors of phytoplankton and biogenic dimethylated sulfur compounds in a changing marine environment, which will enable the effective modeling of future climate change.

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Short-term responses of Corallina officinalis (rhodophyta) to global-change drivers in a stressful environment of Patagonia, Argentina

Published 28 April 2023 Science Closed
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Over the last two decades, an increasing interest has arisen in the responses of primary producers to global-change drivers and, more recently, in the need to consider how those various drivers may interact. To understand how Corallina officinalis (hereafter Corallina) can be affected by future changing conditions, we investigated the short-term direct effects of co-occurring increased nutrient loads, solar radiation, and lower pH, assessing how these clustered drivers affected Corallina‘s overall physiological performance in a harsh Patagonian coastal environment. To describe the seasonal trend of the physiological parameters in the field, we sampled subtidal Corallina to determine their net oxygen production (NOP), pigments, and carbonate content (CC). Furthermore, we conducted seasonal 10-days experiments, simulating the conditions predicted for the year 2100 by the IPCC (RCP 8.5) —manipulating pH, nutrients, and irradiance—along with the current conditions. The pigments and carotenoids/chlorophyll-a ratio were, in general, constant in the field over the seasons; but the NOP and CC dropped in spring, when the carotenoids peaked. After the experiment, the highest carotenoid/chlorophyll-a ratio was registered in summer under both the currentand the predictedconditions and in winter under the predictedcondition. This lower physiological status was also reflected in almost all other variables. Thus, Corallina may display an acclimatation strategy to cope with high ultraviolet-radiation levels by adjusting its pigment composition to avoid photoinhibition. An understanding of how Corallina, as a habitat-forming species, will respond to future global-change may provide clues about the extent of effects on the ecosystem functions and services.

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Reallocation of elemental content and macromolecules in the coccolithophore Emiliania huxleyi to acclimate to climate change

Published 14 April 2023 Science Closed
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Global climate change leads to simultaneous changes in multiple environmental drivers in the marine realm. Although physiological characterization of coccolithophores has been studied under climate change, there is limited knowledge on the biochemical responses of this biogeochemically important phytoplankton group to changing multiple environmental drivers. Here, we investigate the interactive effects of reduced phosphorus availability (4 to 0.4 µmol L−1), elevated pCO2 concentrations (426 to 946 µatm), and increasing light intensity (40 to 300 µmol photons m−2 s−1) on elemental content and macromolecules of the cosmopolitan coccolithophore Emiliania huxleyi. Reduced phosphorus availability reduces particulate organic nitrogen (PON) and protein contents per cell under 40 µmol photons m−2 s−1 but not under 300 µmol photons m−2 s−1. Reduced phosphorus availability and elevated pCO2 concentrations act synergistically to increase particulate organic carbon (POC) and carbohydrate contents per cell under 300 µmol photons m−2 s−1 but not under 40 µmol photons m−2 s−1. Reduced phosphorus availability, elevated pCO2 concentrations, and increasing light intensity act synergistically to increase the allocation of POC to carbohydrates. Under elevated pCO2 concentrations and increasing light intensity, enhanced carbon fixation could increase carbon storage in the phosphorus-limited regions of the oceans where E. huxleyi dominates the phytoplankton assemblages. In each type of light intensity, elemental-carbon-to-phosphorus (C:P) and nitrogen-to-phosphorus (N:P) ratios decrease with increasing growth rate. These results suggest that coccolithophores could reallocate chemical elements and energy to synthesize macromolecules efficiently, which allows them to regulate their elemental content and growth rate to acclimate to changing environmental conditions.

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Increased food resources help eastern oyster mitigate the negative impacts of coastal acidification

Published 11 April 2023 Science Closed
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Oceanic absorption of atmospheric CO2 results in alterations of carbonate chemistry, a process coined ocean acidification (OA). The economically and ecologically important eastern oyster (Crassostrea virginica) is vulnerable to these changes because low pH hampers CaCO3 precipitation needed for shell formation. Organisms have a range of physiological mechanisms to cope with altered carbonate chemistry; however, these processes can be energetically expensive and necessitate energy reallocation. Here, the hypothesis that resilience to low pH is related to energy resources was tested. In laboratory experiments, oysters were reared or maintained at ambient (400 ppm) and elevated (1300 ppm) pCO2 levels during larval and adult stages, respectively, before the effect of acidification on metabolism was evaluated. Results showed that oysters exposed to elevated pCO2 had significantly greater respiration. Subsequent experiments evaluated if food abundance influences oyster response to elevated pCO2. Under high food and elevated pCO2 conditions, oysters had less mortality and grew larger, suggesting that food can offset adverse impacts of elevated pCO2, while low food exacerbates the negative effects. Results also demonstrated that OA induced an increase in oyster ability to select their food particles, likely representing an adaptive strategy to enhance energy gains. While oysters appeared to have mechanisms conferring resilience to elevated pCO2, these came at the cost of depleting energy stores, which can limit the available energy for other physiological processes. Taken together, these results show that resilience to OA is at least partially dependent on energy availability, and oysters can enhance their tolerance to adverse conditions under optimal feeding regimes.

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Sediment-seawater exchange altered adverse effects of ocean acidification towards marine microalgae

Published 30 March 2023 Science Closed
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Highlights

  • Five marine microalgal species showed different sensitivities to OA.
  • OA promoted algal growth except I. galbana after introducing sediments.
  • N, P and Fe released from sediments mitigated OA-induced toxicity to E. huxleyi.
  • OA-induced algal community instability was alleviated by the presence of sediments.

Abstract

Ocean acidification (OA) exhibits high threat to marine microalgae. However, the role of marine sediment in the OA-induced adverse effect towards microalgae is largely unknown. In this work, the effects of OA (pH 7.50) on the growth of individual and co-cultured microalgae (Emiliania huxleyi, Isochrysis galbana, Chlorella vulgaris, Phaeodactylum tricornutum, and Platymonas helgolandica tsingtaoensis) were systematically investigated in the sediment-seawater systems. OA inhibited E. huxleyi growth by 25.21 %, promoted P. helgolandica (tsingtaoensis) growth by 15.49 %, while did not cause any effect on the other three microalgal species in the absence of sediment. In the presence of the sediment, OA-induced growth inhibition of E. huxleyi was significantly mitigated, because the released chemicals (N, P and Fe) from seawater-sediment interface increased the photosynthesis and reduced oxidative stress. For P. tricornutum, C. vulgaris and P. helgolandica (tsingtaoensis), the growth was significantly increased in the presence of sediment in comparison with those under OA alone or normal seawater (pH 8.10). For I. galbana, the growth was inhibited when the sediment was introduced. Additionally, in the co-culturing system, C. vulgaris and P. tricornutum were the dominant species, while OA increased the proportions of dominant species and decreased the community stability as indicated by Shannon and Pielou’s indexes. After the introduction of sediment, the community stability was recovered, but remained lower than that under normal condition. This work demonstrated the role of sediment in the biological responses to OA, and could be helpful for better understanding the impact of OA on marine ecosystems.

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Effects of ocean acidification and eutrophication on the growth and photosynthetic performances of a green tide alga Ulva prolifera

Published 17 March 2023 Science Closed
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With the impact of fossil fuel burning and industrialization, atmospheric CO2 concentration will reach about 1000 ppmv in 2100, and more and more CO2 will be absorbed by ocean, resulting in ocean acidification. The Chinese coastal waters are showing unexpectedly high levels of acidification due to a combination of global ocean acidification and severe regional eutrophication, which is caused by natural accumulation or human activities such as aquacultural tail water input, potentially affecting macroalgal blooms. However, little is known about the combined effects of ocean acidification and entrophication on the eco-physiology of bloom-forming macroalgae. This study investigated Ulva prolifera, a dominant species causing green tide in the South Yellow Sea, and explored its growth and physiological responses under the combination conditions of ocean acidification and enriched nutrients. In this study, U. prolifera thalli were cultured under two CO2 conditions (air and 1000 μatm) and two nutrient conditions (High Nutrient, HN, 135 μmol L-1 N and 8.5 μmol L-1 P; Normal Nutrient, NN, 27 μmol L-1 N and 1.7 μmol L-1 P). The results showed that eutrophication conditions obviously enhanced the relative growth rate and photosynthetic performance of U. prolifera. Elevated pCO2 had no significant effect on U. prolifera growth and photosynthetic performance under normal nutrient conditions. However, under eutrophication conditions elevated pCO2 inhibited U. prolifera growth. Moreover, eutrophication conditions markedly improved the contents of chlorophyll a, chlorophyll b and nitrate reductase activity and inhibited the soluble carbohydrate content, but elevated pCO2 had no significant effect on them under nutrient-replete conditions. In addition, elevated pCO2 significantly reduced the carotenoid content under eutrophication conditions and had no effect on it under normal nutrient conditions. These findings indicate that seawater eutrophication would greatly accelerate U. prolifera bloom, which may also be suppressed to a certain extent by ocean acidification in the future. The study can provide valuable information for predicting the future outbreaks of U. prolifera green tide in nearshore regions.

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Aquatic productivity under multiple stressors

Published 13 March 2023 Science Closed
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Aquatic ecosystems are responsible for about 50% of global productivity. They mitigate climate change by taking up a substantial fraction of anthropogenically emitted CO2 and sink part of it into the deep ocean. Productivity is controlled by a number of environmental factors, such as water temperature, ocean acidification, nutrient availability, deoxygenation and exposure to solar UV radiation. Recent studies have revealed that these factors may interact to yield additive, synergistic or antagonistic effects. While ocean warming and deoxygenation are supposed to affect mitochondrial respiration oppositely, they can act synergistically to influence the migration of plankton and N2-fixation of diazotrophs. Ocean acidification, along with elevated pCO2, exhibits controversial effects on marine primary producers, resulting in negative impacts under high light and limited availability of nutrients. However, the acidic stress has been shown to exacerbate viral attacks on microalgae and to act synergistically with UV radiation to reduce the calcification of algal calcifiers. Elevated pCO2 in surface oceans is known to downregulate the CCMs (CO2 concentrating mechanisms) of phytoplankton, but deoxygenation is proposed to enhance CCMs by suppressing photorespiration. While most of the studies on climate-change drivers have been carried out under controlled conditions, field observations over long periods of time have been scarce. Mechanistic responses of phytoplankton to multiple drivers have been little documented due to the logistic difficulties to manipulate numerous replications for different treatments representative of the drivers. Nevertheless, future studies are expected to explore responses and involved mechanisms to multiple drivers in different regions, considering that regional chemical and physical environmental forcings modulate the effects of ocean global climate changes.

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Predicting effects of multiple interacting global change drivers across trophic levels

Published 8 March 2023 Science Closed
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Global change encompasses many co-occurring anthropogenic drivers, which can act synergistically or antagonistically on ecological systems. Predicting how different global change drivers simultaneously contribute to observed biodiversity change is a key challenge for ecology and conservation. However, we lack the mechanistic understanding of how multiple global change drivers influence the vital rates of multiple interacting species. We propose that reaction norms, the relationships between a driver and vital rates like growth, mortality, and consumption, provide insights to the underlying mechanisms of community responses to multiple drivers. Understanding how multiple drivers interact to affect demographic rates using a reaction-norm perspective can improve our ability to make predictions of interactions at higher levels of organization—that is, community and food web. Building on the framework of consumer–resource interactions and widely studied thermal performance curves, we illustrate how joint driver impacts can be scaled up from the population to the community level. A simple proof-of-concept model demonstrates how reaction norms of vital rates predict the prevalence of driver interactions at the community level. A literature search suggests that our proposed approach is not yet used in multiple driver research. We outline how realistic response surfaces (i.e., multidimensional reaction norms) can be inferred by parametric and nonparametric approaches. Response surfaces have the potential to strengthen our understanding of how multiple drivers affect communities as well as improve our ability to predict when interactive effects emerge, two of the major challenges of ecology today.

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Fresh and saline submarine groundwater discharge as sources of carbon and nutrients to the Japan Sea

Published 7 February 2023 Science Closed
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Highlights

  • Fresh groundwater was comparable to the discharge from rivers and the main source of carbon, phosphate, and nitrate to coastal waters.
  • Groundwater-derived alkalinity fluxes were 7 times greater than river inputs, buffering the coastal ocean.
  • Nutrient and chlorophyll observations revealed the strong influence of groundwater discharge on primary productivity.

Abstract

Submarine groundwater discharge (SGD) is an important pathway for carbon and nutrients to the coastal ocean, sometimes exceeding river inputs. SGD fluxes can have implications for long-term carbon storage, ocean acidification and nutrient dynamics. Here, we used radium (223Ra and 226Ra) isotopes to quantify SGD-derived fluxes of dissolved inorganic (DIC) and organic (DOC) carbon, nitrate (NO3−), nitrite (NO2−), ammonium (NH4+) and phosphate (PO43−) in a spring-fed coastal bay in the Japan Sea. The average coastal water residence times using 223Ra/226Ra ratios was 32.5 ± 17.9 days. Fresh and saline SGD were estimated using a radium mixing model with short- and long-lived isotopes. The volume of fresh SGD entering the bay (4.6 ± 4.6 cm day−1) was more than twice that of the volume of saline SGD (1.9 ± 2.1 cm day−1). Fresh SGD (mmol m2 day−1) was the main source of DOC (2.7 ± 2.6), DIC (13.9 ± 13.7), PO43− (0.3 ± 0.3) and NO3− (6.6 ± 6.5) to the coastal ocean, whereas saline SGD was the main source of NH4+ (0.2 ± 0.2). Total SGD-derived carbon and nutrient fluxes were 4 – 7 and 2–16 times greater than local river inputs. Positive correlations between chlorophyll-a, 226Ra and δ13C-DIC indicate that SGD significantly (p < 0.05) enhances primary productivity nearshore. Overall, fresh SGD of nitrogen and carbon to seawater drove chlorophyll-a, decreased DIC/Alkalinity ratios, and modified the carbonate biogeochemistry of the coastal ocean.

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No effect of ocean acidification on growth, photosynthesis, or dissolved organic carbon release by three temperate seaweeds with different dissolved inorganic carbon uptake strategies

Published 27 January 2023 Science Closed
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In a future ocean, dissolved organic carbon (DOC) release by seaweed has been considered a pathway for organic carbon that is not incorporated into growth under carbon dioxide (CO2) enrichment/ocean acidification (OA). To understand the influence of OA on seaweed DOC release, a 21-day experiment compared the physiological responses of three seaweed species, two which operate CO2 concentrating mechanisms (CCMs), Ecklonia radiata (C. Agardh) J. Agardh and Lenormandia marginata (Hooker F. and Harvey) and one that only uses CO2 (non-CCM), Plocamium cirrhosum (Turner) M.J. Wynne. These two groups (CCM and non-CCM) are predicted to respond differently to OA dependent on their affinities for Ci (defined as CO2 + bicarbonate, HCO3−). Future ocean CO2 treatment did not drive changes to seaweed physiology—growth, Ci uptake, DOC production, photosynthesis, respiration, pigments, % tissue carbon, nitrogen, and C:N ratios—for any species, regardless of Ci uptake method. Our results further showed that Ci uptake method did not influence DOC release rates under OA. Our results show no benefit of elevated CO2 concentrations on the physiologies of the three species under OA and suggest that in a future ocean, photosynthetic CO2 fixation rates of these seaweeds will not increase with Ci concentration.

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Ocean acidification and ammonium enrichment interact to stimulate a short-term spike in growth rate of a bloom forming macroalga

Published 10 January 2023 Science Closed
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Introduction: The coastal macroalgal genus, Ulva, is found worldwide and is considered a nuisance algal genus due to its propensity for forming vast blooms. The response of Ulva to ocean acidification (OA) is of concern, particularly with nutrient enrichment, as these combined drivers may enhance algal blooms because of increased availability of dissolved inorganic resources.

Methods: We determined how a suite of physiological parameters were affected by OA and ammonium (NH4+) enrichment in 22-day laboratory experiments to gain a mechanistic understanding of growth, nutrient assimilation, and photosynthetic processes. We predicted how physiological parameters change across a range of pCO2 and NH4+ scenarios to ascertain bloom potential under future climate change regimes.

Results: During the first five days of growth, there was a positive synergy between pCO2 and NH4+ enrichment, which could accelerate initiation of an Ulva bloom. After day 5, growth rates declined overall and there was no effect of pCO2, NH4+, nor their interaction. pCO2 and NH4+ acted synergistically to increase NO3– uptake rates, which may have contributed to increased growth in the first five days. Under the saturating photosynthetically active radiation (PAR) used in this experiment (500 μmol photon m-2 s-1), maximum photosynthetic rates were negatively affected by increased pCO2, which could be due to increased sensitivity to light when high CO2 reduces energy requirements for inorganic carbon acquisition. Activity of CCMs decreased under high pCO2 and high NH4+ conditions indicating that nutrients play a role in alleviating photodamage and regulating CCMs under high-light intensities.

Discussion: This study demonstrates that OA could play a role in initiating or enhancing Ulva blooms in a eutrophic environment and highlights the need for understanding the potential interactions among light, OA, and nutrient enrichment in regulating photosynthetic processes.

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Species-dependent effects of seawater acidification on alkaline phosphatase activity in dinoflagellates

Published 14 December 2022 Science Closed
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Global climate change is widely shown to cause ocean acidification (OA) and projected to intensify nutrient limitation. Phosphorus (P) is an essential nutrient for phytoplankton to grow. While dissolved inorganic phosphate (DIP) is the preferred form of P, phytoplankton have evolved alkaline phosphatase (AP) to utilize dissolved organic phosphorus (DOP) when DIP is deficient. Although AP is known to require pH>7, how OA may affect AP activity and hence the capacity of phytoplankton to utilize DOP is poorly understood. Here, we examine the effects of pH conditions (5.5 to 11) on AP activity from six species of dinoflagellates, an important group of marine phytoplankton. We observed a general pattern that AP activity declined sharply at pH 5.5, peaked between pH 7 and 8, and dropped at pH>8. However, our data revealed remarkable interspecific variations in optimal pH and niche breadth of pH. Among the species examined, Fugacium kawagutii and Prorocentrum cordatum had an optimal pH at 8, Alexandrium pacificum, Amphidinium carterae, Effrenium voratum, and Karenia mikimotoi showed an optimal pH of 7. However, whereas A. pacificum, F. kawagutii and K. mikimotoi had a broad pH range for AP (7-11), A. carterae, E. voratum, and P. cordatum exhibited a narrow pH range. The response of AP activity of A. carterae to pH changes was verified using purified AP heterologously expressed in Escherichia coli. These findings suggest that OA will likely differentially impact the capacity of different phytoplankton species to utilize DOP in the projected acidified and nutrient-limited future ocean.

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Impact of ocean acidification and ocean warming on the oxidation of dissolved Fe(II) in coastal and open Southern Ocean water

Published 8 December 2022 Science Closed
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The Southern Ocean is the largest region where major nutrients such as nitrate, silicate and phosphate are present in excess, yet the crucial micronutrient element iron (Fe) is scarce. It is well established that the Southern Ocean is key in exporting carbon to greater depths through biomass production by phytoplankton, but Fe is metabolically required for photosynthesis. Changes in uptake of carbon and heat to the ocean will impact ocean acidification and ocean warming. These anthropogenically linked processes are projected to lead to a drop in ocean pH by 0.2 units and an increase in the ocean’s temperature by 2°C by the end of the century and are already known to have tremendous ecological impacts on the ocean’s flora and fauna. However, little is known about how changes in ocean temperature and pH could alter the nutrient composition in future oceans.

Regarding nutrients, this work focuses on the dissolved (d) element Fe. It is essential for photosynthesis, but also a limiting element in the Southern Ocean due to limiting sources leading to low availability. Iron exists in two redox states in seawater. While the species dFe(III) is stable in seawater and occurs in relatively higher concentrations, its redox partner dFe(II) is tied to several physico-chemical processes impacting its oxidation time and overall presence. The importance of dFe(II) also lies with its accessibility for phytoplankton in its reduced oxidative state. The overall aim of this study was to investigate changes in concentration, speciation, and availability of the ‘more’ bioavailable, rapidly oxidizing Fe species dFe(II) under a changing Southern Ocean scenario.

Chapter 2 addressed the redox behaviour of dFe(II) and dFe(III), where several questions were explored for further experimental planning. The main question was how the coastal and open ocean systems differ in their dFe(II) concentrations and how ocean acidification and ocean warming impact Fe redox chemistry in both systems. I therefore performed controlled acidification and temperature alteration experiments in coastal and open ocean water taken from the Tasmanian coast and the Southern Ocean. This large dataset enabled us to project for future ocean dFe(II) concentrations and oxidation rates. I observed that a reduction in ocean pH by 0.2 units doubles the dFe(II) oxidation time in the open ocean and tripled in coastal water through model-based experiments. In contrast to these high impacts from pH, an increase in temperature by 1°C accelerated the oxidation by ~ 1.1 times (13% in coastal water and 8% in open ocean water). Therefore, realistic changes in temperature are likely to have small impacts on the oxidation of dFe(II) in both water systems compared to the proposed changes in pH.

For phytoplankton, these results pose contradicting outcomes, and studies display mixed results once parameters such as ocean warming, and acidification are combined. An increase in temperature might lead to less or no growth once a certain temperature threshold is crossed. Similarly, a decrease in pH is also thought to impact phytoplankton physiology. It also depends on the severity of acidification and the phytoplankton species itself. Ocean warming could reduce phytoplankton growth, despite increased Fe availability due to higher solubility in warmer water. Regarding ocean acidification, on the other hand, dFe(II) could become available for an extended time, therefore enabling further uptake of dFe(II) by phytoplankton for that time. When comparing mixed effects of ocean acidification and warming, a reduction in pH might have a greater impact on the dFe(II) oxidation than just temperature. Temperature changes, however, might be a greater concern in the near future before ocean acidification becomes relevant.

Due to this projection of temperature being a more imminent concern, I targeted the limiting element Fe in its less investigated form dFe(II). I observed how temperature alone impacts growth of two Southern Ocean phytoplankton species. I therefore ran an dFe(II)-enrichment incubation experiment in Chapter 3 with differing temperatures (3°C, 5°C, and 7°C) in coastal and open ocean water from the Southern Ocean using the well-studied haptophyte Phaeocystis antarctica and the diatom Fragilariopsis cylindrus. These enrichment experiments with altered temperatures overall confirmed that phytoplankton growth was elevated once 5 nM dFe(II) were added. In other words, freely available dFe(II) was present, almost regardless of the temperature increase from 3°C to 7°C. This could implicate that an increase in temperature has beneficial effects on growth in the case of higher concentrations of freely available dFe(II). However, these values of future dFe(II) concentrations and oxidation rates under acidified and warmer scenarios are only laboratory-based projections, to better understand the dFe(II) presence and demand by phytoplankton species in a future Southern Ocean.

In Chapter 4, a one-month field study onboard the RV Investigator was conducted east of the Australian continent along the East Australian Current (EAC) into nutrient-rich but Fe poor water in the Southern Ocean. I observed the overall distribution of dFe(II) and hydrogen peroxide in this understudied region. The findings suggest that dFe(II) concentrations are very low in the observed area of the open Southern Ocean (< 0.1 nM) compared to coastal waters (> 0.5 nM), likely driven by differences in terrestrial Fe inputs. Hydrogen peroxide was generally higher in the southern stations within the upper 200 m (~60 nM) while the dFe(II) : dFe ratios are 10 % higher than reported for previous Southern Ocean studies. High biological activity in the upper water extending to the frontal mixing zone where the two major currents meet (EAC and STF), may further have led to the observed low dFe concentrations and high H22O22 concentrations. Occasional higher dFe(II) peaks found in this area in surface water may be the result of several external sources such as rain or vertical transport from seamounts but also due to biological or physico-chemical impacts such as photochemical reduction or uptake by phytoplankton.

Overall, the work in this study advances our understanding of the coupled effects of the climate change parameters ocean acidification and ocean warming on the dFe(II) oxidation, with implications for its availability to phytoplankton and overall sources in the region east and south-east of Tasmania in coastal and open ocean water. The experimental approaches taken suggest a higher impact of ocean acidification compared to ocean warming and a potential benefit for phytoplankton species preferring dFe(II).

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Thalassiosira weissflogii grown in various Zn levels shows different ecophysiological responses to seawater acidification

Published 15 November 2022 Science Closed
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Highlights

  • Zn deficient encouraged cellular silicon and sinking rate under normal pCO2.
  • Higher pCO2 decreased cellular silicon and sinking rate of Zn-deficient T. weissflogii.
  • Higher pCO2 increased cellular silicon and sinking rate in Zn-replete T. weissflogii.
  • Silica and carbon cycle could be impacted by acidification and Zn levels.

Abstract

The presence of zinc (Zn), a vital element for algal physiological functions, coupled with the silicification of diatoms implies that it plays an integral role in the carbon and silicon cycles of the sea. In this study, we examined the effects of different pCO2 and Zn levels on growth rate, elemental compositions and silicification by Thalassiosira weissflogii. The results showed that under normal pCO2 (400 μatm), cultures of T. weissflogii were depressed for growth rate and silica incorporation rate, but encouraged for cellular silicon content, Si/C, Si/N, and sinking rate when Zn deficient (0.3 pmol L−1). However, cellular silicon and sinking rate of Zn-deficient and Zn-replete (25 pmol L−1) T. weissflogii were decreased and increased at higher pCO2 (800 μatm), respectively. Thus, acidification may affect diatoms significantly differently depending on the Zn levels of the ocean and then alter the biochemical cycling of carbon and silica.

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Cascading effects augment the direct impact of CO2 on phytoplankton growth in a biogeochemical model

Published 11 November 2022 Science Closed
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Atmospheric and oceanic CO2 concentrations are rising at an unprecedented rate. Laboratory studies indicate a positive effect of rising CO2 on phytoplankton growth until an optimum is reached, after which the negative impact of accompanying acidification dominates. Here, we implemented carbonate system sensitivities of phytoplankton growth into our global biogeochemical model FESOM-REcoM and accounted explicitly for coccolithophores as the group most sensitive to CO2. In idealized simulations in which solely the atmospheric CO2 mixing ratio was modified, changes in competitive fitness and biomass are not only caused by the direct effects of CO2, but also by indirect effects via nutrient and light limitation as well as grazing. These cascading effects can both amplify or dampen phytoplankton responses to changing ocean pCO2 levels. For example, coccolithophore growth is negatively affected both directly by future pCO2 and indirectly by changes in light limitation, but these effects are compensated by a weakened nutrient limitation resulting from the decrease in small-phytoplankton biomass. In the Southern Ocean, future pCO2 decreases small-phytoplankton biomass and hereby the preferred prey of zooplankton, which reduces the grazing pressure on diatoms and allows them to proliferate more strongly. In simulations that encompass CO2-driven warming and acidification, our model reveals that recent observed changes in North Atlantic coccolithophore biomass are driven primarily by warming and not by CO2. Our results highlight that CO2 can change the effects of other environmental drivers on phytoplankton growth, and that cascading effects may play an important role in projections of future net primary production.

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Phosphate limitation intensifies negative effects of ocean acidification on globally important nitrogen fixing cyanobacterium

Published 10 November 2022 Science Closed
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Growth of the prominent nitrogen-fixing cyanobacterium Trichodesmium is often limited by phosphorus availability in the ocean. How nitrogen fixation by phosphorus-limited Trichodesmium may respond to ocean acidification remains poorly understood. Here, we use phosphate-limited chemostat experiments to show that acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. The increased phosphorus requirements were attributed primarily to elevated cellular polyphosphate contents, likely for maintaining cytosolic pH homeostasis in response to acidification. Alongside the accumulation of polyphosphate, decreased NADP(H):NAD(H) ratios and impaired chlorophyll synthesis and energy production were observed under acidified conditions. Consequently, the negative effects of acidification were amplified compared to those demonstrated previously under phosphorus sufficiency. Estimating the potential implications of this finding, using outputs from the Community Earth System Model, predicts that acidification and dissolved inorganic and organic phosphorus stress could synergistically cause an appreciable decrease in global Trichodesmium nitrogen fixation by 2100.

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High sclerobiont calcification in marginal reefs of the eastern tropical Pacific

Published 8 November 2022 Science Closed
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Graphical abstract.

A sclerobiont is any organism capable of fouling hard substrates. Sclerobionts have recently received attention due to their notable calcium carbonate contributions to reef structures and potential to offset drops in carbonate budgets in degraded reefs. However, due to their encrusting nature, it is difficult to quantify net calcium carbonate production at the level of individual taxonomic groups, and knowledge regarding the main environmental factors that regulate their spatial distributions is limited. In addition, the material types used to create experimental substrates, their orientations, and their overall deployment times can influence settlement and the composition of the resulting communities. Thus, comparative evaluations of these variables are necessary to improve future research efforts. In this study, we used calcification accretion units (CAUs) to quantify the calcium carbonate contributions of sclerobionts at the taxonomic group level and evaluated the effects of two frequently used materials [i.e., polyvinyl chloride (PVC) and terracotta (TCT) tiles] on the recruitment and calcification of the sclerobiont community in the tropical Mexican Pacific and the Midriff Island Region of the Gulf of California over 6 and 15 months [n = 40; 5 CAUs x site (2) x deployment time (2) x material type (2)]. The net sclerobiont calcification rate (mean ± SD) reached maximum values at six months and was higher in the Mexican Pacific (2.15 ± 0.99 kg m−2 y−1) than in the Gulf of California (1.70 ± 0.67 kg m−2 y−1). Moreover, the calcification rate was slightly higher on the PVC-CAUs compared to that of the TCT-CAUs, although these differences were not consistent at the group level. In addition, cryptic microhabitats showed low calcification rates when compared to those of exposed microhabitats. Crustosecoralline algae and barnacles dominated the exposed experimental surfaces, while bryozoans, mollusks, and serpulid polychaetes dominated cryptic surfaces. Regardless of the site, deployment time, or material type, barnacles made the greatest contributions to calcimass production (between 41 and 88%). Our results demonstrate that the orientation of the experimental substrate, and the material to a lesser extent, influence the sclerobiont community and the associated calcification rate. Upwelling-induced surface nutrient levels, low pH levels, and the aragonite saturation state (ΩAr) limit the early cementation of reef-building organisms in the tropical Mexican Pacific and promote high bioerosion rates in corals of the Gulf of California. Our findings demonstrate that sclerobionts significantly contribute to calcium carbonate production even under conditions of high environmental variability.

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Design of a low-cost pH-Stat to study effects of ocean acidification on growth and nutrient consumption of diatoms

Published 31 October 2022 Science Closed
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Highlights

  • A low-cost pH-stat was designed to evaluate the effect of pH variations on the growth rate and nutrient consumption in multiple microalgae cultures.
  • The current pH of the ocean resulted in the highest growth rate for P. tricornutum.
  • Nitrate was the limiting nutrient in the three pH levels evaluated.
  • Phosphate and iron were related to the acclimatization response of the microalgae.
  • Efficient pH control allowed for the observation of some of the effects of climate change on diatoms related to nutrient consumption.

Abstract

Increasing CO2 emissions has modified oceanic pH levels. These pH changes affect phytoplankton growth and composition. Diatom cells constitute almost 50% of phytoplankton, and they have significant importance in the ocean food chains and biotechnology industries. Therefore, knowledge of their response to pH changes could be useful for conservation and aquaculture of these species. There are different pH-Stat systems to supply CO2 gas to the culture medium, however, it is common to use one unit or pH probe for each culture. In this study, we designed a low-cost pH-stat to regulate the pH level in fifteen simultaneous cultures. It was evaluated with Phaeodactylum tricornutum at three pH setpoints:7.5 and 7.8 as acid treatments and 8.1 as control; each experiment lasted seven days, and growth rates, latency phases and nutrient consumption rates were determined. The accuracy and precision of the pH regulated was in an acceptable level compared with other systems. The growth rate and consumption of nitrate were higher at pH 8.1, moreover differences were observed in the duration of the latency phase, suggesting a longer acclimation process at lower pH. Changes in phosphate and iron consumption indicated a higher availability in acid treatments, however they did not enhance the growth. These denoted unfavorable effects of ocean acidification on diatoms growth.

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Quantification of the dominant drivers of acidification in the coastal Mid-Atlantic Bight

Published 28 October 2022 Science Closed
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Abstract

In shallow coastal shelves like the Mid-Atlantic Bight (MAB), ocean acidification due to increased atmospheric carbon dioxide (CO2) is compounded by highly variable coastal processes including riverine freshwater inputs, nutrient loading, biogeochemical influence, coastal currents and water mass mixing, and seasonal transitions in physical parameters. Past deconstructions of carbonate system drivers in the MAB have focused on nearshore zones or single season data, and thus lack the spatial and temporal resolution required to assess impacts to important species occupying the shelf. Deconstructing highly resolved data collected during four seasonal Slocum glider deployments in the MAB, this study uses a Taylor Series decomposition to quantify the influence of temperature, salinity, biogeochemical activity, and water mass mixing on pH and aragonite saturation state from sea surface to bottom. Results show that water mass mixing and biogeochemical activity were the most significant drivers of the carbonate system in the MAB. Nearshore water was more acidic year-round due to riverine freshwater input, but photosynthesis reduced acidity at certain depths and times. Water mass mixing increased acidity in bottom water on the shelf, particularly in summer. Gulf Stream intrusions at the shelf break during fall acted to mitigate acidification on the shelf in habitats occupied by carbonate-bearing organisms. The relationships quantified here can be used to improve biogeochemical forecast models and determine habitat suitability for commercially important fin and shellfish species residing in the MAB.

Key Points

  • Water mass mixing and biogeochemical activity are the major drivers of seasonal carbonate system dynamics in the MAB
  • Water mass mixing has opposing effects on carbonate chemistry in the nearshore and at the continental shelf break

Plain Language Summary

The coastal ocean is experiencing changes in chemistry due to human activities, including carbon dioxide emissions, nutrient runoff, and seasonal changes in temperature, salinity, and coastal currents. These drivers have been studied close to shore and/or only during single seasons, leaving a gap in our understanding of seasonal changes across the entire economically important shelf region. Here, we use high-resolution data collected by a deep-sea robot that measures chemistry from ocean surface to the sea floor. We determined the importance of four key influences (temperature, salinity, water mass mixing, and biological activity) on changes in coastal chemistry over the course of a year. We found that the most important driver of shelf chemistry was mixing of freshwater at the coast and warm, salty water at the edge of the shelf. Biological activity was a secondary influence, which caused smaller scale changes in chemistry. These results can help to predict how coastal chemistry might change in the future, so that we can prepare for the effects on economically important animals and industries.

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