Posts Tagged 'sediments'

Warming and ocean acidification may decrease estuarine dissolved organic carbon export to the ocean

Estuaries make a disproportionately large contribution of dissolved organic carbon (DOC) to the global carbon cycle, but it is unknown how this will change under a future climate. As such, the response of DOC fluxes from microbially dominated unvegetated sediments to individual and combined future climate stressors of warming (from Δ−3 °C to Δ+5 °C on ambient mean temperatures) and ocean acidification (OA, ~2 times the current partial pressure of CO2, pCO2) was investigated ex situ. Warming alone increased sediment heterotrophy, resulting in a proportional increase in sediment DOC uptake, with sediments becoming net sinks of DOC (3.5 to 8.8 mmol-C m−2 d−1) at warmer temperatures (Δ+3 °C and Δ+5 °C, respectively). This temperature response changed under OA conditions, with sediments becoming more autotrophic and a greater sink of DOC (1 to 4 times greater than under current-pCO2). This response was attributed to the stimulation of heterotrophic bacteria with the autochthonous production of labile organic matter by microphytobenthos. Extrapolating these results to the global area of unvegetated subtidal estuarine sediments, the future climate of warming (Δ+3 °C) and OA may decrease the estuarine export of DOC by ~80 % (~150 Tg-C yr−1) and have a disproportionately large impact on the global DOC budget.

Continue reading ‘Warming and ocean acidification may decrease estuarine dissolved organic carbon export to the ocean’

Siderophore production by bacteria isolated from mangrove sediments: a microcosm study


  • Siderophores are organic ligands produced by bacteria primarily for iron sequestration.
  • In this study, siderophore production was independent of warmer temperatures that helped growth of the bacterial isolates.
  • Ocean acidification (pH 6.5 to 7.5) did not suppress siderophore production in these strains.
  • In this study, bacterial isolates used diverse carbon sources to produce siderophores.
  • Such responses of pathogenic strains may help in their survival in changing global environment, hence is of concern.


Mangroves are one of the most productive ecosystems worldwide covering up to 75% of the coastline in the tropics and subtropics. They support a highly diverse community (marine and terrestrial) and serves as reservoirs of nutrients for coastal and shelf waters. Bacterial diversity in mangroves includes heterotrophs, autotrophs (nitrogen fixation) and pathogens (phytopathogens, marine, and human). All these bacterial groups require sequestration of bioavailable iron, which is largely done by the production of siderophores. In this study, microcosm experiments were conducted to test the effect of incubation conditions (temperature, iron concentration, pH, and carbon source) on growth and siderophore production in four mangrove sediment bacterial isolates- Escherichia vulneris, Enterobacter cancerogenus, Pantoea agglomerans, and Enterobacter bugandensis. Our study showed that all isolates produce more siderophores (30 to 60%) at low iron concentrations (10 nM to 1 μM) during lag-phase and early log-phase of growth. Low temperature suppressed bacterial growth without significantly altering the siderophore production, whereas low pH suppressed both growth and siderophore production in these isolates. Although all isolates could produce siderophores when using different carbon sources, glucose served as an ideal carbon source. The observed changes in growth and siderophore production may be attributed to species-specific physiological traits, changes in bioavailability of iron and/or combination of both. Our results suggest that in a changing global environment, warming of the surrounding waters may not reduce the siderophore production and hence, they will be essential in sustaining bacterial activity in sediments.

Continue reading ‘Siderophore production by bacteria isolated from mangrove sediments: a microcosm study’

Effect of Theora lubrica on the response of coastal soft sediment nitrogen cycling to ocean acidification

Ocean acidification could influence nitrogen cycling in coastal soft sediments, which are moderated by bioturbating macrofauna. The functioning of coastal ecosystems has a strong connection with nitrogen fluxes that occur at the sediment–seawater interface; the disturbance of the sediment matrix via bioturbation can significantly alter these fluxes. To investigate how decreasing seawater pH affects the fluxes of 02, NH4+, NO2– and NO3–, I incubated sediment core samples of intact coastal subtidal silt in four seawater recirculating systems and injected CO2 to adjust their pH to 8.0, 7.8, 7.6 and 7.4. I also incorporated bioturbation via a Bivalve treatment by adding 10 Theora lubrica (introduced infaunal bivalve) to a sediment core. Furthermore, the experiment was done in full darkness to eliminate photosynthesis, and salinity and temperature were controlled variables. Initial measurements at in situ pCO2 indicated, that the Bivalve treatment significantly increased NH4+ and NO3– effluxes, and O2 influxes, but had no effect on NO2- fluxes. After a 20-day incubation, the final measurements revealed, that seawater acidification significantly increased NH4+ and NO2– effluxes, but had no effect on the fluxes of NO3- and O2. Furthermore, I detected no significant effects on nitrogen fluxes by the interaction between the pH and Bivalve treatments; however, the interaction significantly decreased O2 influxes. I hypothesise that the addition of T. lubrica stimulated ammonification and nitrification at in situ pCO2 during the initial measurements. I also suspect that seawater acidification decreased coupled nitrification-denitrification during the final measurements. Furthermore, I suggest that T. lubrica caused both direct and indirect effects on the sediment matrix, leading to the significant decrease in O2 influxes during lower seawater pH within the Bivalve treatment cores. Overall, my study was conclusive because I was able to prove that T. lubrica had no influence on coastal soft sediment nitrogen cycling during seawater acidification. Furthermore, I demonstrated that seawater acidification significantly affected sediment nitrogen cycling, which means ocean acidification could have a profound impact on coastal ecosystem functioning in the future.

Continue reading ‘Effect of Theora lubrica on the response of coastal soft sediment nitrogen cycling to ocean acidification’

Nutrient enrichment promotes eutrophication in the form of macroalgal blooms causing cascading effects in two anthropogenically disturbed coastal ecosystems

Humans are impacting almost every major ecological process that structures communities and ecosystems. Examples of how human activity can directly control key processes in ecosystems include destruction of habitat changing trophic structure, nutrient pollution altering competitive outcomes, overharvesting of consumers reducing top down control, and now climate change impacting virtually every global biogeochemical cycle. These human impacts may have an independent effect on the ecosystem, but they also have the potential to cause cascading effects and promote subsequent stressors. Also, these impacts are not limited to a particular system or geographic location making research on their overall effects vital for management practices. For example, tropical reefs have been transitioning from coral to mixed communities dominated by macroalgae, motivating research on how macroalgae respond to anthropogenic stressors and interact with each other during these stressful events. Further, while eutrophication of coastal estuaries due to increased anthropogenic supplies of nutrients has been of critical global concern for decades, the potential for eutrophication to drive new stressors is a growing concern. To address these knowledge gaps, I investigated how human stressors impact two different and major coastal ecosystems known to be vulnerable to anthropogenic disturbances.

In chapter 1, I demonstrate that anthropogenic stressors in the form of increased nutrients in the water and sediments have strong impacts on interspecific interactions of coral reef macroalgae. Abiotic stressors such as nutrients have been linked to phase-shifts from coral to algal domination on tropical reefs. However, few studies have considered how these stressors impact changes in the biotic and abiotic constituents of dominant species of calcifying macroalgae, and how this may be mediated by species-species interactions. I conducted 4 mesocosm experiments to examine whether different nutrient sources (water column vs. terrestrial sediment) as well as species interactions (alone vs. mixed species) affected total mass (biomass + calcium carbonate (CaCO3)) of two common calcifying macroalgae (Padina boryana and Galaxaura fasciculata). P. boryana gained total mass with increased water column nutrients but declined with increased nutrients supplied by the sediment. Conversely, G. fasciculata gained total mass with increased nutrients in the sediment but declined with increased water column nutrients. In both interactions, the “winner” (i.e., G. fasciculata in the sediment experiment) also had a greater % of thallus mass comprised of CaCO3, potentially due to the subsequent decomposition of the “loser” as this result was not found in the alone treatments. These findings ultimately suggest that nutrient stressors can cause cascading effects, such as promoting calcification and biomass growth or loss in these macroalgal communities, and the potential for domination or decline is based on the nutrient source and community composition.

In chapter 2, I demonstrate that decomposition of macroalgal blooms cause a sequence of biogeochemical processes that can drive acidification in shallow coastal estuaries, and that these processes are mediated by a dynamic microbial community. Eutrophication and ocean acidification are both widely acknowledged as major human-induced stressors in marine environments. While the link between eutrophication and acidification has been established for phytoplankton, it is unclear whether eutrophication in the form of macroalgal blooms can cause cascading effects like acidification in shallow eutrophic estuaries. I conducted seasonal field surveys and assessed microbial communities and functional genes to evaluate changes in biotic and abiotic characteristics between seasons that may be associated with acidification in Upper Newport Bay, CA, USA. Acidification, measured as a drop in pH of 0.7, occurred in summer at the site with the most macroalgal cover. Microbial community composition and functional gene expression provide evidence that decomposition processes contributed to acidification, and also suggest that other biogeochemical processes like nitrification and degradation of polyphosphate also contributed to acidification. To my knowledge, my findings represent the first field evidence that eutrophication of shallow coastal estuaries dominated by green macroalgal blooms can cascade to acidification.

In chapter 3, I demonstrate that macroalgal blooms in shallow estuaries are strong drivers of key microbially-mediated biogeochemical processes that can cause cascading effects, such as acidification and nutrient fluxing, regardless of simulated tidal flushing. Estuaries are productive and diverse ecosystems and are vulnerable to eutrophication from increased anthropogenic nutrients. While it is known that enhanced tidal flushing can reduce adverse effects of anthropogenic disturbances in larger, deeper estuarine ecosystems, this is unexplored for eutrophication in shallow coastal estuaries where macroalgae usually dominate. I simulated eutrophication as a macroalgal bloom in a mesocosm experiment, varied tidal flushing (flushed daily vs unflushed), and assessed the effects on water column and sediment biogeochemical processes and the sediment microbial community. While flushing did not ameliorate the negative effects of the macroalgal bloom, it caused transient differences in the rate of change in biogeochemical processes and promoted increased fluxes of nutrients from the sediment. In the beginning, the macroalgal bloom induced basification and increased total alkalinity, but during decomposition, acidification and the accumulation of nutrients in the sediment and water column occurred. The findings from this chapter ultimately suggest that macroalgal blooms have the potential to be the cause of, yet may also offer a partial solution to, global ecological changes to biogeochemical processes.

Overall, my results indicate that anthropogenic disturbances, particularly in the form of increased nutrients, can cause cascading effects like macroalgal blooms that in turn cause acidification, basification, increased interspecific interactions, nutrient depletion, and nutrient fluxing in multiple ecosystems. These data advance our current understanding of the ecological consequences of eutrophication in the form of macroalgal blooms in different ecosystems. It also provides mechanistic links to microbial communities and biogeochemical processes not previously identified for shallow coastal estuaries. As human population and subsequent nutrient pollution increases in watersheds globally, ecological phenomenon such as eutrophication will only be intensified, and macroalgal communities will continue to dominate. Consequently, this dominance, especially during decomposition as shown here, can drive a multitude of subsequent stressors that can impact the entire ecosystem.

Continue reading ‘Nutrient enrichment promotes eutrophication in the form of macroalgal blooms causing cascading effects in two anthropogenically disturbed coastal ecosystems’

Carbonates dissolution and precipitation in hemipelagic sediments overlaid by supersaturated bottom-waters – Gulf of Aqaba, Red Sea

Whether CaCO3 dissolves within the top centimeters of marine sediments overlaid by deep, supersaturated bottom waters remains an area of debate in geochemistry. This uncertainty stems from the fact that different methods used to assess CaCO3 dissolution rates often provide what appear to be profoundly different results. Here we combine microelectrode and porewater chemistry profiles, core incubation experiments, mineral characterizations and observations of the state of preservation of coccolithophorid exoskeletons for a holistic view of carbonate reactions within the top 30 centimeters of hemipelagic sediments from the Gulf of Aqaba, Red Sea. Calculations based on pH and O2 microelectrode data suggest that rapid metabolic dissolution of carbonate minerals occurs in these sediments within the top two millimeters. Porewater chemistry supports these calculations. The porewater-based observations are further supported by sedimentological characteristics such as aragonite content, and dissolution pitting and fragmentation of coccoliths in sediment layers deposited over the last 200 y. Dissolution appears to be occurring today within surface sediments despite the bulk porewater solution being supersaturated with respect to aragonite and Mg-calcite. In spite of intense dissolution within the sediments, there is no evidence for significant alkalinity and/or calcium fluxes (transport) into bottom waters. It appears that the supersaturated bottom water promotes the removal of all excess alkalinity and calcium produced within the sediment, by CaCO3 precipitation at or above the sediment/ bottom water interface. The precipitation mechanism may be by either benthic organisms (biogenic precipitation) or inorganically (direct precipitation on settling CaCO3 grains). We suggest that authigenic precipitation of (Ca,Mn)CO3 as it becomes supersaturated below 3 cm in the sediments can reconcile the evidence for carbonate dissolution in what appears to be supersaturated conditions. This means that MnCO3 replaces CaCO3 within the nanofossils below ∼3 cm, and that part of the manganese rich CaCO3 is bioturbated upwards into undersaturated conditions, facilitating dissolution of these fossils. Diminished calcite and aragonite concentrations in sediments deposited in recent decades are proposed to be a result of increased manganese cycling rates and greater rates of coupled dissolution within the interfacial sediments, possibly combined with diminished calcareous plankton productivity, in response to increased surface water primary productivity.

Continue reading ‘Carbonates dissolution and precipitation in hemipelagic sediments overlaid by supersaturated bottom-waters – Gulf of Aqaba, Red Sea’

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Ocean acidification in the IPCC AR5 WG II

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