Posts Tagged 'protists'

Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact

Debate lingers over what caused the last mass extinction 66 million years ago, with intense volcanism and extraterrestrial impact the most widely supported hypotheses. However, without empirical evidence for either’s exact environmental effects, it is difficult to discern which was most important in driving extinction. It is also unclear why recovery of biodiversity and carbon cycling in the oceans was so slow after an apparently sudden extinction event. In this paper, we show (using boron isotopes and Earth system modeling) that the impact caused rapid ocean acidification, and that the resulting ecological collapse in the oceans had long-lasting effects for global carbon cycling and climate. Our data suggest that impact, not volcanism, was key in driving end-Cretaceous mass extinction.

Continue reading ‘Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact’

Reduced continental weathering and marine calcification linked to late Neogene decline in atmospheric CO2

The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO3 preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide.

Continue reading ‘Reduced continental weathering and marine calcification linked to late Neogene decline in atmospheric CO2’

Evaluating the planktic foraminiferal B/Ca proxy for application to deep time paleoceanography

Highlights

  • Paleocene seawater chemistry affects planktic foraminifer boron/calcium proxy sensitivity.
  • T. sacculifer and O. universa shell boron content is similar to that of Paleogene species.
  • We present a new framework for applying B/Ca calibrations to the early Cenozoic.
  • Our new approach allows application of calibrations from modern species to extinct ones.

Abstract

The Cenozoic Era has been characterized by large perturbations to the oceanic carbon cycle and global climatic changes, but quantifying the magnitude and cause of these shifts is still subject to considerable uncertainty. The boron/calcium (B/Ca) ratio of fossil planktic foraminifera shells is a promising tool for reconstructing surface ocean carbonate chemistry during such events. Previous studies indicate that symbiont-bearing, planktic foraminiferal B/Ca depends on the [B(OH)4− /DIC] ratio of seawater and potentially, when combined with foraminiferal δ11 B proxy reconstructions of B(OH)4− , an opportunity to reconstruct surface ocean DIC in the geologic past. There are, however, two barriers towards interpreting records from the pre-Pleistocene era: (1) changes in seawater major ion chemistry in the past might have affected foraminiferal B/Ca; and (2) modern foraminifera species show variable B/Ca calibration sensitivities that cannot be constrained in now-extinct species. Here we address these challenges with new experiments in which we have cultured modern, symbiont-bearing foraminifera Globigerinoides ruber (pink) and Trilobatus sacculifer in seawater with simulated early Cenozoic seawater chemistry (high [Ca], low [Mg], and low [B]T). We explore mechanisms that can account for the inter-species trends that are observed in foraminiferal B/Ca, and propose a framework that can be used to apply B/Ca calibrations to now-extinct species for reconstructing climate perturbations under varying seawater chemistries.

Continue reading ‘Evaluating the planktic foraminiferal B/Ca proxy for application to deep time paleoceanography’

Marine microbial community dynamics and responses to ocean acidification

Marine microbes, including both eukaryotes and prokaryotes, are the basal components of marine food webs and play a fundamental role in global biogeochemical cycling. Marine phytoplankton are responsible for approximately 50% of Earth’s primary production, while heterotrophic bacteria and archaea modulate carbon and nutrient cycling in the marine environment. The structure and function of marine microbial communities are closely coupled. Consequently, understanding the factors which govern the distribution of marine microbes through space and time has key implications for food webs and biogeochemical cycling. The development of high-throughput sequencing technologies has revolutionised marine microbial ecology by facilitating the profiling of microbial communities in high taxonomic resolution. In this thesis high-throughput sequencing of the 16S and 18S rRNA genes was used to achieve two major aims. The first aim was to investigate the ecological processes which underpin microbial community assembly in the marine environment. The second aim was to investigate the responses of marine microbial communities to near- future ocean acidification.

Two studies were performed towards the first aim of this thesis. In the first study, the microbial biogeography of the South Pacific Gyre was characterised across three depths at 22 stations along a 2,000 km longitudinal transect of the region. Microbial community composition was homogenous across horizontal spatial scales in the surface waters of the South Pacific Gyre, but varied significantly between surface waters and the deep chlorophyll maximum. A null model approach was used to unveil the ecological processes driving microbial community assembly in the region. Microbial communities in the surface waters were assembled primarily through the deterministic process of homogeneous selection, indicating that selection pressures were sufficient to overwhelm the influence of dispersal effects and ecological drift across vast horizontal spatial distances in the region. Dispersal limitation was comparatively more influential in the assembly of microbial communities between the surface waters and the deep chlorophyll maximum, indicating that stochastic processes play a significant role in microbial community assembly between these contiguous water masses.

In the second study, the bacterioplankton and protist biogeography of the Southland Front system was characterised in surface waters at 24 stations spanning four water masses. Both bacterioplankton and protist communities displayed significant structuring according to water mass, although this effect was most pronounced in bacterioplankton communities. A null model approach revealed that bacterioplankton communities were primarily assembled through homogeneous selection, while protist communities were primarily assembled through dispersal limitation and ecological drift across the Southland Front system. These findings highlight that distinct ecological processes can underpin the assembly of co- occurring bacterioplankton and protist communities, and that hydrographic features such as oceanic fronts play an important role in structuring both bacterioplankton and protist communities.

Two studies were conducted towards the second aim of this thesis. In the first study, the effect of ocean acidification and warming on bacterioplankton communities was investigated at the fringe and ultra-oligotrophic centre of the South Pacific Gyre using trace-metal clean deckboard incubation experiments. Bacterioplankton community composition and function were resistant to ocean acidification alone, and combined with warming, at the fringe of the South Pacific Gyre. Subtle but significant responses of bacterioplankton community composition to ocean acidification were observed at the ultra- oligotrophic centre of the South Pacific Gyre. These results suggest that bacterioplankton community responses to ocean acidification may be modulated by nutrient regimes. Nonetheless, the findings of this study did not diverge substantially from the narrative that bacterioplankton communities are resistant to near-future acidification.

In the second study, the effect of ocean acidification on both prokaryotic and eukaryotic biofilm communities was investigated at the Shikine-Jima CO2 seep system in Japan. The composition of both prokaryotic and eukaryotic communities was profoundly affected by ocean acidification through early successional stages, though these responses were not associated with shifts in community diversity or evenness. Notably, the relative abundance of the nuisance algae Prymnesium sp. and Biddulphia biddulphiana were enhanced under high CO2 conditions. These findings suggest that benthic biofilm communities may be vulnerable to near-future ocean acidification, and that changes in biofilm community composition may contribute to the reorganisation of coastal ecosystem observed at CO2 seeps globally.

In its entirety, this thesis significantly contributes to our understanding of the spatial dynamics of marine microbial communities by revealing the highly deterministic nature of bacterioplankton community assembly in the coastal waters and central gyre of the South Pacific Ocean. Furthermore, the findings of this thesis highlight the dominance of stochastic processes in structuring marine protist communities across short spatial scales, which may contribute to challenges in correlating abiotic environmental variables with marine protist community composition through space. The resistance of bacterioplankton communities to ocean acidification at the fringe of the South Pacific Gyre, and subtle responses to ocean acidification at the ultra-oligotrophic centre of the South Pacific Gyre broadly support the notion that bacterioplankton communities are resilient to near-future ocean acidification. In contrast, the composition of both prokaryotic and eukaryotic biofilm communities was profoundly affected by ocean acidification, leading to the proliferation of harmful algae with potentially severe consequences for coastal marine environments.

Continue reading ‘Marine microbial community dynamics and responses to ocean acidification’

The influence of paleo-seawater chemistry on foraminifera trace element proxies and their application to deep-time paleo-reconstructions

The fossilized remains of the calcite shells of foraminifera comprise one of the most continuous and reliable records of the geologic evolution of climate and ocean chemistry. The trace elemental composition of foraminiferal shells has been shown to systematically respond to seawater properties, providing a way to reconstruct oceanic conditions throughout the last 170 million years. In particular, the boron/calcium ratio of foraminiferal calcite (B/Ca) is an emerging proxy for the seawater carbonate system, which plays a major role in regulating atmospheric CO2 and thus Earth’s climate. In planktic foraminifera, previous culture studies have shown that shell B/Ca increases with seawater pH, which is hypothesized to result from increased incorporation of borate ion (B(OH)4 -) at high pH; increasing pH increases the [B(OH)4 -] of seawater. However, further experiments showed that B/Ca responds to both pH and seawater dissolved inorganic carbon concentration (DIC), leading to the hypothesis that B/Ca is driven by the [B(OH)4 -/DIC] ratio of seawater. Because pH (and thus B(OH)4 -) can be determined via the δ11B composition of foraminiferal calcite, B/Ca therefore may provide an opportunity to determine seawater DIC in the geologic past.

Continue reading ‘The influence of paleo-seawater chemistry on foraminifera trace element proxies and their application to deep-time paleo-reconstructions’

Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, scientists are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR) remains entirely unknown. Here we describe, for the first time, 2 extreme GBR mangrove lagoons (Woody Isles and Howick Island), exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (7°C) conditions. Coral cover was typically low (0.5 m diameter), with net photosynthesis and calcification rates of 2 dominant coral species (Acropora millepora, Porites lutea) reduced (20-30%), and respiration enhanced (11-35%), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic ‘strategy’) and flexibility of Symbiodiniaceae taxa associations appear crucial in supporting coral capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress tolerance.

Continue reading ‘Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions’

Impacts of ocean acidification on intertidal benthic foraminiferal growth and calcification

Foraminifera are expected to be particularly susceptible to future changes in ocean carbonate chemistry as a function of increased atmospheric CO2. Studies in an experimental recirculating seawater system were performed with a dominant benthic foraminiferal species collected from intertidal mudflats. We investigated the experimental impacts of ocean acidification on survival, growth/calcification, morphology and the biometric features of a calcareous species Elphidium williamsoni. Foraminifera were exposed for 6 weeks to four different pH treatments that replicated future scenarios of a high CO2 atmosphere resulting in lower seawater pH. Results revealed that declining seawater pH caused a decline in foraminiferal survival rate and growth/calcification (mainly through test weight reduction). Scanning electron microscopy image analysis of live specimens at the end of the experimental period show changes in foraminiferal morphology with clear signs of corrosion and cracking on the test surface, septal bridges, sutures and feeding structures of specimens exposed to the lowest pH conditions. These findings suggest that the morphological changes observed in shell feeding structures may serve to alter: (1) foraminiferal feeding efficiency and their long-term ecological competitiveness, (2) the energy transferred within the benthic food web with a subsequent shift in benthic community structures and (3) carbon cycling and total CaCO3 production, both highly significant processes in coastal waters. These experimental results open-up the possibility of modelling future impacts of ocean acidification on both calcification and dissolution in benthic foraminifera within mid-latitude intertidal environments, with potential implications for understanding the changing marine carbon cycle.

Continue reading ‘Impacts of ocean acidification on intertidal benthic foraminiferal growth and calcification’


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