Recent research led by a team from MARUM at the University of Bremen focuses on the sensitivity of high-latitude phytoplankton to environmental shifts during the PETM. High-latitude marine ecosystems are particularly important yet historically underrepresented in paleoceanographic research, despite their ecological sensitivity and biogeographic distinctiveness. The researchers utilized sediment cores retrieved from the Campbell Plateau in the Southern Ocean during International Ocean Discovery Program Expedition 378, facilitating a novel examination of calcareous nannoplankton assemblages preserved in deep-sea deposits. These microscopic algae biomineralize calcium carbonate shells, leaving detailed fossil records that chronicle shifts in community composition and abundance across climatic perturbations.

Calcareous nannoplankton species exhibit distinct ecological preferences, with some taxa adapted to warmer, oligotrophic surface waters, while others favor cooler, nutrient-rich conditions. By quantifying fossil nannoplankton assemblages preceding and during the PETM, the researchers reconstructed community adaptations to ocean warming and acidification. Contrary to expectations of dramatic PETM-driven turnover, the study reveals a more nuanced response, marked by prior destabilization of communities approximately 200,000 years before the PETM onset. This earlier warming episode appears to have primed phytoplankton assemblages for subsequent environmental stressors, suggesting that background climatic variability plays a critical yet often overlooked role in mediating ecosystem resilience.

Dr. Heather L. Jones, first author of the study, emphasizes the importance of incorporating pre-event intervals when assessing paleobiological responses to climatic crises. The findings highlight that even modest, incremental environmental changes can exert outsized ecological impacts, particularly in sensitive polar marine environments. The research calls for a broader temporal framework in paleoecological investigations to capture the cumulative effects of successive and overlapping stress events on marine communities, which may have direct relevance to forecasting ongoing planktonic responses under progressive anthropogenic climate change.

The study’s identification of this previously undocumented pre-PETM warming event invites further exploration within the extensive global repository of legacy deep-sea sediment cores. The Bremen Core Repository (BCR), housed within MARUM, offers an invaluable archive enabling comparative analyses to determine the spatiotemporal extent and ecological ramifications of this early phase climatic disturbance across multiple ocean basins. Such endeavors will refine paleoceanographic models, adding depth and resolution to our understanding of ecosystem dynamics at critical transitional intervals in Earth’s climate history.

These findings underscore the intricacy of biotic responses to rapid environmental change and emphasize the utility of calcareous nannoplankton as sensitive bioindicators for reconstructing past ocean conditions. The MARUM team’s work contributes significantly to the broader Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface,” which seeks to unravel the complex interactions at the junction of geosphere and biosphere. Investigating how fundamental productivity drivers react to stressors enhances predictive capacity for future ocean health and carbon cycle feedbacks under continued warming and acidification.

The revelation of the pre-PETM event also prompts reconsideration of vulnerability thresholds in marine ecosystems. It appears that ecosystems may exhibit cumulative stress effects, where prior exposure to moderate environmental fluctuations modulates subsequent ecological trajectories. This has significant implications for current climate change impacts in regional high-latitude seas, where warming is occurring at an accelerated pace, and ecosystems may already be operating near critical tipping points.

Furthermore, the study illustrates the value of integrating fossil evidence with present-day ecological theory to develop holistic understandings of how marine life adapts or succumbs to rapid environmental shifts. The documentation of such ecological preliminary changes offers a magnified lens for interpreting contemporary observations, where rapid yet subtle shifts in plankton composition can have cascading effects through food webs and global biogeochemical cycles.

By providing a temporal context extending well before the PETM interval, the research challenges the notion of abrupt biotic change confined narrowly to peak warming periods. Instead, a protracted prelude of environmental destabilization may underlie the most severe ecosystem transformations, emphasizing the need for long-term, multidimensional perspectives in climate impact assessments.

As the ocean continues to absorb anthropogenic CO2, the structured analysis of fossil plankton communities holds promise for deciphering the evolutionary and ecological mechanisms that will govern the resilience or decline of marine primary producers. The MARUM team’s pioneering insights form a cornerstone for future high-resolution paleoecological studies, bridging past and present in the quest to understand climate-driven ecosystem shifts in a warming world.

Subject of Research:
High-latitude phytoplankton community responses to Paleocene-Eocene Thermal Maximum warming and precursor climatic disturbances.

Article Title:
Palaeoecological change preceded the Palaeocene-Eocene Thermal Maximum by 200 kyr in the high latitude south-west Pacific Ocean

News Publication Date:
12-Sep-2025

Web References:
http://dx.doi.org/10.1038/s43247-025-02749-5

Image Credits:
MARUM – Center for Marine Environmental Sciences, University of Bremen; M. Toyos Simón

Keywords:
Paleocene-Eocene Thermal Maximum, ocean acidification, calcareous nannoplankton, high-latitude phytoplankton, paleoceanography, climate warming, deep-sea sediment cores, Southern Ocean, carbon cycle, marine ecosystems, International Ocean Discovery Program, paleoecology

Ocean acidification refers to the ongoing decrease in ocean pH levels due to the absorption of excess CO2 emitted by human activities such as fossil fuel combustion and deforestation. Presently, the average pH of the world’s oceans hovers around 8.1, which is slightly alkaline and conducive to the maintenance of various marine life forms. However, projections indicate that if current emission trends continue unchecked, by the year 2300, the ocean pH could drop to approximately 7.3. This seemingly small numerical change reflects an almost tenfold increase in acidity, creating a hostile chemical environment for calcified and mineralized structures in marine organisms.

The team of researchers, led by Maximilian Baum and senior author Professor Sebastian Fraune from Heinrich Heine University Düsseldorf (HHU), sought to investigate how this fundamental shift in ocean chemistry affects shark tooth morphology, focusing on the Blacktip reef shark (Carcharhinus melanopterus). Utilizing over 600 teeth discarded from sharks housed at Sealife Oberhausen aquarium, the investigators selected 16 pristine and undamaged specimens for a controlled acidification experiment. The shark teeth were submerged for eight weeks in seawater tanks with two different pH settings: 8.1 to simulate current ocean conditions and 7.3 to represent future acidified oceans.

Upon completion of the incubation period, the research team employed microscopic and imaging analyses to assess tooth surface morphology and structural integrity. The results were stark and revealing. Teeth subjected to acidified conditions exhibited pronounced surface degradation manifesting as cracks, holes, and erosion predominantly on the roots where mineralization is crucial. Moreover, these teeth displayed significant alterations in their circumference, appearing larger under two-dimensional imaging due to a roughened and irregular surface texture. These morphological changes imply a weakening of the tooth’s mechanical properties, thereby compromising their ability to withstand the physical stress of capturing prey.

While shark teeth are composed of highly mineralized phosphates—components that generally confer hardness and durability—the study demonstrates that even this biochemical robustness offers limited protection against the corrosive effects of increased acidity. Fraune emphasized that shark teeth, though ingeniously designed biological weapons optimized for slicing through flesh, are far less adapted to endure prolonged exposure to harsh chemical environments. The findings suggest that as the oceans become more acidic, shark teeth may degrade faster, potentially leading to higher incidences of tooth breakage or loss.

This alteration in tooth morphology and functional resilience may have profound ecological consequences. Sharks occupy apex predator roles in marine ecosystems, shaping community structures and maintaining the balance of prey species. A reduction in their efficiency at hunting due to compromised teeth could trigger cascade effects throughout the marine food web. Moreover, Blacktip reef sharks frequently swim with their mouths partly open to facilitate respiration, which leads to constant exposure of their dental surfaces to seawater. This behavior potentially increases their susceptibility to acid damage, making the issue even more pressing.

The study notably focused on non-living mineralized tissue since it used discarded shark teeth detached from the animal. Consequently, the natural reparative processes that living sharks might employ, such as rapid tooth regeneration or remineralization, were not accounted for. The researchers acknowledge that living sharks may compensate for increased dental damage by faster tooth replacement cycles; however, this adaptation might incur higher energetic costs in acidified waters, potentially affecting overall health and fitness. Baum commented that slight reductions in seawater pH, even less severe than projected for 2300, could disproportionately affect species with slower tooth replacement rates or impose cumulative damage over longer periods.

Beyond the direct implications for sharks, this research sheds light on the broader vulnerabilities of marine calcifiers facing environmental change. Much of the attention on ocean acidification has traditionally centered on shelled invertebrates and corals, whose calcium carbonate-based structures are known to be sensitive to acidity. This study extends concern to phosphate-based mineralized tissues, revealing a more widespread potential impact. The microscopic surface irregularities and corrosion observed could compromise functional properties vital for survival, such as cutting efficacy and resistance to mechanical stress, turning nature’s most efficient predatory tools into liabilities.

Looking forward, the authors advocate for expanded research to include live specimens and more nuanced biochemical and biomechanical analyses. Understanding how living sharks manage and potentially mitigate dental corrosion in acidified oceans, including changes in tooth chemistry, regeneration rates, and associated energetic costs, remains a critical frontier. Such insights will be crucial to assess whether sharks can adapt to shifting environmental baselines or face population declines driven by deteriorating foraging ability.

In the context of global climate change, this research offers a sobering reminder that the impact extends well beyond rising temperatures, encompassing chemical alterations of the ocean’s fundamental properties with cascading effects on ecosystems. Magnified by the centrality of sharks in marine food chains, any threat to their survival tools—teeth—is tantamount to destabilizing entire oceanic communities. The degradation of shark teeth under simulated future ocean acidification scenarios underscores the necessity for urgent mitigation of CO2 emissions to preserve marine biodiversity and ecosystem function.

Ultimately, maintaining oceanic pH values near current levels is vital to safeguard the physical integrity of predatory tools such as shark teeth, pivotal for feeding and survival. The observed chemical corrosion and structural degradation, even at the microscopic level, could precipitate profound changes in predator-prey dynamics, with unknown but potentially severe consequences. As Baum eloquently concluded, this study exemplifies how climate change permeates through every link in ecological networks, threatening species reliant on biomechanical adaptations finely tuned over millions of years. It is a compelling call to action to address environmental changes before the sharpness of the ocean’s top predators is irreversibly dulled.

Subject of Research: Animals
Article Title: Simulated ocean acidification affects shark tooth morphology
News Publication Date: 27-Aug-2025
Web References: http://dx.doi.org/10.3389/fmars.2025.1597592
Image Credits: Max Baum
Keywords: Ocean acidification, Shark teeth, Blacktip reef shark, Tooth morphology, Ocean pH, Climate change impact, Marine predators, Phosphate mineralization, Structural degradation, Marine ecosystems