Plankton Evolution Research Articles

Effects of mixotroph evolution on trophic transfer

Abstract Plankton form the foundation of marine food webs, playing fundamental roles in mediating trophic transfer and the movement of organic matter. Increasing ocean temperatures have been documented to drive evolution of plankton, resulting in changes to metabolic traits that can affect trophic transfer. Despite this, there are few direct tests of the effects of such evolution on predator–prey interactions. Here, we used two thermally adapted strains of the marine mixotroph (organism that combines both heterotrophy and autotrophy to obtain energy) Ochromonas as prey and the generalist dinoflagellate predator Oxyrrhis marina to quantify how evolved traits of mixotrophs to hot and cold temperatures affects trophic transfer. Evolution to hot temperatures reduced the overall ingestion rates of both mixotroph strains, consequently weakening predator–prey interactions. We found variability in prey palatability and predator performance with prey thermal adaptation and between strains. Further, we quantified how ambient temperature affects predator grazing on mixotrophs thermally adapted to the same conditions. Increasing ambient temperatures led to increased ingestion rates but declines in clearance rates. Our results for individual, pairwise trophic interactions show how climate change can alter the dynamics of planktonic food webs with implications for carbon cycling in upper ocean ecosystems.

Temporal evolution of plankton and particles distribution across a mesoscale front during the spring bloom

AbstractThe effect of mesoscale features on the distribution of planktonic organisms are well documented. Yet, the interaction between these spatial features and the temporal scale, which can result in sudden increases of the planktonic biomass, is less known and not described at high resolution. A permanent mesoscale front in the Ligurian Sea (north‐western Mediterranean) was repeatedly sampled between January and June 2021 using a SeaExplorer glider equipped with an Underwater Vision Profiler 6 (UVP6), a versatile in situ imager. Both plankton and particle distributions were resolved throughout the spring bloom to assess whether the front was a location of increased zooplankton concentration and whether it constrained particle distribution. Over the 5 months, the glider performed more than 5000 dives and the UVP6 collected 1.1 million images. We focused our analysis on shallow (300 m) transects, which gave a horizontal resolution of 900 m. About 13,000 images of planktonic organisms were retained. Ordination methods applied to particles and plankton concentrations revealed strong temporal variations during the bloom, with a succession of various zooplankton communities. Changes in particle abundance and size could be explained by changes in the plankton community. The front had a strong influence on particle distribution, while the signal was not as clear for plankton, probably because of the relatively small number of imaged organisms. This work confirms the need to sample both plankton and particles at fine scale to understand their interactions, a task for which automated in situ imaging is particularly adapted.

Concepts towards Functional Eukaryotic Microbial Biogeography in the Ocean

High-throughput sequencing technologies have revolutionized microbial diversity studies, shedding light on the oceans’ plankton evolution, distribution, and biological activity. Whereas marine prokaryotes have been more extensively studied and specific methods developed, the research on microbial eukaryotes (protists) is falling behind, with major groups still largely unknown regarding their ecology and function. Because of numerous anthropogenic pressures, it is increasingly important to highlight the functional roles of protists in marine ecosystems. This review outlines the practices, challenges, and opportunities of high-throughput sequencing approaches (i.e., metabarcoding, metagenomics, and metatranscriptomics) to disentangle evolutionary, ecological, and functional aspects of protists in the ocean. These multidimensional approaches allow us to move from the classic picture of microbial biogeography towards functional microbial biogeography, explicitly highlighting the role of protists therein. We provide resources for functional classification and reflect on the current and future potential. We outline aspects of detecting and describing ecosystem changes at the species, population, and community levels, advancing methodological approaches for studying taxonomic diversity towards functional and evolutionary biodiversity concepts, seeking a more complete understanding and monitoring of ocean ecosystems.

Early Paleozoic Plankton Evolution in the Paleo-Asian Ocean: Insights from New and Reviewed Fossil Records from the Gorny Altai, West Siberia

The mountainous Gorny Altai in southern Siberia constitutes the western part of the Altai–Sayan Folded Belt in the northwestern part of the Central Asian orogenic belt, which contains precious records of the lost major Proterozoic–Paleozoic ocean called the Paleo-Asian Ocean (PAO). This paper briefly introduces the latest microfossil (radiolarian and conodont) information recovered from the Lower Paleozoic siliceous and carbonate sequences of the Gorny Altai. The fossils of planktonic biota inhabited in PAO range back to the early Cambrian, in which the world's oldest radiolarians are included. In addition, numerous well-preserved conodonts as well as graptolites were recovered from Cambrian, Ordovician, and Silurian strata in the Gorny Altai. These recorded the Early Paleozoic biodiversity and their secular change both in pelagic and continental margin settings within PAO.

Taxonomic review of living planktonic foraminifera

Abstract. Applications of fossil shells of planktonic foraminifera to decipher past environmental change and plankton evolution require a robust operational taxonomy. In this respect, extant planktonic foraminifera provide an opportunity for benchmarking the dominantly morphological species concepts and classification of the group by considering ecological, physiological and genetic characters. Although the basic framework of the taxonomy of extant planktonic foraminifera has been stable for half a century, many details have changed, not the least in light of genetic evidence. In this contribution, we review the current taxonomy of living planktonic foraminifera, presenting a comprehensive standard that emerged from the meetings and consultations of the SCOR/IGBP Working Group 138 “Planktonic foraminifera and ocean changes”. We present a comprehensive annotated list of 50 species and subspecies recognized among living planktonic foraminifera and evaluate their generic and suprageneric classification. As a result, we recommend replacing the commonly used names Globorotalia menardii by G. cultrata and Globorotalia theyeri by G. eastropacia, recognize Globorotaloides oveyi as a neglected but valid living species, and propose transferring the three extant species previously assigned to Tenuitella into a separate genus, Tenuitellita. We review the status of types and designate lectotypes for Globoturborotalita rubescens and Globigerinita uvula. We further provide an annotated list of synonyms and other names that have been applied previously to living planktonic foraminifera and outline the reasons for their exclusion. Finally, we provide recommendations on how the presented classification scheme should be used in operational taxonomy for the benefit of producing replicable and interoperable census counts.

The Eocene−Oligocene transition in Nanggulan, Java: lithostratigraphy, biostratigraphy and foraminiferal stable isotopes

The Nanggulan section in south central Java comprises open marine sediments and volcanic deposits of Eocene–Oligocene age that accumulated in a marginal basin within the young Sunda Arc complex. A new borehole captures the stratigraphy and showcases the exceptional preservation of calcareous microfossils across an apparently complete Eocene–Oligocene Transition (EOT), a time interval significant for the initiation of continental-scale glaciation on Antarctica. Low-resolution benthic and planktonic foraminifera oxygen and carbon stable isotopes ( δ 18 O and δ 13 C) record increasing δ 18 O and δ 13 C in the basal Oligocene, allowing correlation to global records. Isotopic values imply warm temperatures and relatively high nutrients along the SE Java margin. The Nanggulan EOT is a valuable archive for reconstructing ocean–climate behaviour and plankton evolution and extinction in the Indo-Pacific Warm Pool. The borehole also adds to understanding of the early stages of Sunda Arc volcanism. Supplementary material: Supplementary figures, tables and appendices are available at https://doi.org/10.6084/m9.figshare.c.5429453

SPEAD 1.0 – Simulating Plankton Evolution with Adaptive Dynamics in a two-trait continuous fitness landscape applied to the Sargasso Sea

Abstract. Diversity plays a key role in the adaptive capacity of marine ecosystems to environmental changes. However, modelling the adaptive dynamics of phytoplankton traits remains challenging due to the competitive exclusion of sub-optimal phenotypes and the complexity of evolutionary processes leading to optimal phenotypes. Trait diffusion (TD) is a recently developed approach to sustain diversity in plankton models by introducing mutations, therefore allowing the adaptive evolution of functional traits to occur at ecological timescales. In this study, we present a model called Simulating Plankton Evolution with Adaptive Dynamics (SPEAD) that resolves the eco-evolutionary processes of a multi-trait plankton community. The SPEAD model can be used to evaluate plankton adaptation to environmental changes at different timescales or address ecological issues affected by adaptive evolution. Phytoplankton phenotypes in SPEAD are characterized by two traits, the nitrogen half-saturation constant and optimal temperature, which can mutate at each generation using the TD mechanism. SPEAD does not resolve the different phenotypes as discrete entities, instead computing six aggregate properties: total phytoplankton biomass, the mean value of each trait, trait variances, and the inter-trait covariance of a single population in a continuous trait space. Therefore, SPEAD resolves the dynamics of the population's continuous trait distribution by solving its statistical moments, wherein the variances of trait values represent the diversity of ecotypes. The ecological model is coupled to a vertically resolved (1D) physical environment, and therefore the adaptive dynamics of the simulated phytoplankton population are driven by seasonal variations in vertical mixing, nutrient concentration, water temperature, and solar irradiance. The simulated bulk properties are validated by observations from Bermuda Atlantic Time-series Studies (BATS) in the Sargasso Sea. We find that moderate mutation rates sustain trait diversity at decadal timescales and soften the almost total inter-trait correlation induced by the environment alone, without reducing the annual primary production or promoting permanently maladapted phenotypes, as occur with high mutation rates. As a way to evaluate the performance of the continuous trait approximation, we also compare the solutions of SPEAD to the solutions of a classical discrete entities approach, with both approaches including TD as a mechanism to sustain trait variance. We only find minor discrepancies between the continuous model SPEAD and the discrete model, with the computational cost of SPEAD being lower by 2 orders of magnitude. Therefore, SPEAD should be an ideal eco-evolutionary plankton model to be coupled to a general circulation model (GCM) of the global ocean.

Temperature controls carbon cycling and biological evolution in the ocean twilight zone.

Theory suggests that the ocean's biological carbon pump, the process by which organic matter is produced at the surface and transferred to the deep ocean, is sensitive to temperature because temperature controls photosynthesis and respiration rates. We applied a combined data-modeling approach to investigate carbon and nutrient recycling rates across the world ocean over the past 15 million years of global cooling. We found that the efficiency of the biological carbon pump increased with ocean cooling as the result of a temperature-dependent reduction in the rate of remineralization (degradation) of sinking organic matter. Increased food delivery at depth prompted the development of new deep-water niches, triggering deep plankton evolution and the expansion of the mesopelagic "twilight zone" ecosystem.

Getting more pumped

Carbon Cycle It is thought that the ocean's biological carbon pump, the process that transfers organic matter from the surface to the deep ocean, should be sensitive to climate change because temperature controls photosynthesis and respiration rates. Boscolo-Galazzo et al. show that the efficiency of the biological carbon pump increased over the past 15 million years as the oceans cooled because of a reduction in the rate of the breakdown of sinking organic matter (see the Perspective by Bopp). The resulting redistribution of nutrients at depth could have affected plankton evolution and expanded the mesopelagic “twilight zone” ecosystem. Science , this issue p. [1148][1]; see also p. [1099][2] [1]: /lookup/doi/10.1126/science.abb6643 [2]: /lookup/doi/10.1126/science.abg5994

Evolution of mid-Cretaceous radiolarians in response to oceanic anoxic events in the eastern Tethys (southern Tibet, China)

Oceanic anoxic events (OAEs) are well-known from their widespread black shale and carbon isotopic excursions in the (western and eastern) Tethys and Atlantic Ocean. However, the weakest link in the studies of the OAEs is biological evolution. Sedimentological, biostratigraphical and geochemical data from Albian to Coniacian strata in southern Tibet recorded turnover events and fluctuations in diversity of the radiolarian fauna within the eastern Tethys during OAE 1d and OAE 2. Abundant radiolarian fossils were obtained from the Gyabula Formation, with 93 species from 43 genera identified and assigned to the mid to Upper Cretaceous Acaeniotyle umbilicata, Archaeospongoprunum tehamaensis, Crucella cachensis, Alievium superbum, and Dictyomitra formosa zones. The association of carbon isotopic excursions, black shale and radiolarian turnover indicates extensive changes in the ocean-climate system. Nutrients are made increasing available to the marine plankton through submarine volcanic activity and rising sea-level, which were a likely cause of radiolarian turnover at/or near the OAEs. Active submarine tectonism-volcanism leads to the expansion of the hypoxic zone and may cause many deeper dwelling forms to become extinct whereas most of the shallower dwelling radiolarians survive. Radiolarian evolution thus provides a useful record with which to seek understand relationships between climate, paleoceanographic processes and plankton evolution.

A Global Perspective on Iron and Plankton Through the Tara Oceans Lens

AbstractThe Tara Oceans program has delivered major advances in our knowledge of ocean plankton diversity and complexity, shedding light on key interactions that explain their success on a planetary scale. In this issue, Caputi et al. (2019, https://doi.org/10.1029/2018GB006022) further contribute to this knowledge through combining comprehensive bio‐oceanographic genomic and transcriptomic Tara Oceans data sets with iron distributions derived from two global‐scale biogeochemical models. Their findings reveal the prevalence of iron as a limiting nutrient in pelagic ecosystems at both local and global scales, exerting a considerable force that drives plankton evolution and shapes community structure. Integration of omics data (i.e., genomics, transcriptomics, proteomics, and metabolomics) with oceanographic properties and biogeochemical models will transform our view of the ocean ecosystem and its role on a changing planet.

Temperature is a poor proxy for synergistic climate forcing of plankton evolution.

Changes in biodiversity at all levels from molecules to ecosystems are often linked to climate change, which is widely represented univariately by temperature. A global environmental driving mechanism of biodiversity dynamics is thus implied by the strong correlation between temperature proxies and diversity patterns in a wide variety of fauna and flora. Yet climate consists of many interacting variables. Species probably respond to the entire climate system as opposed to its individual facets. Here, we examine ecological and morphological traits of 12 633 individuals of two species of planktonic foraminifera with similar ecologies but contrasting evolutionary outcomes. Our results show that morphological and ecological changes are correlated to the interactions between multiple environmental factors. Models including interactions between climate variables explain at least twice as much variation in size, shape and abundance changes as models assuming that climate parameters operate independently. No dominant climatic driver can be identified: temperature alone explains remarkably little variation through our highly resolved temporal sequences, implying that a multivariate approach is required to understand evolutionary response to abiotic forcing. Our results caution against the use of a ‘silver bullet’ environmental parameter to represent global climate while studying evolutionary responses to abiotic change, and show that more comprehensive reconstruction of palaeobiological dynamics requires multiple biotic and abiotic dimensions.

Ecology, Virulence, and Phylogeny of Blastulidium paedophthorum, a Widespread Brood Parasite of Daphnia spp.

Parasitism is now recognized as a major factor impacting the ecology and evolution of plankton, including Daphnia. Parasites that attack the developing embryos of daphniids, known as brood parasites, were first described in the early 1900s but have received relatively little study. Here, we link previous morphological descriptions of the oomycete brood parasite Blastulidium paedophthorum with information on its phylogenetic placement, ecology, and virulence. Based on the morphology and phylogenetic relationship with other members of the Leptomitales, we show that a brood parasite observed in daphniids in the Midwestern United States is B. paedophthorum. We used morphology, DNA sequences, and laboratory infection experiments to show that B. paedophthorum is a multihost parasite that can be transmitted between species and genera. A field survey of six hosts in 15 lakes revealed that B. paedophthorum is common in all six host taxa (present on 38.3% of our host species-lake-sampling date combinations; the maximum infection prevalences were 8.7% of the population and 20% of the asexual adult female population). Although B. paedophthorum was observed in all 15 lakes, presence and infection prevalence varied among lakes. Infection with B. paedophthorum did not reduce host life span but significantly impacted host fecundity. Theory predicts that parasites that affect host fecundity without affecting host life span should have the strongest impact on host population dynamics. Based on its virulence and commonness in natural populations and on the central role of daphniids in freshwater food webs, we predict that B. paedophthorum will influence daphniid ecology and evolution, as well as the larger food web.

The Characterization of RNA Viruses in Tropical Seawater Using Targeted PCR and Metagenomics

ABSTRACTViruses have a profound influence on the ecology and evolution of plankton, but our understanding of the composition of the aquatic viral communities is still rudimentary. This is especially true of those viruses having RNA genomes. The limited data that have been published suggest that the RNA virioplankton is dominated by viruses with positive-sense, single-stranded (+ss) genomes that have features in common with those of eukaryote-infecting viruses in the order Picornavirales (picornavirads). In this study, we investigated the diversity of the RNA virus assemblages in tropical coastal seawater samples using targeted PCR and metagenomics. Amplification of RNA-dependent RNA polymerase (RdRp) genes from fractions of a buoyant density gradient suggested that the distribution of two major subclades of the marine picornavirads was largely congruent with the distribution of total virus-like RNA, a finding consistent with their proposed dominance. Analyses of the RdRp sequences in the library revealed the presence of many diverse phylotypes, most of which were related only distantly to those of cultivated viruses. Phylogenetic analysis suggests that there were hundreds of unique picornavirad-like phylotypes in one 35-liter sample that differed from one another by at least as much as the differences among currently recognized species. Assembly of the sequences in the metagenome resulted in the reconstruction of six essentially complete viral genomes that had features similar to viruses in the families Bacillarna-, Dicistro-, and Marnaviridae. Comparison of the tropical seawater metagenomes with those from other habitats suggests that +ssRNA viruses are generally the most common types of RNA viruses in aquatic environments, but biases in library preparation remain a possible explanation for this observation.

Evidence for isolated evolution of deep-sea ciliate communities through geological separation and environmental selection

BackgroundDeep hypersaline anoxic basins (DHABs) are isolated habitats at the bottom of the eastern Mediterranean Sea, which originate from the ancient dissolution of Messinian evaporites. The different basins have recruited their original biota from the same source, but their geological evolution eventually constituted sharp environmental barriers, restricting genetic exchange between the individual basins. Therefore, DHABs are unique model systems to assess the effect of geological events and environmental conditions on the evolution and diversification of protistan plankton. Here, we examine evidence for isolated evolution of unicellular eukaryote protistan plankton communities driven by geological separation and environmental selection. We specifically focused on ciliated protists as a major component of protistan DHAB plankton by pyrosequencing the hypervariable V4 fragment of the small subunit ribosomal RNA. Geospatial distributions and responses of marine ciliates to differential hydrochemistries suggest strong physical and chemical barriers to dispersal that influence the evolution of this plankton group.ResultsCiliate communities in the brines of four investigated DHABs are distinctively different from ciliate communities in the interfaces (haloclines) immediately above the brines. While the interface ciliate communities from different sites are relatively similar to each other, the brine ciliate communities are significantly different between sites. We found no distance-decay relationship, and canonical correspondence analyses identified oxygen and sodium as most important hydrochemical parameters explaining the partitioning of diversity between interface and brine ciliate communities. However, none of the analyzed hydrochemical parameters explained the significant differences between brine ciliate communities in different basins.ConclusionsOur data indicate a frequent genetic exchange in the deep-sea water above the brines. The “isolated island character” of the different brines, that resulted from geological events and contemporary environmental conditions, create selective pressures driving evolutionary processes, and with time, lead to speciation and shape protistan community composition. We conclude that community assembly in DHABs is a mixture of isolated evolution (as evidenced by small changes in V4 primary structure in some taxa) and species sorting (as indicated by the regional absence/presence of individual taxon groups on high levels in taxonomic hierarchy).

Ecological and evolutionary response of Tethyan planktonic foraminifera to the middle Eocene climatic optimum (MECO) from the Alano section (NE Italy)

The enigmatic middle Eocene climatic optimum (MECO) is a transient (∼ 500 kyr) warming event that significantly interrupted at ∼ 40 Ma the long-term cooling through the middle and late Eocene, eventually resulting in establishment of permanent Antarctic ice-sheet . This event is still poorly known and data on the biotic response are so far scarce. Here we present a detailed planktonic foraminiferal analysis of the MECO interval from a marginal basin of the central-western Tethys (Alano section, northeastern Italy). The expanded and continuous Alano section provides an excellent record of this event and offers an appealing opportunity to better understand the role of climate upon calcareous plankton evolution. A sapropel-like interval, characterized by excursions in both the carbon and oxygen bulk-carbonate isotope records, represents the lithological expression of the post-MECO event in the study area and follows the δ 18O negative shift, interpreted as representing the MECO warming. High-resolution quantitative analysis performed on both > 38 μm and > 63 μm fractions reveals pronounced and complex changes in planktonic foraminiferal assemblages indicating a strong environmental perturbation that parallels the variations of the stable isotope curves corresponding to the MECO and post-MECO intervals. These changes consist primarily in a marked increase in abundance of the relatively eutrophic subbotinids and of the small, low-oxygen tolerant Streptochilus, Chiloguembelina and Pseudohastigerina. At the same time, the arrival of the abundant opportunist eutrophic Jenkinsina and Pseudoglobigerinella bolivariana, typical species of very high-productivity areas, also occurs. The pronounced shift from oligotrophic to more eutrophic, opportunist, low-oxygen tolerant planktonic foraminiferal assemblages suggests increased nutrient input and surface ocean productivity in response to the environmental perturbation associated with the MECO. Particularly critical environmental conditions have been reached during the deposition of the sapropel-like beds as testified by the presence of common giant and/or odd morphotypes. This is interpreted as evidence of transient alteration in the ocean chemistry. The enhanced surface water productivity inferred by planktonic foraminiferal assemblages at the onset of the event should have resulted in heavier δ 13C values. The recorded lightening of the carbon stable isotope preceding the maximum warmth therefore represents a robust indication that it derives principally by a conspicuous increase of pCO 2. The increased productivity of surface waters, also supported by geochemical data, may have acted as mechanism for pCO 2 reduction and returned the climate system to the general Eocene cooling trend. The oxygen-depleted deep waters and the organic carbon burial following the peak of the MECO event represent the local response to the MECO warming and suggest that high sequestration of organic matter, if representing a widespread response to this event, might have contributed to the decrease of pCO 2 as well. Though the true mechanisms are still obscure, several lines of evidence indicate a potential pressure on planktonic foraminiferal evolution during the MECO event including permanent changes besides transient and ecologically controlled variations.

Morphological and Physiological Characteristics of Gephyrocapsa oceanica var. typica Kamptner 1943 in Culture Experiments: Evidence for Genotypic Variability

In order to test whether morphological variations within Gephyrocapsa oceanica var. typica Kamptner 1943 reflect genotypic variation or phenotypic responses to environmental conditions, culture experiments of six strains of G. oceanica collected at different locations in the North Atlantic and Mediterranean have been carried out under different temperature and nutrients conditions. All morphological, and physiological data suggest the presence of two species or subspecies within G. oceanica var typica that correspond morphologically to Gephyrocapsa "Larger" and possibly to Gephyrocapsa "Equatorial" as previously defined from Holocene sediments. Given the importance of Gephyrocapsa species for the carbon cycle in the past, genetic studies on this group are of major interest to the understanding of past climate change and plankton evolution.

Spatial heterogeneity and genetic variation in the copepod Neocalanus cristatus along two transects in the North Pacific sampled by the Continuous Plankton Recorder

We present a macrogeographic study of spatial heterogeneity in an important subarctic Pacific copepod and describe the first genetic analysis of population structure using Continuous Plankton Recorder (CPR) samples. Samples of Neocalanus cristatus were collected at a constant depth of similar to 7 m from two CPR tow-routes, (i) an east-west similar to 6500-km transect from Vancouver Island, Canada to Hokkaido Island, Japan, and (ii) a north-south transect of similar to 2250 km from Anchorage, Alaska to Tacoma, Washington. Analysis of these samples revealed three features of the biology of N. cristatus. First, N. cristatus undergoes small-scale diel vertical migration that is larger among stages CV- adult (3-6 times more abundant at 7 m at night), than stages CI-CIV (only 2-4 times higher at night). Secondly, while there were no regions where N. cristatus did not appear, each transect sampled a few large-scale macrogeographic patches. Thirdly, an analysis of molecular variation, using a partial sequence of the N. cristatus cytochrome oxidase I gene, revealed that 7.3% (P < 0.0001) of the total genetic variation among N. cristatus sampled from macrogeographic patches by the CPR could be explained by spatial heterogeneity. We suggest that spatial heterogeneity at macrogeographic scales may be important in plankton evolution.

A review of calcareous nannofossil astrobiochronology encompassing the past 25 million years

This paper presents a review of (astrobiochronological) calibration of Recent to late Oligocene calcareous nannofossil datum events. Biohorizons included in the paper are those of the widely used “standard” nannofossil zonations of Martini, E. [1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. In: Farinacci, A. (Ed.), Proceedings of the Second International Conference on Planktonic Microfossils Roma. Rome, Ed. Tecnosci. vol. 2, pp. 739–785], and Okada, H., and Bukry, D. [1980. Supplementary Modification and Introduction of Code Numbers to the Low-Latitude Coccolith Biostratigraphy Zonation (Bukry, 1973, 1975). Marine Micropaleontology 51, 321–325], as well as supplementary biohorizons proposed in the literature. The biohorizons have been selected on the basis of the unambiguous taxonomy of the index taxa and their biostratigraphic usefulness. We emphasise the application of rigorous methodology in nannofossil studies which permits an evaluation of biohorizons in terms of reliability, and calibrates their potential correlatability. Astrochronological age estimates rely on the Geologic timescale developed by the ICS in 2004, with some new calibrations included. We provide an overview of the relative position of biohorizons versus the astronomically calibrated ages of magnetic reversals and reference isotope stratigraphies. Surprisingly, there are still few high-resolution quantitative biostratigraphic studies of astrochronologically tuned sections in spite of the central role of such studies in addressing fundamental problems such as the tempo and mode of plankton evolution.

Abiotic forcing of plankton evolution in the Cenozoic.

We characterize the evolutionary radiation of planktic foraminifera by the test size distributions of entire assemblages in more than 500 Cenozoic marine sediment samples, including more than 1 million tests. Calibration of Holocene size patterns with environmental parameters and comparisons with Cenozoic paleoproxy data show a consistently positive correlation between test size and surface-water stratification intensity. We infer that the observed macroevolutionary increase in test size of planktic foraminifera through the Cenozoic was an adaptive response to intensifying surface-water stratification in low latitudes, which was driven by polar cooling.