Jeremy R. Young

Species‐specific responses of calcifying algae to changing seawater carbonate chemistry

Uptake of half of the fossil fuel CO2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO2-related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO2 levels (∼360 ppm CO2). With particulate organic carbon (POC) remaining constant over the range of CO2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO2, POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.

Pan genome of the phytoplankton Emiliania underpins its global distribution

Coccolithophores have influenced the global climate for over 200 million years. These marine phytoplankton can account for 20 per cent of total carbon fixation in some systems. They form blooms that can occupy hundreds of thousands of square kilometres and are distinguished by their elegantly sculpted calcium carbonate exoskeletons (coccoliths), rendering them visible from space. Although coccolithophores export carbon in the form of organic matter and calcite to the sea floor, they also release CO2 in the calcification process. Hence, they have a complex influence on the carbon cycle, driving either CO2 production or uptake, sequestration and export to the deep ocean. Here we report the first haptophyte reference genome, from the coccolithophore Emiliania huxleyi strain CCMP1516, and sequences from 13 additional isolates. Our analyses reveal a pan genome (core genes plus genes distributed variably between strains) probably supported by an atypical complement of repetitive sequence in the genome. Comparisons across strains demonstrate that E. huxleyi, which has long been considered a single species, harbours extensive genome variability reflected in different metabolic repertoires. Genome variability within this species complex seems to underpin its capacity both to thrive in habitats ranging from the equator to the subarctic and to form large-scale episodic blooms under a wide variety of environmental conditions.

Pseudo-cryptic speciation in coccolithophores.

Coccolithophores are a group of calcifying unicellular algae that constitute a major fraction of oceanic primary productivity, play an important role in the global carbon cycle, and are key biostratigraphic marker fossils. Their taxonomy is primarily based on the morphology of the minute calcite plates, or coccoliths, covering the cell. These are diverse and include widespread fine scale variation, of which the biological/taxonomic significance is unknown. Do they represent phenotypic plasticity, genetic polymorphisms, or species-specific characters? Our research on five commonly occurring coccolithophores supports the hypothesis that such variation represents pseudocryptic speciation events, occurring between 0.3 and 12.9 million years ago from a molecular clock estimation. This finding suggests strong stabilizing selection acting on coccolithophorid phenotypes. Our results also provide strong support for the use of fine scale morphological characters of coccoliths in the fossil record to improve biostratigraphic resolution and paleoceanographic data retrieval.

The Cretaceous-Tertiary biotic transition

Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversity. Both approaches have inherent flaws, and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachio-pods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the K-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.

Calculation of coccolith volume and its use in calibration of carbonate flux estimates

Abstract Estimates of coccolith volume help in determining total coccolith carbonate fluxes and the relative contributions of various coccolith species. It is argued here that the best approach to deriving such coccolith volumes is to calculate shape factors, k s , for each species separately based on reconstruction of cross profiles and to determine mean sizes from measurement of size variation in the sample of interest. Then volume= k s l 3 . Values of k s are given for the most important extant coccolith species based on reconstruction of cross-sections, followed by calculation of a volume of rotation using an iterative routine implemented in an image analysis package. Substantial errors, ca. 50%, are inevitable in such calculations, but the resultant data are nonetheless of value. Coccolith assemblages from North Atlantic (47°N 20°W) JGOFS 1989 sediment trap samples are analysed as a test case; calculated coccolith PIC fluxes constitute 30–80% of the chemically determined total PIC fluxes.

Global correlation of Cenomanian (Upper Cretaceous) sequences: Evidence for Milankovitch control on sea level

We have investigated the sequence stratigraphy of two widely separated marine Cenomanian successions in southeast India and northwest Europe, and used high-resolution ammonite biostratigraphy to demonstrate that sea-level changes are globally synchronous and therefore must be eustatically controlled. Sequence-scale sea-level changes in the Cenomanian were driven by the long eccentricity cycle (400 k.y.) in the Milankovitch band. We hypothesize that, during pre-Quaternary time, the third-order sequences of Vail and Haq are essentially a sediment response to sea-level changes driven by the 400 k.y. cycle. Construction of a relative sea-level curve for the marginal marine succession in India demonstrates that the short-term sea-level changes are rapid (10-100 m/m.y.) and have a magnitude of 2-20 m. Glacioeustasy is a possible but unproven driving mechanism.

Genotypic variation in the coccolithophorid speciesEmiliania huxleyi

Abstract Culture strains ofEmiliania huxleyi have been used for a case-study of intra-specific variation in coccolith morphology. Five types of variation are recognised: degree of completion, degree of calcification, size, malformation and genotypic variation. The genotypic variation is described in detail. It also effects cell physiology and immunological cross-reactions. The different criteria consistently divide the strains into two groups termed here type A and type B. The variation has been stable for several years and under widely varying growth conditions. Biometric analysis is used to investigate the nature of the variation in coccolith morphology. Review of data on oceanicE. huxleyi suggests that the strain types represent two of a limited number of varieties within the global population.

Life-cycle associations involving pairs of holococcolithophorid species: intraspecific variation or cryptic speciation?

New holococcolith-heterococcolith life-cycle associations are documented based on observations of combination coccospheres. Daktylethra pirus is shown to be a life-cycle phase of Syracosphaera pulchra and Syracolithus quadriperforatus a life-cycle phase of Calcidiscus leptoporus. In addition, new observations from cultures confirm the life-cycle associations of Crystallolithus braarudii with Coccolithus pelagicus and of Zygosphaera hellenica with Coronosphaera mediterranea. In all four cases previous work has shown that the heterococcolithophorid species is associated with another holococcolithophorid. Two other examples of a heterococcolithophorid being associated with two holococcolithophorids have previously been identified, so this seems to be a common phenomenon. The six examples are reviewed to determine whether a single underlying mechanism is likely to be responsible for all cases. It is concluded that there is no single mechanism but rather that the six examples fall into three categories: (a) in two cases the holococcolith types are probably simply ecophenotypic morphotypes; (b) in two other cases the holococcolith types are discrete and are paralleled by morphometric differences in the heterococcolith types; (c) in the final two cases the holococcolith types are discrete but are not paralleled by any obvious morphological variation in the heterococcolith morphology. We infer that cryptic speciation may be widespread in heterococcolithophorid phases and that study of holococcolithophorid phases can provide key data to elucidate this phenomenon.

The origin of Heinrich layers: evidence from H2 for European precursor events

Recent well-dated isotopic (Sr–Nd) fingerprinting of Heinrich layer ice-rafted detritus (IRD) on the European margin indicates supply from European ice sheets as precursors to Laurentide Ice Sheet (LIS) supply [F.E. Grousset et al., Geology 28 (2000) 123–126, H. Snoeckx et al., Mar. Geol. 158 (1999) 197–208]. These precursorevents lead LIS input by up to 1.5 ka [F.E. Grousset et al., Geology 28 (2000) 123–126] and have been interpreted to indicate LIS collapse during Heinrichevents stimulated by events originating on the European side of the Atlantic [F.E. Grousset et al., Geology 28 (2000) 123–126]. Such phasing of IRD supply from different sources within Heinrich layers therefore has implications for the origin and mechanics of Heinrichevents. We present evidence here that the IRD comprising Heinrich layer 2 (H2; ∼20–21 14C ka BP) on the European continental margin contains detrital Campanian Upper Chalk deriving from bedrock sources eroded on the Celtic shelf by the British Ice Sheet (BIS) in addition to lithic material sourced from the LIS. High-resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of LIS-sourced detritus. The specificity of the chalk fingerprint to the BIS enables a 700–1000 yr lag between the BIS and LIS events to be identified. This phasing indicates a more rapid response of the outlet lobes draining the smaller BIS than those draining the LIS and implicates external climatic forcing of Heinrichevents. It is unlikely that this precursorevent represents IRD event 18, the recently identified 1–2 ka IRD cycle event which immediately precedes H2, because the lag between precursor and main event is here less than 1.5 ka and because such pervasive periodicity is not apparent in European continental margin IRD records. The later synchroneity between the BIS and LIS input in H2 identifies glacio–eustatic sea-level rise associated with LIS discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during Heinrichevents.

Calibration of the random settling technique for calculation of absolute abundances of calcareous nannoplankton

We describe a device for random settling preparation of calcareous nannoplankton (coccolith) samples. The device allows easy draining and cleaning, as well as sedimentation at different heights of the water column through the use of different slide carriers. Reproducibility and accuracy of the device has been tested with standardised microbeads as spiking material. In applying counting techniques with different equations for determining absolute abundances, we discovered major anomalies in the calculated results that we interpreted as being due to the effect of convection currents within the device and the usage of elevated cover slides. A modified formula that corrects for the influence of elevated cover slides in random settling experiments is therefore proposed. Although the settling method is more time-consuming than standard smear-slide techniques, additional information is gained about the spatial and temporal distribution of coccoliths. These are important for palaeoecological and palaeobiogeographical interpretations.