Precambrian palaeontology in the light of molecular phylogeny – an example: the radiation of the green algae

International audienceThe problem of the antiquity of the radiation of the green algae (phylum Viridiplantae) has been hotly debated and is still controversial today. A method combining Precambrian paleontology and molecular phylogeny is applied to shed light on this topic. As a critical method, molecular phylogeny is essential for avoiding taxonomic mistakes. As a heuristic method, it helps us to discern to what extent the presence of such and such clade is likely at such and such time, and it may even suggest the attribution of some fossil to a clade whose taxonomic position will be distinctly defined even though it has no previously known representative. Some well characterized Precambrian fossils of green algae are Palaeastrum and Proterocladus at Svanbergfjellet (ca. 750 Ma), Tasmanites and Pterospermella at Thule (ca. 1200 Ma), Spiromorpha at Ruyang (ca. 1200 Ma) and Leiosphaeridia crassa at Roper (ca. 1450 Ma). The position of these fossils in the taxonomy and the phylogeny of the Viriplantae is discussed. The conclusions are that the Chlorophyceae and the Ulvophyceae were separated long before 750 Ma, that the Chlorophyta and the Streptophyta were separated long before 1200 Ma and that the last common ancestor of the Viridiplantae and the Rhodophyta was possibly two billion years old

New organic-walled dinoflagellate cysts from recent sediments of central Asian seas

Recent to sub-recent sediments from the Caspian Sea, the Kara-Bogaz-Gol Bay, the Enseli lake and the Aral Sea contain the new organic-walled dinoflagellate cysts Caspidinium, Caspidinium rugosum and Impagidinium caspienense. Caspidinium rugosum has S-type paratabulation, dextral torsion and low relief intratabular ornamentation. Impagidinium caspienense has low intratabular suturo-cavate relief, parasutural septa irregular in height and a high septum at the junction of paraplate 1'''' and the sulcus. The accompanying species consist of Spiniferites cruciformis, Lingulodinium machaerophorum, Pyxidinopsis psilata, cysts of Pentapharsodinium dalei and Brigantedinium spp. Spiniferites cruciformis varies from specimens with a cruciform body with a well-developed postero-lateral membranous flange to specimens with a pear-shaped body, reduced processes and no flange. Sea-surface data from these Central Asian seas suggests that the two new taxa Caspidinium rugosum and Impagidinium caspienense are probably related to low salinity conditions (12-13)

Proterozoic and Early Cambrian Protists: Evidence for Accelerating Evolutionary Tempo

In rocks of late Paleoproterozoic and Mesoproterozoic age (ca. 1700-1000 million years ago), probable eukaryotic microfossils are widespread and well preserved, but assemblage and global diversities are low and turnover is slow. Near the Mesoproterozoic-Neoproterozoic boundary (1000 million years ago), red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of ''higher'' eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates. Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude. Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms.Organismic and Evolutionary Biolog

Phylogenetic Investigation of the Aliphatic, Non-hydrolyzable Biopolymer Algaenan, with a Focus on Green Algae

Algaenan, an aliphatic biopolymer found in various microalgae, has been implicated as the source of a sizable proportion of the aliphatic refractory organic matter in sedimentary rocks. Because of its recalcitrant nature, algaenan is thought to be preserved selectively in the formation of kerogen and microfossils. Its taxonomic distribution in organisms has not been studied in detail or in a phylogenetic context. Here, we evaluate the distribution and phylogenetic relationships of algaenan-producing organisms from a broad, eukaryote-wide perspective down to the level of genus and species. We focus on the kingdom Plantae, as most described algaenan producers belong to this superkingdom. The phylogenetic distribution of algaenan producers within the Plantae is actually quite limited and a detailed phylogenetic analysis of the two classes that include all green algal algaenan producers suggests that there is no finer-grained pattern of phylogenetic distribution to the production of this biopolymer. Our results suggest that the biopolymer is not widespread ecologically or phylogenetically, is not found abundantly in marine organisms and likely represents a functional description of molecular class, rather than a biomarker for green algae. This adds to a growing body of literature that questions the selective preservation hypothesis for insoluble organic matter and calls for a more detailed chemical and structural analysis of algaenan.Organismic and Evolutionary Biolog

Upward nitrate transport by phytoplankton in oceanic waters : balancing nutrient budgets in oligotrophic seas

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PeerJ 2 (2014): e302, doi:10.7717/peerj.302.In oceanic subtropical gyres, primary producers are numerically dominated by small (1–5 µm diameter) pro- and eukaryotic cells that primarily utilize recycled nutrients produced by rapid grazing turnover in a highly efficient microbial loop. Continuous losses of nitrogen (N) to depth by sinking, either as single cells, aggregates or fecal pellets, are balanced by both nitrate inputs at the base of the euphotic zone and N2-fixation. This input of new N to balance export losses (the biological pump) is a fundamental aspect of N cycling and central to understanding carbon fluxes in the ocean. In the Pacific Ocean, detailed N budgets at the time-series station HOT require upward transport of nitrate from the nutricline (80–100 m) into the surface layer (∼0–40 m) to balance productivity and export needs. However, concentration gradients are negligible and cannot support the fluxes. Physical processes can inject nitrate into the base of the euphotic zone, but the mechanisms for transporting this nitrate into the surface layer across many 10s of m in highly stratified systems are unknown. In these seas, vertical migration by the very largest (102–103 µm diameter) phytoplankton is common as a survival strategy to obtain N from sub-euphotic zone depths. This vertical migration is driven by buoyancy changes rather than by flagellated movement and can provide upward N transport as nitrate (mM concentrations) in the cells. However, the contribution of vertical migration to nitrate transport has been difficult to quantify over the required basin scales. In this study, we use towed optical systems and isotopic tracers to show that migrating diatom (Rhizosolenia) mats are widespread in the N. Pacific Ocean from 140°W to 175°E and together with other migrating phytoplankton (Ethmodiscus, Halosphaera, Pyrocystis, and solitary Rhizosolenia) can mediate time-averaged transport of N (235 µmol N m-2 d-1) equivalent to eddy nitrate injections (242 µmol NO3− m-2 d-1). This upward biotic transport can close N budgets in the upper 250 m of the central Pacific Ocean and together with diazotrophy creates a surface zone where biological nutrient inputs rather than physical processes dominate the new N flux. In addition to these numerically rare large migrators, there is evidence in the literature of ascending behavior in small phytoplankton that could contribute to upward flux as well. Although passive downward movement has dominated models of phytoplankton flux, there is now sufficient evidence to require a rethinking of this paradigm. Quantifying these fluxes is a challenge for the future and requires a reexamination of individual phytoplankton sinking rates as well as methods for capturing and enumerating ascending phytoplankton in the sea.This work has been funded by the National Science Foundation: OCE-0726726, OCE-0094591, OCE-9414372, OCE-9100888 and OCE-9415923 to TAV, and OCE-9423471 to CHP

Non-Skeletal Biomineralization by Eukaryotes: Matters of Moment and Gravity

Skeletal biomineralisation by microbial eukaryotes significantly affects the global biogeochemical cycles of carbon, silicon and calcium. Non-skeletal biomineralisation by eukaryotic cells, with precipitates retained within the cell interior, can duplicate some of the functions of skeletal minerals, e.g., increased cell density, but not the mechanical and antibiophage functions of extracellular biominerals. However, skeletal biomineralisation does not duplicate many of the functions of non-skeletal biominerals. These functions include magnetotaxis (magnetite), gravity sensing (intracellular barite, bassanite, celestite and gypsum), buffering and storage of elements in an osmotically inactive form (calcium as carbonate, oxalate, polyphosphate and sulfate; phosphate as polyphosphate) and acid-base regulation, disposing of excess hydroxyl ions via an osmotically inactive product (calcium carbonate, calcium oxalate). Although polyphosphate has a wide phylogenetic distribution among microbial eukaryotes, other non-skeletal minerals have more restricted distributions, and as yet there seems to be no definitive evidence that the alkaline earth components (Ba and Sr) of barite and celestite are essential for completion of the life cycle in organisms that produce these minerals.Organismic and Evolutionary Biolog

River inflow and salinity changes in the Caspian Sea during the last 5500 years

Pollen, spores and dinoflagellate cysts have been analysed on three sediment cores (1.8–1.4 m-long) taken from the south and middle basins of the Caspian Sea. A chronology available for one of the cores is based on calibrated radiocarbon dates (ca 5.5–0.8 cal. ka BP). The pollen and spores assemblages indicate fluctuations between steppe and desert. In addition there are some outstanding zones with a bias introduced by strong river inflow. The dinocyst assemblages change between slightly brackish (abundance of Pyxidinopsis psilata and Spiniferites cruciformis) and more brackish (dominance of Impagidinium caspienense) conditions. During the second part of the Holocene, important flow modifications of the Uzboy River and the Volga River as well as salinity changes of the Caspian Sea, causing sea-level fluctuations, have been reconstructed. A major change is suggested at ca 4 cal. ka BP with the end of a high level phase in the south basin. Amongst other hypotheses, this could be caused by the end of a late and abundant flow of the Uzboy River (now defunct), carrying to the Caspian Sea either meltwater from higher latitudes or water from the Amu-Daria. A similar, later clear phase of water inflow has also been observed from 2.1 to 1.7 cal. ka BP in the south basin and probably also in the north of the middle basin

Caracterización taxonómica y paleoecológica de la ingresión del Holoceno en el Canal Beagle (Tierra del Fuego) en base a las asociaciones de dinoquistes y otros palinomorfos acuáticos

On the southern coast of Tierra del Fuego, Argentina, the Holocene marine ingression took place around 8,000 C14 years BP, and it is represented by several discontinuous marine terraces along the north coast of the Beagle Channel. During the beginning of the marine incursion, the aquatic assemblages developed in Bahía Lapataia and Río Varela localities, were characterized by a scarce participation of marine components, especially of dinofl agellate cysts. After 6,000 C14 years BP, in Río Ovando and Albufera Lanushuaia localities, an increase in the number of aquatic species was observed with a major participation of Peridiniales dinocysts in the associations, suggesting the establishment of marginal marine environments with low to moderate salinities and high nutrient content in the water. The analysis of the dinofl agellate cysts and other aquatic palynomorphs assemblages recorded in the marine sediments from northern coast of the Beagle Channel, indicate that the environmental conditions during the middle-late Holocene correspond to fjord (estuarine) environments close to ice fi elds affected by glacier meltwater discharge with anomalously low salinities. These aquatic assemblages identifi ed have a similar composition to those observed in modern sediments of the Beagle Channel.Na costa sul da Tierra del Fuego, Argentina, a ingressão marinha holocena (datada em torno de 8.000 anos AP pelo método C14) é representada por vários terraços marinhos descontínuos ao longo da costa norte do Canal de Beagle. No início da ingressão marinha, nas localidades de Baía Lapataia e Rio Varela, as associações aquáticas caracterizavam-se por uma escassa participação de elementos marinhos, em especial de cistos de dinofl agelados. A partir de 6.000 anos AP, nas localidades de Albufera Lanushuaia e Rio Ovando, foi observado um aumento no número de palinomorfos aquáticos, com maior participação das associações de dinocistos da Ordem Peridiniales, sugerindo o estabelecimento de ambientes marinhos marginais com salinidade baixa a moderada e alto conteúdo de nutrientes. A análise das associações de dinocistos e de outros palinomorfos aquáticos, registrados na costa norte do Canal de Beagle, indica que as condições ambientais estabelecidas durante o Holoceno médio-tardio correspondem a ambientes de fi ordes (estuarinos), próximos a glaciares infl uenciados pela descarga de águas de degelo, com salinidades anomalamente baixas. As associações aquáticas identificadas nos sedimentos holocênicos apresentam uma composição similar àquelas observadas nos sedimentos atuais do Canal de Beagle.Fil: Candel, Maria Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Cientificas; Argentina;Fil: Borromei, Ana Maria. Consejo Nacional de Invest.cientif.y Tecnicas. Centro Cientifico Tecnol.Conicet - Bahia Blanca. Instituto Geológico del Sur; Argentina

Ediacaran Microfossils from the Ura Formation, Baikal-Patom Uplift, Siberia: Taxonomy and Biostratigraphic Significance

Abundant and diverse microfossils from shales of the uppermost Ura Formation, central Siberia, document early to middle Ediacaran life along the southeastern margin of the Siberian Platform. The Ura Formation is well exposed in a series of sections in the Lena River basin but the best microfossil assemblages come from a locality along the Ura River. Here, the uppermost twenty meters of the formation contain diverse microfossils exceptionally well preserved as organic compressions. Fossils include nearly two dozen morphospecies of large acanthomorphic microfossils attributable to the Ediacaran Complex Acanthomorph Palynoflora (ECAP), a distinctive assemblage known elsewhere only from lower, but not lowermost, to middle Ediacaran rocks. Discovery of ECAP in strata previously considered Mesoproterozoic through Cryogenian confirms inferences from chemostratigraphy, dramatically changing stratigraphic interpretation of sedimentary successions and Proterozoic tectonics on the Siberian Platform. Systematic paleontology is reported for 36 taxa (five described informally) assigned to 23 genera of both eukaryotic and prokaryotic microfossils. One new genus and two new species are proposed: Ancorosphaeridium magnum n. gen. n. sp. and A. minor n. gen. n. sp.Organismic and Evolutionary Biolog