Prymnesiophyte Algae Research Articles

Distinct ecological niches of marine symbiotic N2 -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa sublineages.

A recently described symbiosis between the metabolically streamlined nitrogen-fixing cyanobacterium UCYN-A and a single-celled eukaryote prymnesiophyte alga is widely distributed throughout tropical and subtropical marine waters, and is thought to contribute significantly to nitrogen fixation in these regions. Several UCYN-A sublineages have been defined based on UCYN-A nitrogenase (nifH) sequences. Due to the low abundances of UCYN-A in the global oceans, currently existing molecular techniques are limited for detecting and quantifying these organisms. A targeted approach is needed to adequately characterize the diversity of this important marine cyanobacterium, and to advance understanding of its ecological importance. We present findings on the distribution of UCYN-A sublineages based on high throughput sequencing of UCYN-A nifH PCR amplicons from 78 samples distributed throughout many major oceanic provinces. These UCYN-A nifH fragments were used to define oligotypes, alternative taxonomic units defined by nucleotide positions with high variability. The data set was dominated by a single oligotype associated with the UCYN-A1 sublineage, consistent with previous observations of relatively high abundances in tropical and subtropical regions. However, this analysis also revealed for the first time the widespread distribution of the UCYN-A3 sublineage in oligotrophic waters. Furthermore, distinct assemblages of UCYN-A oligotypes were found in oligotrophic and coastally influenced waters. This unique data set provides a framework for determining the environmental controls on UCYN-A distributions and the ecological importance of the different sublineages.

Changes in gene expression of Prymnesium parvum induced by nitrogen and phosphorus limitation.

Prymnesium parvum is a globally distributed prymnesiophyte alga commonly found in brackish water marine ecosystems and lakes. It possesses a suite of toxins with ichthyotoxic, cytotoxic and hemolytic effects which, along with its mixotrophic nutritional capabilities, allows it to form massive Ecosystem Disruptive Algal Blooms (EDABs). While blooms of high abundance coincide with high levels of nitrogen (N) and phosphorus (P), reports of field and laboratory studies have noted that P. parvum toxicity appears to be augmented at high N:P ratios or P-limiting conditions. Here we present the results of a comparative analysis of P. parvum RNA-Seq transcriptomes under nutrient replete conditions, and N or P deficiency to understand how this organism responds at the transcriptional level to varying nutrient conditions. In nutrient limited conditions we found diverse transcriptional responses for genes involved in nutrient uptake, protein synthesis and degradation, photosynthesis, and toxin production. As anticipated, when either N or P was limiting, transcription levels of genes encoding transporters for the respective nutrient were higher than those under replete condition. Ribosomal and lysosomal protein genes were expressed at higher levels under either nutrient-limited condition compared to the replete condition. Photosynthesis genes and polyketide synthase genes were more highly expressed under P-limitation but not under N-limitation. These results highlight the ability of P. parvum to mount a coordinated and varied cellular and physiological response to nutrient limitation. Results also provide potential marker genes for further evaluating the physiological response and toxin production of P. parvum populations during bloom formation or to changing environmental conditions.

Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host.

Symbiotic interactions between nitrogen-fixing prokaryotes and photosynthetic eukaryotes are an integral part of biological nitrogen fixation at a global scale. One of these partnerships involves the cyanobacterium UCYN-A, which has been found in partnership with an uncultivated unicellular prymnesiophyte alga in open-ocean and coastal environments. Phylogenetic analysis of the UCYN-A nitrogenase gene (nifH) showed that the UCYN-A lineage is represented by three distinct clades, referred to herein as UCYN-A1, UCYN-A2 and UCYN-A3, which appear to have overlapping and distinct geographic distributions. The relevance of UCYN-A's genetic diversity to its symbiosis and ecology was explored through combining flow cytometric cell sorting and molecular techniques to determine the host identity, nifH expression patterns and host cell size of one newly discovered clade, UCYN-A2, at a coastal site. UCYN-A2 nifH expression peaked during daylight hours, which is consistent with expression patterns of the UCYN-A1 clade in the open ocean. However, the cell size of the UCYN-A2 host was significantly larger than UCYN-A1 and host, suggesting adaptation to different environmental conditions. Like the UCYN-A1 host, the UCYN-A2 host was closely related to the genus Braarudosphaera; however, the UCYN-A1 and UCYN-A2 host rRNA sequences clustered into two distinct clades suggesting co-evolution of symbiont and host.

Inter-season and interannual variations in fish and macrocrustacean community structure on a eastern English Channel sandy beach: Influence of environmental factors

The intertidal zone of an exposed sandy beach on the French coast of the English Channel was sampled with a 1.5 m beam-trawl over five years (2000 and 2003–2006) at weekly intervals. The fish and macrocrustacean catches were analysed to determine the inter-season and interannual variation in community structure and relate these variations to changes in the major environmental variables. Only six species (plaice Pleuronectes platessa, common goby Pomatoschistus microps, sprat Sprattus sprattus, sand eel Ammodytes tobianus, brown shrimp Crangon crangon and shore crab Carcinus maenas) from the 27 species captured can be considered as dominant species of the intertidal zone, and they accounted for >90% of total numbers. Most individuals caught were young-of-the-year or juvenile. Analysis of similarities (ANOSIM) and similarities percentage (SIMPER) indicated that inter-season variability of community structure (mean average similarity = 47%) was more pronounced than interannual variability (mean average similarity = 65%). Canonical correspondence analysis (CCA) indicates that a substantial component (32.2%) of the measured inter-season variation in community structure can be explained by environmental factors (mainly water temperature). The main inter-season changes in the abundance and community structure were due to the variation of the six key species and reflect the different times of their recruitment. During the five years of the study, the structure of the fish and macrocrustacean spring community persisted from year to year, with the dominant species reappearing consistently even though their abundances fluctuated from year to year. This interannual variation probably reflects variable recruitment success influenced by physico-chemical conditions. In spite of the considerable interannual variation (40 times) in the spring bloom of the prymnesiophyte alga Phaeocystis globosa we found no effect of this bloom on either fish and macrocrustacean species densities or diversity index.

The temporal dynamics of the flagellated and colonial stages of Phaeocystis antarctica in the Ross Sea

Abstract Phaeocystis antarctica in the Ross Sea forms colonies, and blooms of colonial P. antarctica often occur over large areas in the southern Ross Sea. Sites where colonies occur often have significant vertical fluxes of carbon in the form of aggregated and flocculent material. P. antarctica also is a key component of the sulfur cycle; therefore, the species is critically important in many of the biogeochemical cycles in the Ross Sea. Despite this fundamental role, the life history and temporal dynamics of this species are poorly known. This study investigated the contribution of solitary, flagellated forms and colonial cells of P. antarctica to phytoplankton abundance and autotrophic carbon, and the factors that might control the relative importance of these two morphological forms. Solitary P. antarctica cells numerically dominated the phytoplankton assemblage early in austral spring, although colony formation occurred almost immediately upon the onset of net population growth. The percentage of solitary cells relative to total cells (colonial+solitary) was high in early austral spring but decreased to a minimum during late spring; specifically, nearly 98% of the P. antarctica cells were in colonies in late spring, coinciding with the seasonal chlorophyll maximum. Significant phytoplankton mortality rates were positively correlated with high ratios of solitary: total P. antarctica cells. Highest mortality rates were observed during austral spring when solitary cells dominated the P. antarctica population. The abundance of solitary P. antarctica cells began to increase again during late summer, but colonial cell numbers were always greater than those of solitary cells during this period. This increase in the contribution of solitary forms during summer may have been a consequence of more severe micronutrient limitation for the colonies (relative to solitary cells), life history processes of P. antarctica , reduced microzooplankton grazing at that time, or a combination of these and other factors. We conclude that the relative abundance of solitary and colonial forms of this prymnesiophyte alga may be a consequence of seasonal changes in these factors. The outcome of these interactions affects the contribution of this alga to the vertical flux of carbon and the degree to which P. antarctica participates in the microbial food web of the Ross Sea.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (36°N–75°N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from −0.27 to −0.15°C kyr −1. Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.9°C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition from the Hypsithermal interval (∼9–5.7 ka B.P.) to the Neoglaciation (∼5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T from early to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records.

Formation of insoluble, nonhydrolyzable, sulfur-rich macromolecules via incorporation of inorganic sulfur species into algal carbohydrates

The process of sulfur incorporation into organic matter was simulated in the laboratory by sulfurization of cell material of the prymnesiophyte alga Phaeocystis in sea water with inorganic polysulfides at 50°C. Flash pyrolysis of the residue, obtained after extraction and several hydrolysis steps, yielded mainly C 1–C 4 alkylbenzenes and C 1–C 4 alkylphenols and, in contrast to control and blank experiments, relatively high amounts of C 0–C 4 alkylthiophenes. The distribution of the thiophenes is very similar to that in pyrolysates of type II-S kerogens. The formation of high-molecular-weight sulfur-rich macromolecules co-occurs with a marked drop in the content of hydrolyzable carbohydrates. This indicates that sulfurization results in the preservation of algal carbohydrate carbon in a macromolecular structure composed of (poly)sulfidic cross-linked carbohydrate skeletons, which upon pyrolysis yields alkylthiophenes. Sulfurization of glucose under similar conditions resulted in the formation of a nonhydrolyzable, solid material, which yielded high amounts of organic sulfur compounds upon pyrolysis, mainly short-chain alkylthiophenes, although with a different distribution than that in the pyrolysate of the sulfurized algal material. The carbon numbers of these organic sulfur compounds extend beyond six, indicating that the length of the carbon skeleton of the pyrolysis products is not limited by the length of the carbon skeleton of the substrate. These results suggest that the sulfurization of carbohydrates may be an important pathway in the preservation of organic matter in euxinic depositional environments.

Influence of nutrients on carbon isotope fractionation by natural populations of Prymnesiophyte algae in NE Pacific

Isotope effects by Prymnesiophyte algae were determined on samples of marine particulate organic material collected by MULVFS pumps during three C-JGOFS cruises along Line P in the northeast Pacific. Carbon isotope fractionation associated with photosynthetic carbon uptake was calculated from 13 C/ 12 C ratios of dissolved carbon dioxide and the C 37:2 alkenone, a biomarker originating from Prymnesiophyte algae. In the presence of nitrate (N) the magnitude of the carbon isotope fractionation ( ε p) is dependent on dissolved phosphate concentration (PO 4) normalized to the concentration of dissolved carbon dioxide (C e), i.e. ε p∝PO 4/C e for N≠0. In nitrate-depleted waters, the magnitude of carbon isotope fractionation is lower than expected from the ε p vs PO 4/C e relationship. These results suggest that growth conditions approaching nitrogen starvation can severely affect carbon isotope fractionation in these paleo-climatically important algae.

Sulphur and oxygen sequestration of n-C 37 and n-C 38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation

Sedimentary rock from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) containing immature ( R o = 0.25%) S-rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330°C for 72 h to study the diagenetic fate of n-C 37 and n-C 38 di- and tri-unsaturated methyl and ethyl ketones (alkenones) biosynthesised by several prymnesiophyte algae. During early diagenesis, the alkenones are incorporated into the kerogen by both sulphur and oxygen cross-linking as indicated by chemical degradation experiments with the kerogen of the unheated sample. Heating at temperatures between 160 and 260°C, which still represents early stages of thermal maturation, produces large amounts (up to 1 mg/g TOC) of S-bound, O-bound, and both S- and O-bound n-C 37 and n-C 38 skeletons, saturated n-C 37 and n-C38 methyl, ethyl, and mid-chain ketones, C 37 and C 38 mid-chain 2,5-di- n-alkylthiophenes, C 37 and C 38 1,2-di- n-alkylbenzenes, and C 37 and C 38 n-alkanes. With increasing thermal maturation, three forms of the n-C 37 and n-C 38 skeletons are relatively stable (saturated hydrocarbons, 1,2-di- n-alkylbenzenes and saturated ketones), whereas the S- and O-bound skeletons are relatively labile. These results suggest that in natural situations saturated ketones with an n-C 37 and n-C 38 skeleton can be expected as well as the corresponding hydrocarbons.

Lipídios como Indicadores de Mudanças Climáticas no Passado 1: Biomarcadores Marinhos

The distribution and abundance of specific lipid biomarkers in marine sediments can play an important rule for the reconstruction of past climatic changes. Here, it is shown the use of a widely known paleo Sea Surface Temperature (SST) proxy based on the unsaturation ratio of a suite of long-chain C37 alkenones: the alkenone unsaturation index, UK37'. These compounds are biosynthesised by only a restricted group of prymnesiophyte algae, mostly notably the coccolithophorid Emiliania huxleyi. To ilustrate the importance of the UK37' index as a molecular tool, the correlation between the SST signal derived from the UK37' and other paleoclimatic records examined in a core from the North Atlantic is discussed.

Faunal and alkenone reconstructions of subtropical North Atlantic surface hydrography and paleotemperature over the last 28 kyr

Two techniques for estimating past variations in sea surface temperature (SST) have been used to investigate climatic change in Biogeochemical Oceanic Flux Study (BOFS) core 31K (19°N, 20°W) from the eastern subtropical Atlantic. High‐resolution SST records for the last 28 kyr have been produced using planktonic foraminiferal assemblages, based on the Imbrie‐Kipp transfer function technique, and the UK′37 index derived from abundances of C37 alkenones biosynthesized by prymnesiophyte algae. Modern observations suggest that these indices reflect particular hydrographic conditions in the upper ocean: the UK′37 index corresponds to the temperature at the time of maximum coccolith productivity, typically late spring‐early summer in the study area today, whereas the faunal transfer function is calibrated for seasonal maximum and minimum temperatures. In general, the faunal and biomarker paleotemperature records display comparable SST variations during the last glacial and deglacial, but although the overall trends are similar, differences exist in the magnitude and timing of these temperature changes. Most notably, the faunal Twarm and UK′37 SST estimates diverge by 3°C between 8 ka and 6 ka, and this offset persists through the late Holocene. This difference cannot be adequately explained by uncertainties associated with either the calibration data sets or fluctuating preservation levels. We therefore propose that the deviation in SST estimates is linked to a switch in the seasonal timing of maximum coccolith production from the summer in the glacial ocean to the late spring‐early summer in the modern ocean. Our results suggest that a dual approach to SST estimation based on faunal and biomarker proxies can provide a valuable means of evaluating mixed layer and productivity changes associated with the movement of oceanographic frontal zones during the late Quaternary.

Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years.

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C [is equivalent to epsilon(p)] in a central equatorial Pacific sediment core that spans the last approximately 255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic composition of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon(p), derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 [is equivalent to c(e)] based on a new, site-specific calibration of the relationship between epsilon(p) and c(e). The calibration was based on reassessment of existing epsilon(p) versus c(e) data, which support a physiologically based model in which epsilon(p) is inversely related to c(e). Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone-unsaturation index U(K/37). Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon(p) and 1/c(e). These are discussed in detail and it is concluded that the observed record of epsilon(p) most probably reflects significant variations in delta pCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from approximately 110 microatmospheres during glacial intervals (ocean > atmosphere) to approximately 60 microatmospheres during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO2 differences and other parameters revealed significant (p<0.01) inverse correlations of delta pCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.

Small changes in the sea surface temperature during the last 20,000 years: Molecular evidence from the western tropical Pacific

One of the most controversial problems of glacial paleoclimatology is the conservative change in tropical sea surface temperature (SST) in contrast with the large high altitude temperature change in the western tropical Pacific. In this report SST was estimated for the last 20,000 years using alkenones preserved in marine sediments recovered from a site 800 km north of New Guinea. The UK37 (unsaturation degree of long‐chain alkenones synthesized by planktonic prymnesiophyte algae) record suggests that SST decreased at most 1.5°C at the last glacial maximum. This small changes in SST of the western tropical Pacific is concordant with results previously reported in UK37 studies from the eastern equatorial Pacific and the equatorial Atlantic. The UK37‐based SST estimates are in good agreement with the CLIMAP results and planktonic foraminifera δ18O, but much smaller than those (ca. −5°C) obtained from coral Sr/Ca and δ18O thermometries and glacial vegetation reconstruction.

A biomarker perspective on prymnesiophyte productivity in the northeast pacific ocean

Long-chain alkenones derived from prymnesiophyte algae were analysed in 1-year sediment trap time series (September 1987–1988) from three sites along a 630 km offshore transect at ∼42°N in the northeast Pacific Ocean. Biomarker flux monitored at 1000 m water depth was evident throughout the year at all sites and showed a consistent seasonal maximum in late spring which increased in amplitude with distance offshore. The integrated annual biomarker flux was constant along the transect, despite differences in seasonality between sites. Alkenone unsaturation patterns were remarkably uniform throughout the time series, reflecting an algal growth temperature of 10.6 ± 1.1°C. This value corresponds to regional water temterature at the sea-surface in winter. It recurs in seasonal upwelling near the coast and at the depth of the subsurface chlorophyll maximum offshore during seasons of stratification. These biomaker observations, interpreted in view of trap data for total organic (TOC) and inorganic carbon and ancillary hydrographic information, help to clarify seasonal productivity patterns for alkenone-producing prymnesiophytes in the northeast Pacific Ocean. Sediments accumulating with distance offshore along the sampling transect change from suboxic and Mn-reducing at the water-sediment interface to aerobic throughout the depths penetrated by box coring. Comparison of alkenone and TOC accumulation rates in surface (0–1 cm) sediments with corresponding annual fluxes integrated by the trap time series, shows that the fraction of both properties accounted at the seafloor is highest and similar under sub-oxic conditions (∼25%), and declines steeply and disproportionately as aerobic conditions are encountered farther offshore. Only 0.25 and 3.1% of the annual inventory for alkenones and TOC in traps are accountable in surface sediments from the slowest accumulating, most oxidizing site farthest offshore. Despite major loss of biomarker to early diagenesis, surface sediments and trap particles display consistent alkenone unsaturation patterns. Results from this study provide a necessary background for palaeoceanographic reconstruction of the northeast Pacific Ocean from stratigraphic analysis of alkenone abundances, unsaturation patterns and isotopic compositions in sediment cores.

Marine and terrigenous origin of organic matter in modern sediments of the equatorial East Atlantic: the σ 13C and molecular record

The contributions of marine and terrigenous organic carbon in modern organic sediments from the equatorial East Atlantic was quantified, based on the stable carbon isotope composition standardized for sea surface temperature and water depth. In our binary σ 13C mixing model, the marine end member −26 ∞ is redefined at 23°C and 0 m water depth and the terrigenous end member, independent of SST and water depth, at − 26 ∞. Terrigenous carbon fractions account for more than 60% of total organic carbon (TOC) on the shelf off East Liberia and the Ivory Coast and off Gabon. On the upper slope the land-derived fraction decreases in general to less than 20% of the high TOC concentrations, which can reach 3.5 wt%. The distribution of plant wax n-alkanes (C 27, C 29, C 31) and C:N ratios do not parallel those of land-derived organic carbon, but may be controlled largely by carbon degradation and aeolian/aquatic sorting. The ratio of n-alkanols vs n-alkanes (HPA index) varies with water depth in a nonlinear mode. Since both groups of compounds stem from the same source, plant waxes, it is proposed that the HPA index is controlled mainly by degradation and to a lesser extent by sorting prior to degradation. Enhanced n-alkane concentrations (up to 580 μg/gTOC) in the Gambia Basin and in the central Guinea Basin clearly reflect the influx of aeolian organic matter from northeasterly trades near and below the Inter Tropical Convergence Zone (ITCZ). The particle flux from marine plankton is traced by high concentrations of both dinosterol, long-chain unsaturated methyl and ethyl ketones (C 37-C 39), and alkandiols in marine organic matter. Whereas alkenones, synthetized by prymnesiophyte algae generally, reflect upwelling-related productivity off-shore, dinosterol, synthetized by dinoflagellates, is enriched in near-shore areas of high marine productivity linked to fluvial fertilization. All marine biomarker groups show a surprisingly low concentration below the equatorial high productivity belt.

Calibration of alkenone unsaturation ratios ( Uk' 37) for paleotemperature estimation in cold polar waters

C 37–C 39 long-chain unsaturated ketones (alkenones) and alkenes were identified in samples of particulate organic matter obtained from surface waters spanning the temperature range -0.7–12.2°C in the Southern Ocean south of Australia. The carbon number distribution indicates that these compounds are derived from the prymnesiophyte alga Emiliania huxleyi which is the predominant coccolithophorid in these waters. Values of U k' 37 were calculated according to the definition U k' 37 = [37:2] ([37:2] + [37:3]) , where [37: x] is the concentration of the C 37 alkenone with “ x” double bonds, and plotted against sea surface temperature (SST). Our data form a contiguous set with earlier compilations of data from warmer waters. Over much of the temperature range the relationship between U k' 37 and temperature is remarkably linear, and is best fitted by the simple equation U k' 37 = 0.0414 T — 0.156 (for temperatures 4–25°C). This calibration of U k' 37 with temperature for field samples is significantly different from that established from analyses of prymnesiophyte algae in culture, and it diverges especially at the colder temperatures. Data for temperatures below about 4–6°C show increased scatter and the calibration at these low temperatures may be better fitted by a nonlinear exponential function, but this requires further testing. Certainly, the predictive power of the linear calibration is severely constrained at temperatures less than 5.0°C, but at temperatures above this our data establish the usefulness of U k' 37 as an SST indicator for waters north of the polar front in the Southern Ocean and for several regions of the world ocean.

Upwelling and productivity changes inferred from a temperature record in the central equatorial Pacific

A SERIES of C37–39 alkenones synthesized by prymnesiophyte algae is commonly preserved in marine sediments, and can be used for estimating past sea surface temperatures (SSTs)1,2. Here we present an alkenone analysis of sediments taken from the central equatorial Pacific Ocean (core W8402A-14GC; 0° 57′N, 138° 57′W) which shows that SST varied slightly (<2°C) but coherently with Milankovitch insolation cycles over the past 250 kyr. The in-phase response of the SST to the precessional component of insolation indicates that this part of the SST time series may be driven by changes in local trade-wind strength, and the longer 100-kyr eccentricity component indicates a response to basin-wide South Pacific winds. Using a simple heat-balance model, we use our palaeotemperature record to constrain the upwelling rates that have occurred in the past. Similar SST records from the eastern Pacific could constrain the horizontal advection component of the SST record and allow for better estimates of upwelling. Upwelling and palaeoproductivity then could be compared to determine whether changes in equatorial nutrient levels actually cause equivalent changes in productivity.

The genesis and preservation of biomarkers in the sedimentary record: an experimental study of the alga Emiliania huxleyi

The distribution of molecular markers in sediments provides a reservoir of unique information concerning biogeochemical processes in the geological past, and how these processes respond to environmental change. However, sedimentary systems themselves are biologically dynamic and these markers, and their precursors, have been subjected to bacterial degradation and modification. Recent research indicates that key changes in sedimented organic matter take place during the earliest stages of sediment burial and diagenesis where bacterial activity is also intense. Hence, effective interpretation of the distribution of biomarkers from deep sediment layers and sedimentary rocks requires knowledge of the rates and processes of bacterial decomposition under a range of environmental conditions.Algae are important primary producers in the marine environment. The prymnesiophyte alga Emiliania huxleyi was selected as a subject for study as it is a source of long chain ketones which are geochemically important biomarkers. The ratio of the ketones C37:2 to C37:3 is temperature sensitive and has been used as a palaeotemperature indicator (UK37).Preliminary sediment slurry incubations were carried out to optimize experimental design (concentration of decay organism, concentration of sediment in slurry, ability to obtain defined microbial environments over long term incubations, aerobic and anaerobic), and quantitative analytical scheme (extraction and separation technique, type and concentration of internal standards). Subsequent experiments on aerobic bacterial degradation of E. huxleyi demonstrated rapid increase in bacterial activity and biomass. This was accompanied by major changes in lipid classes. The dominant aliphatic hydrocarbons, three isomers of nC31:2, were rapidly degraded and completely removed by 78 days. In contrast, in preliminary anaerobic incubations, the same compounds remained unchanged. By 78 days a significant reduction in the total algal sterols was accompanied by a small increase in total stanols; hence the cholestanol/cholesterol ratio increased markedly. The abundance of the long chain unsaturated ketone C37:3 decreased relative to C37:2 resulting in an increase in the UK37 ratio. The reasons for these changes are unclear. However, they indicate that bacterial degradation may have to be taken into account in the interpretation of UK37 ratios in terms of paleotemperatures.Further experiments are in progress to clarify the interpretation of these results and provide information on the more recalcitrant biomarkers.

A new source of 4-methyl sterols and 5α(H)-stanols in sediments: prymnesiophyte microalgae of the genus Pavlova

Lipid analyses of four species of marine unicellular prymnesiophyte algae of the genus Pavlova have shown that each contains significant amounts of 4α-methyl sterols, the major one being the C 30 sterol 4α-methyl-24-ethyl-5α-cholest-22E-en-3gb-ol. This sterol has a similar structure to dinosterol, which occurs in dinoflagellates, except that the side-chain contains a 24-ethyl group rather than 23,24-dimethyl substitution. Other 4-methyl sterols include 4α,24-dimethyl-5α-cholest-22E-en-3β-ol, 4α,24-dimethyl-5α-cholestan-3β-ol and 4α-methyl-24-ethyl-5α-cholestan-3β-ol. The major 4-dimethyl sterol in each of the species is 24-ethylcholesta-5,22E-dien-3β-ol. Two species also contain significant amounts of the 5α(H)-stanol 24-ethyl-5α-cholest-22E-en-3β-ol. Unusual hydroxylated compounds found in each species were tentatively identified as dihydroxylated 4-methylsteranes. Species of Pavlova are common in marine and brackish water environments and so might be important sources of 4-methyl and 4-desmethyl stenols and stanols in some sediments.

Post-depositional stability of long-chain alkenones under contrasting redox conditions

PRYMNESIOPHYTE algae, which include the coccolithophorid species Emiliania huxleyi1, are the recognized biological source of a series of long-chain (C37, c38, 39, unsaturated methyl and ethyl ketones2 widely observed in marine sediments3. Studies of E. huxleyi in culture have demonstrated that these biomarkers are attractive geochemical tools for palaeoceanographic study. No-tably, unsaturation patterns within the alkenone series change regularly with growth temperature3–5 and the total alkenone abundance in the living plant cell is relatively constant, accounting for 5–10% of the total cellular organic carbon5. If these com-pounds are relatively well-preserved in sediments, profiles of alkenone unsaturation patterns and total alkenone concentration with depth in dated deep-sea cores6–8 provide a temporal record of sea surface temperatures and the productivity of an important group of marine phytoplankton3–5. Here we analyse the long-chain alkenone composition of sediment samples from above and below an oxidation front in an ungraded turbidite layer9 deposited 140 ±12 kyr BP in the Madeira Abyssal Plain10, to evaluate the post-depositional stability of these biomarkers under contrasting redox conditions. The results demonstrate that >85% of the total amount of these compounds is degraded over ∼8 kyr as a consequence of diffusion-controlled oxidation10. Remarkably, such extensive degradative loss has little effect on the unsaturation pattern of the residual biomarker series. Thus, we find that long-chain alkenones provide reliable indicators of sea surface temperature in the ocean. The total abundance of these biomarkers in sediments, however, is controlled not only by prymnesiophyte productivity, but also by their degree of exposure to oxidative degradation in the sedimentary process.