Ralf Goericke

A novel free-living prochlorophyte abundant in the oceanic euphotic zone

The recent discovery of photosynthetic picoplankton has changed our understanding of marine food webs1. Both prokaryotic2,3 and eukaryotic4,5 species occur in most of the worlds oceans and account for a significant proportion of global productivity6. Using shipboard flow cytometry, we have identified a new group of picoplankters which are extremely abundant, and barely visible using traditional microscopic techniques. These cells are smaller than the coccoid cyanobacteria and reach concentrations greater than 105 cells ml–1 in the deep euphotic zone. They fluoresce red and contain a divinyl chlorophyll a-like pigment, as well as chlorophyll b, α-carotene, and zeaxanthin. This unusual combination of pigments, and a distinctive prokaryotic ultrastructure, suggests that these picoplankters are free-living relatives of Prochloron7. They differ from previously reported prochlorophytes—the putative ancestors of the chloroplasts of higher plants—in that they contain α-carotene rather than β-carotene and contain a divinyl chlorophyll a-like pigment as the dominant chlorophyll.

Variations of marine plankton δ13C with latitude, temperature, and dissolved CO2 in the world ocean

Variations of the 13C content of marine participate organic carbon (δ13CPOC) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO2 through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2 in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵp) covaries primarily with concentrations of dissolved molecular CO2 ([CO2]aq). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO2]aq may be predicted with a reasonable degree of accuracy from sea surface temperature for specific regions of the world ocean. The second assumption is shown to be questionable due to the weak covariation of ϵp and [CO2]aq in the modern ocean. The large residual variance for regressions of ϵp against [CO2]aq suggests that factors other than [CO2]aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aq cannot be easily calibrated using δ13CPOC data from the modern ocean.

The marine prochlorophyte Prochlorococcus contributes significantly to phytoplankton biomass and primary production in the Sargasso Sea

The newly-discovered prochlorophyte Prochlorococcus marinus is often numerically dominant in the euphotic zone of the tropical and subtropical ocean; however, its contribution to phytoplankton biomass and primary production is largely unknown. Using its unique pigment divinyl-chlorophyll a (Chl a2) as a chemosystematic marker, we show that Prochlorococcus is present at a station in the Sargasso Sea throughout most of the year. Whereas it is only found at depth during the early summer, it can be found throughout the euphotic zone during the rest of the year. Averaged over the year Prochlorococcus pigment-biomass constitutes about 30% of the total. Its growth rate, estimated from the incorporation of 14C into Chl a2 ranged from values of 0.3 day−1 in the surface layer to values less than 0.1 day−1 at the bottom of the euphotic zone. Averaged over the seasons, approximately 25% of the total productivity was due to Prochlorococcus. Prochlorococcus clearly is an important component of the ecosystem in the Sargasso Sea, and perhaps the world ocean.

Primary productivity and its regulation in the Pacific Sector of the Southern Ocean

We measured primary productivity in the Pacific Sector of the Southern Ocean as part of the Joint Global Ocean Flux Study. We collected data along 170degrees W from 54 degreesS to 72 degreesS on four cruises during the austral growing season of 1997-1998. The cruises crossed the Subantarctic Front, the Antarctic Polar Front (APF), the Southern Antarctic Circumpolar Current (ACC) Front, and the Southern Boundary of the ACC. Primary productivity and chlorophyll a increased rapidly in spring, peaked in summer, and decreased rapidly in fall, following the seasonal pattern of irradiance. In early spring (October), primary productivity was 20 mmol C m(-2) d(-1) and increased to 54 mmol C m(-2) d(-1) within 3 weeks. During peak irradiance (December), productivity reached its maximum throughout the study area with values ranging from 33 to 93 mmol C m(-2) d(-1) depending on station location. By February, average productivity dropped to 20+/-1 mmol C m(-2) d(-1), and individual station values reached a minimum of 13 mmol C m(-2) d(-1). In early spring, chlorophyll was less than 0.5 mg Chl m(-3) throughout the study area. In late spring and early summer, chlorophyll values were between 0.15 and 1.5 mg Chl m(-3) depending on station location. By late summer, chlorophyll decreased to less than 0.3 mg Chl m(-3) across the study region. Highest absolute values of productivity and biomass occurred near the southward-propagating Si gradient (DeltaSi(max)). A spatial gradient in photosynthetic performance correlated with DeltaSi(max): photosynthetic performance was elevated in low silicic acid waters (less than 10 muM) to the north of DeltaSi(max) and was depressed in high silicic acid waters (greater than 30 muM) to the south of DeltaSi(max) Photosynthetic performance also was correlated with iron-enrichment response: when photosynthetic performance was low, iron-enrichment response was high, and when photosynthetic performance was high, iron-enrichment response was low. These results suggest that phytoplankton were iron sufficient north of DeltaSi(max) and iron limited south of DeltaSi(max). We argue that the southward-traveling DeltaSi(max), the APF, and the location of upwelling, iron-rich Upper Circumpolar Deep Water (UCDW) define three regions with differing iron sufficiency. Furthermore, we suggest that a winter recharge of upwelled, iron-rich UCDW within the Antarctic and Southern ACC Zones provides enough iron to support a diatom bloom that annually propagates poleward across the Antarctic and Southern ACC Zones to the Southern Boundary of the ACC, where the absence of UCDW prevents the blooms progression into the Subpolar Regime

Nitrogen and carbon isotopic ratios of chlorophyll from marine phytoplankton

The relationship between the nitrogen and carbon isotopic ratios of chlorophyll a and total biomass was explored in cultured marine phytoplankton to assess the utility of chlorophyll as an isotopic proxy for photoautotrophs. A near constant nitrogen isotopic depletion of 5.06 6 1.13‰ (95% confidence interval) in chlorophyll a relative to total nitrogen was observed in 8 species. This value was similar to isotopic differences between chlorophyll a and marine particles (5.27 6 1.48‰ (1 s); n 5 6) and sediments (5.39 6 0.67‰ (1s); n 5 4) in a variety of settings. These findings suggest that a 5.1‰ isotopic depletion of chlorophyll a relative to total algal nitrogen is a robust relationship that justifies the use of chlorophyll as a nitrogen isotopic surrogate for photoautotrophs. Although interspecies differences in Dd 15 Ncell-Chla exist, and growth rate has a small effect on this parameter, the field data suggest these factors are probably minimized in the ocean where multiple species and growth rates occur. The nitrogen isotopic depletion of chlorophyll a probably occurs during the transamination of glutamic acid in d-aminolevulinic acid biosynthesis, the first committed precursor to chlorophyll. The carbon isotopic composition of chlorophyll from 12 batch cultures and 7 species of marine phyto- plankton was elevated by 0.32 6 1.61‰ (95% confidence interval) relative to total cellular carbon. No significant interspecies variance was observed that was not attributable to intraspecies variance. There was a moderate inverse correlation between growth rate and Dd 13 Ccell-Chla, and it is hypothesized that this parameter is largely responsible for the large range of intraspecies Dd 13 Ccell-Chla values observed in batch cultures. Copyright

Re-assessment of the stock-recruit and temperature-recruit relationships for Pacific sardine (Sardinops sagax)

The harvest guideline for Pacific sardine (Sardinops sagax) incorporates an environmental parameter based on averaged surface temperatures at the Scripps Institution of Oceanography pier (SIO pier) in La Jolla, California, USA, which would be invoked after a series of cool years to reduce commercial catches using a precautionary decision rule. We revisit the stock-recruit and temperature-recruit relationships underpinning the currently used environmental parameter for sardine assessment and found that the temperature-recruit relationship no longer holds for the SIO pier when time series are updated with data from more recent years. The significance of the correlation between temperature and recruitment was also artificially increased by autocorrelation in the time series. In contrast, the stock-recruit relationship was still valid when recent data were added. SIO pier surface temperatures are warmer than 10 m-depth Southern California Bight (SCB) temperatures where the sardine spawn, and the difference has increased since the late 1970s. Sardine recruitment was also not related to offshore temperatures in the SCB. We demonstrate that the environmental proxy derived from SIO pier tem- perature, which has never affected the harvest guideline since its implementation, no longer predicts recruitment of Pacific sardine, and should be removed from sardine management.

Green fluorescent protein regulation in the coral Acropora yongei during photoacclimation

SUMMARY Reef-building corals inhabit high light environments and are dependent on photosynthetic endosymbiotic dinoflagellates for nutrition. While photoacclimation responses of the dinoflagellates to changes in illumination are well understood, host photoacclimation strategies are poorly known. This study investigated fluorescent protein expression in the shallow-water coral Acropora yongei during a 30 day laboratory photoacclimation experiment in the context of its dinoflagellate symbionts. Green fluorescent protein (GFP) concentration measured by Western blotting changed reversibly with light intensity. The first 15 days of the photoacclimation experiment led to a ∼1.6 times increase in GFP concentration for high light corals (900 μmol quanta m–2 s–1) and a ∼4 times decrease in GFP concentration for low light corals (30 μmol quanta m–2 s–1) compared with medium light corals (300 μmol quanta m–2 s–1). Green fluorescence increased ∼1.9 times in high light corals and decreased ∼1.9 times in low light corals compared with medium light corals. GFP concentration and green fluorescence intensity were significantly correlated. Typical photoacclimation responses in the dinoflagellates were observed including changes in density, photosynthetic pigment concentration and photosynthetic efficiency. Although fluorescent proteins are ubiquitous and abundant in scleractinian corals, their functions remain ambiguous. These results suggest that scleractinian corals regulate GFP to modulate the internal light environment and support the hypothesis that GFP has a photoprotective function. The success of photoprotection and photoacclimation strategies, in addition to stress responses, will be critical to the fate of scleractinian corals exposed to climate change and other stressors.

PIGMENT TURNOVER IN THE MARINE DIATOM THALASSIOSIRA WEISSFLOGII. I. THE 14CO2‐LABELING KINETICS OF CHLOROPHYLL a1

The turnover of chlorophyll a (chl a) was investigated in the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle using a new method based on the incorporation of 14C into chl a. The alga was maintained in its exponential growth phase under continuous light; 14C was supplied as bicarbonate. The time course of label accumulation into the tetrapyrrole ring and the phytol side chain was determined for time periods equivalent to 1–2 cell doublings. The labeling kinetics of the tetrapyrrole ring and the phytol side chain were described satisfactorily by a simple precursor‐pigment model with two free parameters, the precursor turnover rate and the pigment turnover rate, both having dimensions of time−1. The model was fit to the experimental data to determine the values of these two free parameters. The turnover rates of the tetrapyrrole ring and the phytol side chain were not significantly different, ranging from 0.01 to 0.1 per day. These rates are equivalent to turnover times ranging from days to weeks. Growth rate‐normalized turnover rates did not vary with irradiance (7.5–825 μE · m−2· s−1). The precursor turnover rates of the tetrapyrrole ring and the phytol side chain differed by an order of magnitude. These results indicate that chl a is not degraded significantly in cultures of T. weissflogii grown under continuous light. Neither irradiance nor growth rate affected growth rate‐normalized chlorophyll turnover rates. Our results are inconsistent with the hypothesis that steady‐state cellular concentrations of chl a are maintained by a dynamic equilibrium between rates of synthesis and degradation.

Reconstructing the phytoplankton community of the Cariaco Basin during the Younger Dryas cold event using chlorin steryl esters

[1] A record of the downcore distribution of chlorin steryl esters (CSEs) through the Younger Dryas was produced from Cariaco Basin sediments in order to assess the potential use of CSEs as recorders of the structure of phytoplankton communities through time. Using an improved high-performance liquid chromatography method for the separation of CSEs, we find significant changes in the distribution of CSEs during the Younger Dryas in the Cariaco Basin. During the Younger Dryas, enhanced upwelling in the Cariaco Basin caused an increase in the diatom population and therefore an increase in the relative abundance of CSEs derived from diatoms. In contrast, the dinoflagellate population, and therefore CSEs derived from dinoflagellates, decreased in response to the climate change during the Younger Dryas. These community shifts agree well with the shifts observed in the present day on a seasonal basis that result from the north-south migration of the Intertropical Convergence Zone over the Cariaco Basin. We also identify changes in the abundance of several CSEs that seem to reflect rapid warming and cooling events. This study suggests that CSEs are useful proxies for reconstructing phytoplankton communities and paleoenvironments. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 4267 Oceanography: General: Paleoceanography; 4855 Oceanography: Biological and Chemical: Plankton; KEYWORDS: Younger Dryas, Cariaco Basin, chlorin steryl esters

Cold induces acute stress but heat is ultimately more deleterious for the reef-building coral Acropora yongei

Climate change driven increases in intensity and frequency of both hot and cold extreme events contribute to coral reef decline by causing widespread coral bleaching and mortality. Here, we show that hot and cold temperature changes cause distinct physiological responses on different time scales in reef-building corals. We exposed the branching coral Acropora yongei in individual aquaria to a ± 5°C temperature change. Compared to heat-treated corals, cold-treated corals initially show greater declines in growth and increases in photosynthetic pressure. However, after 2–3 weeks, cold-treated corals acclimate and show improvements in physiological state. In contrast, heat did not initially harm photochemical efficiency, but after a delay, photosynthetic pressure increased rapidly and corals experienced severe bleaching and cessation of growth. These results suggest that short-term cold temperature is more damaging for branching corals than short-term warm temperature, whereas long-term elevated temperature is more harmful than long-term depressed temperature.