All species of coccolithophore appear to respond to perturbations of carbonate chemistry in a different way. Here, we show that the degree of malformation, growth rate and stable isotopic composition of organic matter and carbonate produced by two contrasting species of coccolithophore (<i>Gephyrocapsa oceanica</i> and <i>Coccolithus pelagicus ssp. braarudii</i>) are indicative of differences between their photosynthetic and calcification response to changing DIC levels (ranging from ~1100 to ~7800 μmol kg⁻¹) at constant pH (8.13 ± 0.02). <i>Gephyrocapsa oceanica</i> thrived under all conditions of DIC, showing evidence of increased growth rates at higher DIC, but <i>C. braarudii</i> was detrimentally affected at high DIC showing signs of malformation, and decreased growth rates. The carbon isotopic fractionation into organic matter and the coccoliths suggests that <i>C. braarudii</i> utilises a common internal pool of carbon for calcification and photosynthesis but <i>G. oceanica</i> relies on independent supplies for each process. All coccolithophores appear to utilize bicarbonate as their ultimate source of carbon for calcification resulting in the release of a proton. But, we suggest that this proton can be harnessed to enhance the supply of CO₂(aq) for photosynthesis either from a large internal HCO₃^- pool which acts as a pH buffer (<i>C. braarudii</i>), or pumped externally to aid the diffusive supply of CO₂ across the membrane from the abundant HCO₃^- (<i>G. oceanica</i>), likely mediated by an internal and external carbonic anhydrase respectively. Our simplified hypothetical spectrum of physiologies may provide a context to understand different species response to changing pH and DIC, the species-specific &epsilon;_p and calcite "vital effects", as well as accounting for geological trends in coccolithophore cell size

HighlightBroad variations in the CO2 fixation kinetics of diatom Rubisco indicate novel mechanistic diversity and large differences in their carbon-concentrating mechanism.

SummaryHigh-latitude oceans are areas of high primary production despite temperatures that are often well below the thermal optima of enzymes, including the key Calvin Cycle enzyme, Ribulose 1,5 bisphosphate carboxylase oxygenase (Rubisco).We measured carbon fixation rates, protein content and Rubisco abundance and catalytic rates during an intense diatom bloom in the Western Antarctic Peninsula (WAP) and in laboratory cultures of a psychrophilic diatom (Fragilariopsis cylindrus).At À1°C, the Rubisco turnover rate, k cat c , was 0.4 C s À1 per site and the half saturation constant for CO 2 was 15 lM (vs c. 3 C s À1 per site and 50 lM at 20°C). To achieve high carboxylation rates, psychrophilic diatoms increased Rubisco abundance to c. 8% of biomass (vs c. 0.6% at 20°C), along with their total protein content, resulting in a low carbon : nitrogen ratio of c. 5. In psychrophilic diatoms, Rubisco must be almost fully active and near CO 2 saturation to achieve carbon fixation rates observed in the WAP. Correspondingly, total protein concentrations were close to the highest ever measured in phytoplankton and likely near the maximum possible. We hypothesize that this high protein concentration, like that of Rubisco, is necessitated by slow enzyme rates, and that carbon fixation rates in the WAP are near a theoretical maximum.

jects. Interferon is of less use in placebo-controlled studies in Lamivudine is a novel 2 ,3 -dideoxy cytosine analogue Chinese HBsAg carriers, possibly because the patients are that has potent inhibitory effects on hepatitis B virus immunotolerant to HBV. [5][6][7][8] However, when Chinese HBsAg replication in vitro and in vivo. We performed a singlecarriers develop elevated transaminase levels (i.e., when they blind, placebo-controlled study to assess its effectiveness are in the phase of viral clearance 9 ), their response to interand safety in Chinese hepatitis B surface antigen feron is similar to that of white patients. 8 (HBsAg) carriers. Forty-two Chinese HBsAg carriers Lamivudine is a novel 2 ,3 -dideoxy cytosine analogue that were randomized to receive placebo (6 patients) or lamihas potent inhibitory effects on HBV replication in vitro, with vudine orally in dosages of 25 mg, 100 mg, or 300 mg an IC 50 value of 5.6 mmol/L. 10,11 It is equally effective against daily (12 patients for each dosage). The drug was given the duck hepatitis B virus. 12 Studies in four chimpanzees for 4 weeks. The patients were closely monitored clinichronically infected with HBV showed that lamivudine was cally, biochemically, and serologically up to 4 weeks effective in suppression of HBV DNA in all four chimpanzees after drug treatment. All 36 patients receiving lamivuin as low a dose as 0.1 mg/kg twice daily. However, in both dine had a decrease in hepatitis B virus (HBV) DNA valducks and chimpanzees, there was a rebound of HBV DNA ues of ú90% (P õ .001 compared with placebo). Although after cessation of treatment. mg of lamivudine was slightly less effective than 100The present study was a single-blind, placebo-controlled mg (P Å .011) and 300 mg (P Å .005), it still induced 94% study designed to investigate (1) the effectiveness of lamivusuppression of HBV DNA after the fourth week of therdine in Chinese HBsAg carriers with a 4-week course, (2) the apy. HBV DNA values returned to pretreatment levels differences in response of HBV DNA to 25 mg, 100 mg, and within 4 weeks of cessation of therapy. There was no 300 mg of lamivudine daily, and (3) and positive for HBeAg for a median of 4.5 years (range, 0.5-10.5 years). HBV DNA was tested using the Abbott genostics assay (Abbott Laboratories, Chicago, IL). The patients all had HBV DNA levels Hepatitis B virus (HBV) infection is one of the most com-of ú10 pg/mL for at least 3 months. To ensure that spontaneous mon viral infections in humans. There are an estimated 300 seroconversion was unlikely, all patients had stable serum alanine and aspartate transaminase (ALT and AST, respectively) levels of million hepatitis B surface antigen (HBsAg) carriers in the less than 2 times the upper limit of the normal range for at least 3world; approximately 75% of these are Chinese. 1 The calcumonths before entry into the study. They had not taken any other lated lifetime risk of death from hepatocellular carcinoma or investigational drugs, antiviral agents, or biological modifiers for at...