Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence-although each with important uncertainties-lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

The role of transparent exopolymer particles (TEP) and dissolved organic carbon (DOC) for organic carbon partitioning under different CO 2 conditions was examined during a mesocosm experiment with the coccolithophorid Emiliania huxleyi. We designed 9 outdoor enclosures (~11 m 3 ) to simulate CO 2 concentrations of estimated 'Year 2100' (~710 ppm CO 2 ), 'present' (~410 ppm CO 2 ) and 'glacial' (~190 ppm CO 2 ) environments, and fertilized these with nitrate and phosphate to favor bloom development. Our results showed fundamentally different TEP and DOC dynamics during the bloom. In all mesocosms, TEP concentration increased after nutrient exhaustion and accumulated steadily until the end of the study. TEP concentration was closely related to the abundance of E. huxleyi and accounted for an increase in POC concentration of 35 ± 2% after the onset of nutrient limitation. The production of TEP normalized to the cell abundance of E. huxleyi was highest in the Year 2100 treatment. In contrast, DOC concentration exhibited considerable short-term fluctuations throughout the study. In all mesocosms, DOC was neither related to the abundance of E. huxleyi nor to TEP concentration. A statistically significant effect of the CO 2 treatment on DOC concentration was not determined. However, during the course of the bloom, DOC concentration increased in 2 of the 3 Year 2100 mesocosms and in 1 of the present mesocosms, but in none of the glacial mesocosms. It is suggested that the observed differences between TEP and DOC were determined by their different bioavailability and that a rapid response of the microbial food web may have obscured CO 2 effects on DOC production by autotrophic cells. KEY WORDS: Emiliana huxleyi · Transparent exopolymer particles · TEP · Dissolved organic carbon · DOC · Carbon overconsumption · CO 2 · Redfield ratios · Mesocosms Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 34: [93][94][95][96][97][98][99][100][101][102][103][104] 2004 raised to explain carbon overconsumption, including the underestimation of new production due to unaccounted for biological N 2 -fixation (Michaels et al. 1996, Hood et al. 2001, the temporary accumulation of carbon-rich dissolved organic matter (DOM) , preferential nutrient recycling (Thomas et al. 1999) or the formation of carbon-rich extracellular particles known as transparent exopolymer particles (TEP) (Engel et al. 2002a).Considering carbon cycling at the cellular level, it is well known that the uptake of carbon continues when nutrient acquisition limits cell division but not primary production. One consequence of the excess assimilation of carbon is the extracellular release (ER) of organic matter ( Fig. 1) (Fogg 1966). Although the mechanisms of ER have not yet been fully elucidated, it can be assumed that low molecular weight (LMW) substances, such as monomer or oligomer sugars and amino acids, penetrate the cell membrane by diffusion (Fogg 1966). The rate of this leakage of LMW substances should ther...

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We introduce a novel, simple method to measure sinking velocity of particles and aggregates in roller tanks. Using this noninvasive method, it is possible to follow changes in sinking velocities on the same aggregates during time and to make paired measurements of aggregate sinking velocity and composition. Particles and aggregates are video recorded in roller tanks, and their sinking velocity is derived from the orbital trajectories. This new method is compared with three other methods (using roller tanks, a vertical flow system, and a sedimentation column), which have not previously been inter-calibrated. Agar spheres and diatom aggregates were used as model particles in all experimental systems. No method showed significantly different sinking velocities of agar spheres compared with those calculated by theory. Paired measurements showed that sinking velocities from 70 to 700 m d -1 were linearly correlated between different methods. Highest sinking velocities were measured in a sedimentation column followed by those measured in roller tanks and in the vertical flow system, respectively. The average difference of sinking velocity measured with the different methods ranged from 8% to 11% for agar spheres, and up to 20% for diatom aggregates.

Chromophoric dissolved organic matter (CDOM) represents the optically active fraction of the bulk dissolved organic matter (DOM) pool. Recent evidence pointed towards a microbial source of CDOM in the aquatic environment and led to the proposal that phytoplankton is not a direct source of CDOM, but that heterotrophic bacteria, through reprocessing of DOM of algal origin, are an important source of CDOM. In a recent experiment designed at looking at the effects of elevated pCO 2 on blooms of the coccolithophorid alga Emiliania huxleyi, we found that despite the 3 different pCO 2 levels tested (190, 414 and 714 ppm), no differences were observed in accumulation of CDOM over the 20 d of incubation. Unlike previous mesocosm experiments where relationships between CDOM accumulation and bacterial abundance have been observed, none was observed here. These results provide some new insights into the apparent lack of effect of pCO 2 on CDOM accumulation in surface waters, and question the previously proposed mechanisms and rates of CDOM production in natural phytoplankton blooms.KEY WORDS: CDOM · DOC · Mesocosms · Viruses · Cyanobacteria · Heterotrophic bacteria · CO 2 Resale or republication not permitted without written consent of the publisher