Major contribution of diatom resting spores to vertical flux in the sub-polar North Atlantic

The mass sinking of phytoplankton cells following blooms is an important source of carbon to the ocean's interior, with some species contributing more to the flux of particulate organic carbon (POC) than others. During the 2008 North Atlantic Bloom Experiment in the Iceland Basin, we examined plankton community composition from surface waters and from sediment traps at depths down to 750 m. Samples collected with neutrally buoyant Lagrangian sediment traps captured a major flux event. Diatoms comprised ≥99% of cell flux into the sediment traps, with vegetative cells and resting spores of the genus Chaetoceros contributing 50–95% of cell flux. Resting spores of one species, identified as Chaetoceros aff. diadema, were dominant, comprising 35–92% of cell flux. The flux of resting spores ranged from 2 to 63 mg C m−2 day−1 and was significantly correlated with POC flux (p=0.003). Over the course of 10 days, the flux of resting spores increased by 26 fold, suggesting that the cells sank en masse, possibly in aggregates. In contrast, vegetative cells of C. aff. diadema sampled from surface waters during the period preceding the flux event generally comprised <1% of the diatom community and never exceeded 5.2%. Resting spores of C. aff. diadema were rarely observed in surface waters but their concentrations increased with depth (to 200 m) below the mixed layer. This increase in resting spore abundance, coupled with increased dissolved silicic acid concentrations at depth, suggest that the morphological changes associated with spore formation may have occurred in the mesopelagic zone, while cells were sinking. The values of variable fluorescence (Fv/Fm) measured on sediment trap material dominated by resting spores were among the highest values measured in the study area at any depth. This, in combination with the rapid germination of resting spores in ship-board incubations, suggests that vegetative cells were not physiologically stressed during spore formation. The degradation-resistant, heavily silicified resting spore valves explain the high relative contribution of C. aff. diadema resting spores to total plankton carbon at depth. These data emphasize the ephemeral nature of organic carbon flux events in the open ocean and highlight how non-dominant species and transient life stages can contribute more to carbon flux than their more abundant counterparts

The <i>Phaeodactylum</i> genome reveals the evolutionary history of diatom genomes

Diatoms are photosynthetic secondary endosymbionts found throughout marine and freshwater environments, and are believed to be responsible for around one-fifth of the primary productivity on Earth. The genome sequence of the marine centric diatom Thalassiosira pseudonana was recently reported, revealing a wealth of information about diatom biology. Here we report the complete genome sequence of the pennate diatom Phaeodactylum tricornutum and compare it with that of T. pseudonan to clarify evolutionary origins, functional significance and ubiquity of these features throughout diatoms. In spite of the fact that the pennate and centric lineages have only been diverging for 90 million years, their genome structures are dramatically different and a substantial fraction of genes (~40%) are not shared by these representatives of the two lineages. Analysis of molecular divergence compared with yeasts and metazoans reveals rapid rates of gene diversification in diatoms. Contributing factors include selective gene family expansions, differential losses and gains of genes and introns, and differential mobilization of transposable elements. Most significantly, we document the presence of hundreds of genes from bacteria. More than 300 of these gene transfers are found in both diatoms, attesting to their ancient origins, and many are likely to provide novel possibilities for metabolite management and for perception of environmental signals. These findings go a long way towards explaining the incredible diversity and success of the diatoms in contemporary oceans