Recycling and uptake of Si(OH)4 when protozoan grazers feed on diatoms.

Herbivory of microzooplankton is an emerging key factor of diatom mortality in the ocean. As part of the microbial loop, protozoan grazers also feed on bacteria that accelerate the degradation of diatom detritus. The potentially pivotal effect of microzooplankton grazing on Si(OH)(4) recycling was investigated with cultures of single-celled diatoms, Thalassiosira pseudonana and Chaetoceros gracilis, and heterotrophic protozoans, the dinoflagellate Oxyrrhis marina and the ciliate Strombidium sp. Both grazers ingested diatoms and the bacteria in the non-axenic cultures. C. gracilis, whose frustule is "armed" with setae, was less suitable as a prey than T. pseudonana. Ingestion rates of T. pseudonana were comparable for O. marina and Strombidium, but the dinoflagellate produced two orders of magnitude more detrital bSiO(2) than the ciliate, due to the higher abundance reached by O. marina. Total net release of Si(OH)(4) was lower in the grazing treatments compared to the control possibly due to the reduced bacterial growth by microzooplankton bacterivory, and to the transient protection of detrital bSiO(2) in discarded feeding vacuoles. Over the first 24h, microzooplankton grazing even led to enhanced uptake of Si(OH)(4) by diatoms, confirming the potential of grazing to influence the silicification of diatom frustules. Subsequently however, the Si dynamics in bottles with grazers turned rapidly from net uptake to net Si(OH)(4) release. Protozoan grazers hence tie Si(OH)(4) recycling into the microbial loop by producing detrital bSiO(2).

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean.

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Grazing-induced changes in cell wall silicification in a marine diatom.

In aquatic environments, diatoms (Bacillariophyceae) constitute a central group of microalgae which contribute to about 40% of the oceanic primary production. Diatoms have an absolute requirement for silicon to build-up their silicified cell wall in the form of two shells (the frustule). To date, changes in diatom cell wall silicification have been only studied in response to changes in the growth environment, with consistent increase in diatom silica content when specific growth rates decrease under nutrient or light limitations. Here, we report the first evidence for grazing-induced changes in cell wall silicification in a marine diatom. Cells grown in preconditioned media that had contained both diatoms and herbivores are significantly more silicified than diatoms grown in media that have contained diatoms alone or starved herbivores. These observations suggest that grazing-induced increase in cell wall silicification can be viewed as an adaptive reaction in habitats with variable grazing pressure, and demonstrate that silicification in diatoms is not only a constitutive mechanical protection for the cell, but also a phenotypically plastic trait modulated by grazing. In turn, our results corroborate the idea that plant-herbivore interactions, beyond grazing sensu stricto, contribute to drive ecosystem structure and biogeochemical cycles in the ocean.