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Explorers Deliver Tea to the Pole Water Ever Enters the Atmosphere

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Explorers Deliver Tea to the Pole Water Ever Enters the Atmosphere news and views of the compound. In the sea this response GBT–DMSP switch exists7, and Gage and might shift grazing pressure away from cells colleagues now provide further support that contain high levels of DMSP. Wolfe and for the idea at the biochemical level. DMSP colleagues’ report is the first of grazing-acti- synthesis is initiated by a transamination vated defence for a unicellular organism. reaction, and this pathway would be favoured In their paper, Gage and co-workers2 in nitrogen-depleted cells, hence effectively describe a series of elegant biochemical favouring a switch to DMSP. experiments through which they have And what of the climate link? Global identified the intermediates in DMSP climate change is a big issue8, and there is gen- biosynthesis, including the novel com- eral awareness of how human activities ar8e pound dimethylsulphoniohydroxybutyrate changing the concentration and distribution (DMSHB). They show that the pathway in a of greenhouse gases. It is less well known that common green seaweed is different from atmospheric aerosols, which are submicro- Figure 2 Scanning electron micrograph of a cell that found in higher plants, which suggests metre particles or droplets, have a cooling of the marine phytoplankton species Emiliania that these pathways evolved independently. effect on global temperature. The link with huxleyi, approximately 5 mm in diameter. The However, macroalgae, which are typically algae is that some of the marine DMS escapes outside of the cell is covered with ornate found in the intertidal and subtidal zones of into the atmosphere where it oxidizes rapidly interlocking plates of calcium carbonate known rocky shores, are thought to have only a small to form aerosol particles; these absorb and as coccoliths. This organism contains high effect on the global sulphur cycle when com- reflect solar energy directly back into space, concentrations of dimethylsulphoniopropionate pared with marine phytoplankton. With this and scatter radiation indirectly by acting as (DMSP), a precursor of DMS. The work in mind, Gage et al. went on to identify cloud-condensation nuclei. discussed here2,3 shows the route by which DMSHB in three phytoplankton species Natural aerosol levels have been substan- DMSP is synthesized in marine algae, and how from different algal classes. DMSHB was tially affected by anthropogenic sulphur, grazing results in its conversion to DMS readily catabolized when added to algal cul- especially in the Northern Hemisphere. and acrylic acid. (Micrograph courtesy of tures, and the authors make the intriguing Nevertheless, DMS oxidation products D. Harbour, SAHFOS, Plymouth.) suggestion that it could be an additional remain the major source of aerosol particles precursor of DMS in vivo. over remote marine areas (that is, over most blages in the vast areas of the open oceans Marine organisms usually have a combi- of the globe). Anything that disturbs the where steady-state populations exist for nation of organic osmolytes. DMSP is similar balance of the microbial ecology of DMS longer periods. Every millilitre of sea water in both structure and function to the nitro- production, such as changes in wind speed contains a large and diverse microbial popu- gen-containing osmoregulatory compound that alter the upwelling of nutrients, or lation of photosynthetic cells, zooplankton, glycine betaine (GBT). It has been proposed increases in sea-water temperature that bacteria and viruses. The processes by which that, under conditions of nitrogen limita- change the spatial distribution of grazers DMSP is released from phytoplankton cells tion6, a common occurrence in the marine relative to their phytoplankton prey, could and DMS is produced are best depicted by environment, organisms might preferential- affect how much of this sulphur compound is a network of production, transformation ly change to making the sulphur-containing emitted to the air, and therefore affect climate. and consumption which involves the entire DMSP molecule. There is evidence from lab- In 1987 Charlson and colleagues1 sparked marine microbial food web4. To date several oratory studies and a field study that such a a flurry of research interest by proposing that key processes have been identified, although turnover rates and what controls them are Cell biology not well understood. What is clear is that less than 10 per cent of the DMS in surface sea Explorers deliver tea to the pole water ever enters the atmosphere. The rest is turned over by bacteria or oxidized by sun- A few weeks ago, a report in these pages light to form non-volatile products. claimed that drinking green tea can Grazing is a major route for the release of prevent cancer (see Nature 387, 561; 1997). NURSE A/P DMSP to sea water. In many regions unicel- Now, in the 13 June issue of Cell, T . MA lular microzooplankton are the principal Juan Mata and Paul Nurse report J herbivores, and they have a particularly (Cell 89, 939–949; 1997) that another type important impact on small marine phyto- of tea could help you to grow properly — protein is tightly localized to both ends of a plankton cells such as Emiliania huxleyi — a if you’re a fission yeast, that is. cell. Not only that but, using species that contains large amounts of Immediately after cell division, the immunofluorescence staining, they saw DMSP (Fig. 2). These protozoa are highly fission yeast Schizosaccharomyces pombe that tea1 (green) colocalizes with the ends selective in what they eat, and use chemical begins to grow. Initially, it extends only at of tubulin (red) microtubules. cues to select and reject prey5. Some DMSP- the old tip that existed in the mother cell. Mata and Nurse believe this to mean containing microalgae have an enzyme But at a certain point in the cell cycle, it that tea1 acts as an end marker, directing known as DMSP lyase, which does not come switches to bipolar growth, and its ends the yeast’s growth machinery to the cell into contact with intracellular DMSP during elongate directly away from one another. poles. Tea1 is delivered to the poles by the normal growth. Wolfe et al.3 propose that How does it do this? microtubular cytoskeleton, the grazing mixes DMSP with the enzyme, To find out, Mata and Nurse studied organization of which may be affected as a thereby liberating DMS and acrylic acid, and yeast mutants in which the tips were not result. This produces a dynamic system in then go further to suggest that DMSP may precisely opposed. Some of these mutants which the space of the cell is explored, then act as a defence precursor. Acrylic acid has were bent, others were T-shaped, but all the two growing tips are identified and signalling properties for a range of organ- had defects in one of two tea (tip maintained by tea1 — a kind of tea for two isms, including bacteria, protozoa and fish, elongation aberrant) genes. Having cloned for yeast. and the herbivorous microzooplankton used tea1, the authors found that the tea1 Alison Mitchell in the study actively retreated from gradients 858 NATURE | VOL 387 | 26 JUNE 1997 Nature © Macmillan Publishers Ltd 1998.
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