1. Burnstock, G. Cell. Mol. Life Sci. 64, 1471–1483 (2007).

2. Surprenant, A. & North, R. A. Annu. Rev. Physiol. 71, 333–359 (2009).

3. Egan, T. M. & Khakh, B. S. J. Neurosci. 24, 3413–3420

(2004).

4. Nicke, A. et al. EMBO J. 17, 3016–3028 (1998).

5. Kawate, T., Michel, J. C., Birdsong, W. T. & Gouaux, E.

Nature 460, 592–598 (2009).

6. Gonzales, E. B., Kawate, T. & Gouaux, E. Nature 460, 599–604 (2009).

7. Li, M., Chang, T.-H., Silberberg, S. D. & Swartz, K. J. Nature

Neurosci. 11, 883–887 (2008).

8. Kobertz, W. R., Williams, C. & Miller, C. Biochemistry 39, 10347–10352 (2000).

9. Long, S. B., Campbell, E. B. & MacKinnon, R. Science 309, 897–903 (2005).

10. Unwin, N. J. Mol. Biol. 346, 967–989 (2005).

11. Sonavane, S. & Chakrabarti, P. PLoS Comput. Biol. 4, e1000188 (2008).

12. Evans, R. J. Eur. Biophys. J. 38, 319–327 (2009).

13. Walker, J. E., Saraste, M., Runswick, M. J. & Gay, N. J.

EMBO J. 1, 945–951 (1982).

14. Jasti, J., Furukawa, H., Gonzales, E. B. & Gouaux, E. Nature

449, 316–323 (2007).

Well over 100 years ago, scientists specu-lated that the ocean was driven by the Sun, the Moon, Earth’s rotation and the combined motion of all the fishes’ tails (M. G. Briscoe, personal communication). Since then, there has been only sporadic interest in the idea that swimming fish and other organisms help to stir the ocean. But, recently, the concept has experienced a resurgence of interest, and the latest example of thinking on the topic appears in the paper by Katija and Dabiri on page 624 of this issue1. They offer a new angle: “a viscos-ity-enhanced mechanism for biogenic ocean mixing”.

The broader context here is that the amounts of heat and carbon stored by the ocean dwarf those held by the atmosphere, and that to understand climate change it is essential to understand the processes that affect ocean–atmosphere exchange. The studies involved

have led to some counter-intuitive results. First, although ocean properties with length scales of thousands of kilometres matter most to climate, they are sensitive to mixing pro cesses on scales of a few centimetres — think of the way you stir cream into your morning coffee; similarly, tiny whorls mix in the ocean. Second, beneath the surface, starting only about one football field deep, the ocean is a very quiet mixing environment. Roughly speaking, all the energy needed to mix a cubic kilometre of subsurface ocean could be provided by a single hand-held kitchen mixer. Through a remark-able interplay of length and timescales, very weak and small-scale mixing helps to set our climate and affects the burial of atmospheric carbon in the ocean.

It was the venerable Walter Munk2 who, in 1966, attempted to quantify the effect that activ-ity in the ocean biosphere might have on ocean

OCEANOGRAPHY

A fishy mix William K. Dewar

Ocean life is in almost constant motion, and such activity must surely stir things up. Innovative investigations into this concept of ‘biogenic mixing’ show a role for jellyfish and their brethren.

Figure 1 | Shoal of Mastigias jellyfish. These are the experimental subjects studied by Katija and Dabiri1, also pictured on the cover of this issue. The study site was a saltwater lake — Jellyfish Lake — on the Pacific island of Palau.

K. K

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channel? Because the P2X structure was solved in the absence of ATP, the binding site isn’t visible. But experiments in which amino acids were mutated12 implicate specific basic and other polar (hydrophilic) residues that line a pocket on the outer surface of the receptor at the interface between each subunit5 (Fig 1a). Although the structure of this site is unlike that of conventional ATP-binding motifs13, such motifs are notoriously diverse and the resi-dues lining the P2X pocket are appropriate for ATP binding. It will be interesting to find out whether ATP binds to this region, to determine whether magnesium ions are required for ATP binding, as it is in many other proteins, and to compare the chemistry of ligand binding with that observed in other ATP-binding pro-teins. Answering these questions will be highly informative for designing new drugs targeting P2X receptors.

The P2X receptor is not the first trimeric ion- channel structure to be solved. A structure had been reported for an acid-sensing ion channel (ASIC)14, but the protein yielding this structure was non-functional and the transmembrane segments adopted a non-native conformation. The new ASIC structure reported by Gonzales and colleagues6 was solved from a functional protein, and so provides several novel details. Although P2X receptors and ASICs have unrelated amino-acid sequences and open in response to different ligands (ATP versus protons), their transmembrane regions are remarkably similar. The pore of the ASIC is closed by an extended plug that is similar to that in the P2X receptor. And although the extra-cellular domains of P2X receptors and ASICs are structurally unrelated, both have the acidic central and upper vestibules. By soaking ASIC crystals in different ions, the authors6 located an ion-binding site in the extracellular vesti-bule, providing a first glimpse of how trimeric channels might coordinate (and thereby select for) different permeating ions.

These two papers5,6 are wonderful exam-ples of how protein structures help to frame a wealth of fascinating questions. How does ligand binding to the extracellular domains trigger opening of the pore, and how does the structure of the pore change during opening? What is the role of the two acidic vestibules in the extracellular domains? Do the similarities in the pore regions of the two channels mean that they gate using similar mechanisms, or are there simply limited ways to construct an ion-conducting pore from so few transmem-brane helices? Answering these fundamental questions will undoubtedly keep nerds busy for years to come. ■

Shai D. Silberberg and Kenton J. Swartz are in the

Molecular Physiology and Biophysics Section,

Porter Neuroscience Research Center, National

Institute of Neurological Disorders and Stroke,

National Institutes of Health, Bethesda, Maryland

20892, USA.

e-mails: shai.silberberg@nih.gov;

kenton.swartz@nih.gov

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mixing. His result neither strongly dismissed nor supported the idea, but for decades after-wards conventional wisdom held that fish could be ignored in ocean mixing. It often happens in science that a flurry of un connected activities on a common topic emerge almost simulta-neously, however, and such has been the case for biogenic ocean mixing.

In 2004, Huntley and Zhou3 pointed out that the expected levels of turbulence in schools of fish are comparable to those associated with storms. In a subsequent paper, my colleagues and I argued4 that the kinetic energy expended by the biosphere is sizeable compared with global mixing requirements; we further suggested that the true swimmers (fish), when all lumped together, provide about half of the biosphere input, with the balance coming from zooplankton. Exciting, direct confirmation of hugely elevated turbulence levels in vertically migrating shrimp-like animals followed from Kunze and colleagues5.

A major question is how efficiently biogenic turbulence actually mixes the ocean. The answer hinges on length scales. Very small whorls introduced into a fluid will be quickly damped by friction, and thus will not mix the fluid. To illustrate, consider a tall coffee cup with a slight gradient in creaminess from top to bottom; small whorls at the bottom would have little effect on the cream at the top before dying a frictional death. Guidance on the size at which turbulence changes from unimpor-tant to important in mixing is provided by the Ozmidov scale, which takes into account how stratified a fluid is and how strong the tur-bulence is. Given that many zooplankton are comparable to or smaller than oceanic Ozmi-dov scales, one view is that biogenic mixing is negligible6. The story is not yet complete, how-ever. It could be that zooplankton schooling introduces larger scales and increases mixing efficiency7. Although an attempt8 to observe such an effect failed to do so, the search continues.

Into this mix comes the paper by Katija and Dabiri1. The authors emphasize that the mere act of swimming implies that some water travels with the swimmer. Whereas viscosity lessens the effect of turbulent mixing, here it is found to increase the total transport. In remarkable videos obtained by scuba divers in shoals of jellyfish (Fig. 1), dye releases clearly show the process (see Supplementary Informa-tion1). One wonders what the jellyfish made of all this, but that would be another story.

The relevance to mixing, however, can be simply described. Suppose a jellyfish is in cold water, and swims vertically to warmer zones. Some amount of cold water will follow (the videos suggest a surprisingly large amount). Once there, mixing of the local fluid proper-ties ensues. From energetics estimates based on the dye’s behaviour, the effect seems to be sizeable. This mechanism is implicit in pre-vious energetics estimates, but it has escaped explicit notice until now and lessens doubts,

based on Ozmidov scales, about the possible strength of biogenic mixing.

Translation of Katija and Dabiri’s results from anecdotes to assessments of possible global impacts remains to be carried out. Should the overall idea of significant biogenic mixing survive detailed scrutiny, climate sci-ence will have experienced a paradigm shift. To quote Carl Wunsch9, modellers will “need to start thinking about the fluid dynamics of biology”, to which he added, “that’s a tough one” — as, indeed, it is. ■

William K. Dewar is in the Department of

Oceanography, Florida State University,

Tallahassee, Florida 32306-4320, USA.

e-mail: dewar@ocean.fsu.edu

1. Katija, K. & Dabiri, J. O. Nature 460, 624–626 (2009).

2. Munk, W. H. Deep-Sea Res. 13, 707–730 (1966).

3. Huntley, M. E. & Zhou, M. Mar. Ecol. Prog. Ser. 273, 65–79

(2004).

4. Dewar, W. K. et al. J. Mar. Res. 64, 541–561 (2006).

5. Kunze, E. et al. Science 313, 1768–1770 (2006).

6. Visser, A. W. Science 316, 838–839 (2007).

7. Catton, K. B., Webster, D. R. & Yen, J. ‘Can krill mix the

ocean?’ 2008 Ocean Sci. Mtg, Orlando, Florida, abstr.

(ASLO, 2008).

8. Gregg, M. C. & Horne, J. K. J. Phys. Oceanogr. (in the press).

9. Schiermeier, Q. Nature 447, 522–524 (2007).

The rate at which a planet rotates is a funda-mental property that informs our understanding of its formation, evolution, internal dynam-ics and meteorology. For planets with solid surfaces, the spin rate can simply be deter-mined by tracking the motion of landforms as they rotate across the surface. But for the gas giants Jupiter, Saturn, Uranus and Neptune, which lack any solid surfaces, determining the rotation rates of their interiors is more difficult. Saturn has proved the most enig-matic, and in recent years our imprecise understanding of its rotation rate has become obvious1. On page 608 of this issue, Read and colleagues2 use clues from Saturn’s dynamic meteorology to derive a new estimate for its rotation rate.

Tracking cloud motions over time shows that Saturn’s atmosphere, like all atmospheres, does not rotate as a solid body but contains

several east–west jet streams. Air at the equator circles the planet once every 10 hours 12 minutes, whereas air at higher latitudes can take up to 30 minutes longer to do so3. These cloud-tracked wind measurements imply that Saturn’s atmosphere contains a broad equato-rial jet — extending from 30° N to 30° S latitude — that flows eastward at speeds that are up to 450 m s−1 faster than air at higher latitudes. High-latitude atmospheric regions (outside the equatorial jet) are further subdivided into differentially rotating latitude bands whose relative speeds typically differ by 100 m s−1.

But what is the rotation rate of Saturn’s interior? Despite the planet’s fluid nature, electro magnetic forces in the electrically conducting interior should keep the interior rotation at nearly a single value. But is this interior rotation rate faster, slower or interme-diate between the wide range of atmospheric

PLANETARY SCIENCE

Windy clues to Saturn’s spinAdam P. Showman

Saturn’s rotation period has been a mystery. An estimate based on its meteorology comes with implications for our understanding of the planet’s atmospheric jet streams and interior structure.

a b

Figure 1 | Saturn’s swinging winds. a, The rotation period of Saturn is traditionally deduced from periodicities in the planet’s radio emission. These measurements have long suggested that the planet’s atmospheric winds move solely in an eastward direction (right-pointing arrows), varying in strength with latitude and interspersed with nulls in speed (dots). b, Read and colleagues’ new estimate2 of Saturn’s rotation period, which is based instead on the planet’s dynamical meteorology, implies that the winds alternate between eastward and westward (left-pointing arrows) with latitude.

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