The presence of cilia on epithelia of the respiratory tract was reported more than 150 yr ago, and the two-layer model of mucus transport was put forward more than 50 yr ago. However, it is only in the last 10 yr or so that the motion of mucus-propelling cilia of the mammalian respiratory system has been adequately described, and fluid dynamic studies have developed far enough to allow descriptions of the mechanisms by which ciliary movement is coupled to mucus transport. In this review, scientific developments on the study of cilia and mucus, and interactions between them, are drawn together to further understanding of mucociliary clearance mechanisms of the respiratory tract. The study of the cilia incorporates a discussion of the internal mechanics and biochemistry of the ciliary axoneme, the physical principles of the beat pattern, and the (weak) metachronal coordination of cilia in the lung. Mucus rheology plays a central role in mucociliary transport with the rheologic properties of the mucus determining the effective functioning of this clearance mechanism. Theoretical models provide information on the mechanical principles of the beat pattern as well as providing reliable estimates of the transport rates. Although airflow is not thought to contribute to mucus transport in the normal state, high frequency ventilation and coughing may make significant contributions.

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'~l y m o u t h Marine Laboratory, Prospect Place, The Hoe, Plymouth PLI 3DH. United Kingdom 3~e p a r t m e n t of Biology. University of Southampton. Medical and Biological Sciences Building. Bassett Crescent East, Southampton S 0 1 6 7PX. United Kingdom ABSTRACT. The coastal sea Ice in the vicinity of Davis Statlon, Antarctica (68' 35' S, 77" 58' E ) , supported a dlverse microbial community which varied in composition and biomass in response to increasing insolatlon and temperature durlng the austral summer To understand more fully the fate of photosynthetically fixed carbon in sea ice, w e examined the dynamics of community composition, biomass and production in autotrophs, heterotrophic protozoa and bacteria. The microbial community inhabiting the bottom few centimeters of land fast ice differed markedly from the interior communities in taxonomic composition and biomass and in the timing and fate of production. Total micl-obial biomass integrated throughout the ice depth declined during the season from a mean of 1150 m g C rn-' on 17November to 628 mg C m-' by 22 December. This largely reflected a decrease in the biomass of the bottom Ice community which was dominated by the diatom Entornoneisspp. In contrast, the biomass of the interlor Ice community increased during summer and was dominated by autotrophic forms <20 pm in length with a small dinoflagellate, Gymnodinjum sp., becoming particularly abundant Heterotrophic protozoa, composed of mainly nanoflagellate, euglenold and dinoflagellate taxa, contributed between 16 and 19% of the total integrated microbial biomass in the interior ice and between 1 and 11 U/u in the bottom Ice. The biomass of heterotrophlc protozoa increased throughout the ice depth during summer and estimated taxon-specific net growth rates ranged between 0.168 d-' for a hcterotrophic euglenold and 0.05 d-' for the heterotrophic nanoflagellate population over a 23 d period. Bacterial biomass varied by several orders of magnitude between ice depths mainly due to the occurrence of a n abundant population of large epiphytic bacteria attached to Entomoneis spp. in the bottom ice. However, bacterial blomass contributed a simllar proportion of between 4 and 16",, of the total microbial biomass in both lnterlor and bottom ice The biomass of unattached bacterla increased throughout the ice depth during summer and exhibited an estimated net growth rate of 0 05 d.' These data are used to quantify autotrophlc production in bottom and interior communities, to estimate the flux of carbon to heterotrophs and to illustrate the complexity of the trophic interactions In coastal sea ice

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