'~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
An annual study of the bacterioplankton community structure was carried out at Stn L4 (50°15' N, 04°13' W) in the western English Channel between August 2003 and July 2004. Bacterioplankton abundance and community structure were assessed using flow cytometry and fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes, respectively. The Eubacteria domain dominated over the Archaea domain (<15%) at the highest phylogenetic level. The Sphingobacteria-Flavobacteria group of the Bacteroidetes phylum (SFB) numerically dominated in spring and early summer. The α-Proteobacteria dominated from late summer to winter. The SAR11 clade represented ~13% of the microbial community throughout the year and accounted for up to 69% of α-Proteobacteria in late spring. Annually, γ-Proteobacteria were 2 or 3 times less abundant than the other groups and showed no obvious seasonal trend. The SAR86 cluster accounted for up to half of γ-Proteobacteria when it peaked in summer. Consequently, we found that community structure at higher taxonomic level did not change dramatically with season but lower level phylogenetic groups showed pronounced seasonal peaks.KEY WORDS: Bacterioplankton · Seasonal variability · Community structure · English Channel · Fluorescence in situ hybridization Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 42: [119][120][121][122][123][124][125][126] 2006 munity biomass, function, structure, and diversity are known to change between winter and summer (Pernthaler et al. 1998, Zaccone et al. 2002. Schauer et al. (2003) showed the overall stability in time of the taxonomic composition of the bacterioplankton in coastal marine system, with gradual changes throughout the year revealing a substitution of closely related phylotypes during the seasonal cycle. Crump et al. (2003) demonstrated that shifts in bacterioplankton community composition were related to seasonal cycles in the source and lability of dissolved organic matter. Similarly, succession in marine bacterioplankton assemblages occurred in response to seasonal shifts in water column stability and water temperature, suggesting that bacterioplankton community composition may demonstrate an annual pattern of variability (Murray et al. 1998). Other studies have demonstrated relationships between bacterioplankton community composition and seasonal dynamics of other members of the aquatic food web (Hofle et al. 1999, Fandino et al. 2001, Hahn & Hofle 2001, Arrieta & Herndl 2002.A typical seasonal dynamic in temporal coastal waters still remains to be established. Previous mesocosm experiments showed dramatic changes in the composition of the bacterial assemblage on daily to weekly time scales (van Hannen et al. 1999, Schäfer et al. 2001. However, it is not clear whether these changes are frequent in the field, although in some situations, such as during phytoplankton blooms, strong changes in numbers and phylogenetic shifts of the bacterial assemblage have been observed (Fandin...
Heterotrophic dinoflagellates and their herbivory were quantified at a coastal site in East
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