Ice-brine and planktonic microheterotrophs from Saroma-ko Lagoon, Hokkaido (Japan): quantitative importance and trophodynamics
Résumé
Copyright (c) 1996 Elsevier Science B.V. All rights reserved. Biologists have rarely had the opportunity to investigate the community characteristics and dynamics of heterotrophic microorganisms in highly productive first-year sea ice. In this study, sterile seawater was used as a salinity buffer to extract the ice-brine microheterotroph communities (bacteria, flagellates and ciliates) from a coastal lagoon in Japan (Saroma-ko, Hokkaido; 44°N, 144°E) during the late winter (February−March) of 1992. This procedure reduced osmotic shock during the melting of ice cores and allowed the recovery of up to 323% more cells than the traditional melting method. Most of the organisms were concentrated in the bottom 3−4 cm of the ice, where abundances were up to 33 times higher than in the plankton. In ice and plankton samples, heterotrophic flagellates were dominated by small species (<8 μm, mainly choanoflagellates) and cryothecomonad-type cells while ciliates were dominated by a photosynthetic species, Mesodinium rubrum. In contrast to higher latitudes, increased snow cover appeared to favor the development of protozoa beneath the relatively thin 30−40 cm ice cover of Saroma-ko Lagoon. Temporally, a successional sequence was observed between protozoa and the bacterial compartment. Bacteria decreased in abundance throughout the sampling period while protozoa increased or attained their maximum number in late winter, toward the end of the sampling period. These observations support previous suggestions of the existence of a functional microbial food web within the sea-ice community. Heterotrophic flagellate biomass greatly exceeded bacterial biomass in the sea ice (30−60×). Coupled with similar potential growth rates, this suggests the utilization of additional (non-bacterial) food items by ice-brine flagellates. Finally, the effects of salinity variations (ranging between 15 and 120 psu) on potential microheterotroph growth rates are discussed.