ASSOCIATION OF BACTERIA WITH PHAEOCYSTIS SP IN MCMURDO-SOUND, ANTARCTICA

We present a microscopic study of bacteria and Phaeocystis sp. during the annual bloom in McMurdo Sound, Antarctica. During the initial phases of the bloom when Phaeocystis sp. was actively growing, bacterial abundance and cell size both increased, suggesting that the bacterial community was also actively growing. At 5 and 25 m, the median density of bacteria associated with Phaeocystis sp. colonies ranged from about 2- to over 11-fold greater than over an equivalent area of a nearby region of the slide. On 7 out of 8 sampling dates, the length of bacteria associated with Phaeocystis sp. colonies was similar to or larger than pelagic bacteria from the same sample, suggesting that bacterial growth rates were not reduced by their proximity to the alga. In short, rather than bacteriocidal effects sometimes ascribed to Phaeocystis sp., we observed a close association between both pelagic and epiphytic bacteria and Phaeocystis sp. throughout the bloom in McMurdo Sound. Initial results with a strain of Phaeocystis pouchetii (CCMP 628) originally collected in Surinam and grown in culture suggest that differences between the results of this and previous studies might, at least in part, reflect strain differences. As the season progressed, disappearance of Phaeocystis sp. colonies in the upper water column corresponded to the appearance of a large bacterial bloom at and below 1 00 m depth. We speculate that in McMurdo Sound, the close alga-bacterial association might enhance remineralization of Phaeocystis sp. thus reducing the amount of organic material originating from the bloom which ultimately reaches the sediments.