Survival strategies of two small marine ciliates and their role in regulating bacterial community structure under experimental conditions

Abstract

Urcmema sp. of ca 12 X 5 p m and Euplotes sp. ca 20 X 10 pm were isolated from surface waters of the English Channel. The rapidly motile Uronerna sp. has a relative growth rate of 3.32 d-' and responds rapidly to the presence of bacterial food with a doubling time of only 5.01 h. Its mortality rate is 0.327 d-' and mortality time is therefore short at 50.9 h once the bacterial food resource has become h i t i n g. Uronema sp. therefore appears to be adapted to exploit transitory patches when bacterial prey abundance exceeds a concentration of ca 6 X 106 cells ml-'. In contrast, Euplotes sp. had a slower relative growth rate of 1.31 d-' and a doubling time of ca. 12.7 h, implying a slower response to peaks in bacterial food supply. The mortality rate of 0.023 d-' is considerably lower than In Uronema and mortality time is as much as 723 h. This suggests that, relative to Uronerna, the slower moving Euplotes has a more persistent strategy w h c h under the conditions of our experiment favours a stable e q u h b n u m wlth its food supply. Grazing activities of these 2 ciliates have an important influence on abundance and size-class structure of their bacterial prey. In the presence of both Uronema and Euplotes, there is some evidence of an initial removal of large rods and squat rods, followed by removal of the smaller cocci before the initial population of bacteria is consumed. This is followed by the appearance of a secondary bactenal assemblage which is associated only with the development of the grazlng chates and is dominated by small thin rods. Gross growth efficiency of Uronema and Euplotes is ca 27 and 19 '10 respectively. Hence as much as 73 to 81 % of the carbon ingested with the bactenal food is dissipated through respiration and excretion. Should this occur in natural euphotic waters this process may supplement the DOM release by primary producers. Extracellular DOM production by the ciliates may therefore be of major significance in the maintenance of the secondary population of bacteria, and emphasizes the close interdependence of ciliate predators and bacterial prey in rnicro-heterotrophic food webs.