Large proportion of marine planktonic ciliates found to contain functional chloroplasts

Abstract

Planktonic ciliates, most importantly tintinnids and oligotrichs (subclass Choreotrichia), are important components of the micro-plankton in coastal waters¹. At times, the biomass of ciliates is equivalent to the biomass of larger zooplankters². The role of ciliates as grazers has been recognized (see ref. 3 for review), but their role as chloroplast-bearing cells has not been widely appreciated⁴. Many planktonic oligotrichs are pigmented because they sequester chloroplasts derived from a variety of chromophytic algae^5–7. We report the first quantitative data on the frequency of chloroplast retention among planktonic oligotrichs and tintinnids. In surface waters during the spring and summer, approximately 42% of the planktonic ciliates have chloroplasts. During the autumn and winter, chloroplast-retaining species are less abundant, but still constitute at least 10% of the combined tintinnid and oligotrich fauna. Water samples containing large numbers of pigmented ciliates have a high chlorophyll content⁸ and fix carbon⁹. Although it has been speculated that sequestered chloroplasts in ciliates are functional^5–7, we have demonstrated photosynthesis by chloroplast-containing ciliates experimentally. The trophic position of ciliates in planktonic food webs needs to be reconsidered.