Marine planktonic ciliates that prey on macroalgae and enslave their chloroplasts
Limnol. Oceanogr., 49(1), 2004, 308-313 | DOI: 10.4319/lo.2004.49.1.0308
ABSTRACT: We found two tide-pool ciliates, Strombidium oculatum and Strombidium stylifer, that ingest ulvaceous green macroalgae and retain their chloroplasts. Sequences of the form I ribulose bisphosphate carboxylase/oxygenase (rubisco) large subunit gene (rbcL1) from chloroplasts found in the ciliates cluster with the Ulvophyceae sequences on GenBank and with those of ulvaceous macroalgae from pools in which the ciliates were collected. In addition, we have cultivated S. stylifer in vitro on filtered seawater supplemented with pieces of Enteromorpha thalli that had been treated with light and temperature shock to maximize production of reproductive unicells (swarmers). An average growth rate of 1.08 ± 0.07 SE [standard error] d-1 was measured when S. stylifer was grown this way. Because both ciliates and the algal swarmers contain eyespots while vegetative cells in the Ulvophyceae do not, we speculate that these reproductive cells are the source of both the chloroplasts and the pigments used in the ciliates eyespots. This ciliate will not grow in the dark and is required to ingest fresh chloroplasts every few days, making it an obligate mixotroph. Ingestion of macroalgae by ciliates constitutes an upside-down trophic link, contrary to the usual pattern for planktonic food webs, in which production passes from very small organisms to successively larger ones. Our finding suggests that macroalgal production, heretofore believed to contribute predominantly to detrital or macroherbivore food chains, may be an important food source in the nearshore plankton and that ciliates may play an important role in this trophic pathway.