Nanoarchaeum equitans and Ignicoccus hospitalis: new insights into a unique, intimate association of two Archaea

Lehrstuhl für Mikrobiologie und Archaeenzentrum, Universität Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany; Center for Electron Microscopy, Faculty of Natural Sciences III, Universität Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany; Lehrstuhl für Biochemie, Technische Universität München, Lichtenbergstr. 4, D-85748 Garching, Germany

^* To whom correspondence should be addressed. Email: Harald.huber{at}biologie.uni-regensburg.de.

	Abstract

Nanoarchaeum equitans and Ignicoccus hospitalis represent a unique, intimate association of two Archaea. Both form a stable coculture which is mandatory for N. equitans but not for the host I. hospitalis. Here, we investigated interactions and mutual influence between these microorganisms. Fermentation studies revealed that during exponential growth only about 25% of I. hospitalis cells are occupied with N. equitans (one to three cells). The latter strongly proliferate in the stationary phase of I. hospitalis, until 80 to 90% of the I. hospitalis cells carry around ten N. equitans cells. Furthermore, the expulsion of H₂S, the major metabolic end product of I. hospitalis, by strong gas stripping yields huge amounts of free N. equitans cells. N. equitans had no influence on doubling times, final cell concentrations, and growth ranges or optima (temperature, pH, salt concentration) of I. hospitalis. However, isolation studies using the optical tweezers revealed that infection with N. equitans inhibited proliferation of individual I. hospitalis cells. This inhibition might be caused by the deprivation of cell components like amino acids from the host as demonstrated by ¹³C labeling studies. The strong dependence of N. equitans on I. hospitalis was affirmed by Live/Dead stains and electron microscopic analyses which indicate a tight physiological and structural connection between both microorganisms. No alternative hosts, including other Ignicoccus species, were accepted by N. equitans. In summary, the data show a highly specialized association of N. equitans and I. hospitalis which so far cannot be assigned to a classical symbiosis, commensalism, or parasitism.