Lichen symbiont interfaces revisited: ultrastructure of intraparietal contacts between fungal and algal cells in several microlichens with non-trebouxialean chlorobionts

Abstract This work attempts to better understand the significance of morphological diversity among fungal-algal contact zones present in lichens. We used TEM to examine a variety of lichen symbioses involving non-trebouxialean green algae that show intraparietal penetration by the mycobiont. A principal focus was on Endocarpon pusillum , a well-known member of a family ( Verrucariaceae ; Eurotiomycetes ) previously reported to be characterized by unwalled haustoria exposing a naked fungal protoplast. Peg-like haustoria arose from an inner layer(s) of the mycobiont cell wall that broke through outer layers and penetrated a short distance into the wall of the green algal symbiont ( Diplosphaera ). In both fungal and algal cells at the contact interface, lomasome-like vesicles and tubules occurred as modifications of the plasmalemma intermixed with wall materials at the inner surface of the cell wall. A fungal cell wall was consistently present around the haustorium, which resembled those depicted in earlier TEM studies of Verrucariaceae. Previously published micrographs of Verrucariaceae purporting to show wall-less haustoria surrounded by an empty space are believed to have been misinterpreted. However, in the isidiose Porina and foliicolous Calopadia , Byssoloma and Fellhanera species ( Lecanoromycetes ), we did observe extreme degrees of reduction in the mycobiont cell wall at symbiont contact interfaces. In those lichens, a broad area of the fungal cell bulged into the adjacent algal symbiont, broadly invaginating the wall of the latter and penetrating it intraparietally without differentiation of a distinct haustorial structure. The mycobiont wall surrounding such protrusions often thinned to near indistinguishability towards its extremity. The protrusion made direct contact with the algal cell wall; no empty space occurred between them. We propose that the short, peg-like intraparietal haustoria bind the symbionts and help maintain cell contacts amid the stresses of tissue expansion and shrinkage, thereby avoiding disruption of the continuous hydrophobic coating that facilitates transfer between them. Broader contact interfaces with extremely thin adjacent walls may facilitate solute flow between symbionts. Reciprocal penetration of algal protrusions into mycobiont cells, noted in Porina as well as other lichens studied previously, is a neglected but potentially significant indication that both symbionts may actively work to maintain functional contact interfaces.