The marine lichen Lichina confinis (O. F. Müll.) C. Ag.: ultrastructure and localization of nitrogenase, glutamine synthetase, phycoerythrin and ribulose 1, 5‐bisphosphate carboxylase/oxygenase in the cyanobiont

Abstract

SUMMARYThallus ultrastructure and the localization of proteins involved in photosynthesis, N^ fixation and animonia assimilation was studied in the Lichina confinis (O. F. Mull) C. Ag.-Calothrix symbiosis. The lichen thallus showed profuse branching. Within this, the cyanobiont was located in the cortical zone. Haustoria were frequently observed in contact with vegetative cells of the cyanobiont but not with heterocysts. An extensive mucilage layer containing numerous vesicles occurred around cyanobiont cells. Immunolabelling showed nitrogenase to be exclusively located in heterocysts of the cyanobiont and the free-living Calothrix sp. Glutamine synthetase (GS) levels in the cyanobiont were found to be drastically decreased compared to the levels in free-living (cultured) Calothrix sp. Such a decrease was more prominent in mature parts of the thallus than in the apical region. Rubisco was mostly located in carboxysomes of the vegetative cells of the cyanobiont. There were relatively low levels of Rubisco in tbe cytoplasm of vegetative cells, wbile heterocysts sbowed negligible levels. In free-living (cultured) Calothrix sp., pbycoerytbrin (PE) was found to be located along tbe tbylakoid membranes both in vegetative cells and beterocysts. Cyanobiont cells sbowed a similar pattern but tbe levels were relatively lower.

Tansley Review No. 116

“…The latter continues even when the cyanobiont population becomes constant (leaf numbers 15-28) (Hill, 1977(Hill, , 1989Braun-Howland & Nierzwicki-Bauer, 1990). A similar pattern is found from young to mature parts of the lichen thallus (Rowell et al, 1985 ;Hill, 1989 ;Rai, 1990c ;Janson et al, 1993), the coralloid roots of cycads , Gunnera plants ( So$ derba$ ck et al, 1990 ;Bergman et al, 1992a) and the thalli of liverworts\hornworts Meeks, 1990). Slime cavities, coralloid roots or stem glands are produced as a fixed proportion of the host biomass, and the cyanobiont population in them is controlled (Halliday & Pate, 1976 ;Stewart et al, 1983 ;Meeks, 1990 ;So$ derba$ ck et al, 1990 ;Steinberg & Meeks, 1991 ;Osborne et al, 1992 ;Meeks et al, 1999).…”

“…The decrease varies from young to more mature symbiotic tissues or organs in a developing symbiosis (higher decrease in older parts), and a maximum decrease in GS might coincide with maximum nitrogenase activity and ammonia release (Rowell et al, 1985 ;Hill, 1989 ;Rai, 1990a ;Bergman et al, 1992b). The decrease in GS activity of the cyanobiont was attributed to the repression of GS synthesis in lichens (Rai, 1990c ;Janson et al, 1993) and to low rates of glnA transcription in Azolla (Braun-Howland & Nierzwicki-Bauer, 1990), but to post-translational modification of the enzyme in Anthoceros (Meeks, 1990). The cyanobiont populations used in these studies were mixtures of cells from both young and old parts.…”

“…Thinning of cell walls and development of fungal haustoria or haustoria-like protrusions with TCU have been observed. In some cases these penetrate the sheath of cyanobiont vegetative cells, but not the heterocysts (Smith & Douglas, 1987 ;Rai 1990c ;Honegger, 1991 ;Janson et al, 1993). The mycobiont is the dominant partner constituting the bulk of the thallus.…”

“…If this results from microaerobiosis, nif genes might be contiguous in vegetative cells of some of the other cyanobionts too. However, despite microaerobiosis and the contiguous nif gene arrangement in vegetative cells (at least in Azolla), nitrogenase has been found to be located exclusively in heterocysts So$ derba$ ck et al, 1990 ;Janson et al, 1993Janson et al, , 1995. Thus, heterocysts seem to be essential for symbiotic N # fixation, and features such as enhanced sugar catabolism (higher activity of enzymes of the oxidative pentose phosphate pathway and higher rates of respiration ; Wolk et al, 1994) and the provision of reductant and ATP to nitrogenase might be more important than protection against oxygen damage.…”

“…In the symbiotic tissue, the filamentous habit of the cyanobiont tends to become aseriate and sometimes a filamentous cyanobiont seems unicellular. Other changes include an increase in cell size, altered cell shape (more rounded), lack of polyphosphate granules, fewer carboxysomes, less sheath material and glycogen, thinner cell wall and altered cell surface antigens and, in some cases, altered thylakoid arrangements (Rai, 1980(Rai, , 1990aStewart et al, 1983 ;Galun & Bubrick, 1984 ;Hill, 1989Hill, , 1994Janson et al, 1993Janson et al, , 1995Schussler et al, 1996 ;Ja$ ger et al, 1997). The growth and cell division of the cyanobiont are significantly lower in symbiotic tissue than in culture.…”