Commentaries on Cyanobacterial Symbioses: Introduction and Overview

Abstract

Bergman, Botany Department, University of Stockholm, SE-1091, Sweden In recent years there has been a resurgence of interest in symbioses To some extent, this is a result of the more widespread recognition, cham pioned in recent populist texts (Sapp 1994, Margulls 1998; Wakeford 2001), that all eukaryotic organisms are almost certainly a consequence of the merger of two or more prokaryotic partners Recent evidence indicates that this probably in volved a single pnmary endosymbiotic event some 2 billion years ago whereas eukaryotes symbiotic with cyanobacteria evolved more recently, approxi mately 600 million years ago Symbioses, particu larly those involving cyanobacteria, are also very common and an important feature of major ecosystems worldwide To a large extent, the orngins and continued existence of life on earth can be viewed as a consequence of the sustained ability of organisms to form symbiotic partnerships Sym biotic associations are the rule rather than the exception. The following short communications are the outcome of keynote papers presented at a com bined workshop and summer school held at Ballyvaughan, Co. Clare, from 27 August to 3 September 2000, as part of a five-year European Science Foundation scientific research programme on cyanobactenal nitrogen fixation, with the acronym CYANOFIX Although the majority of the papers deal specifically with cyanobactenal symbioses, they have been supplemented by two papers on bacterial symbioses-one on signalling, the other on oxygen protection mechanisms-and another paper on the control of heterocyst differ entiation in free-living cyanobacteria. All are of fundamental importance for the establishment and maintenance of functional nitrogen-fixing partner ships An understanding of the way in which heterocyst frequency is regulated is one of the key questions associated with the establishment of ni trogen-fixing cyanobacterial associations This meeting was an attempt to highlight the past importance, current significance and future potential of symbioses involving cyanobacteria It was also an attempt, in part, to correct an imbal ance in emphasis that has existed in favour of investigations of bacterial, rather than cyanobac terial, nitrogen-fixing symbioses In contrast to the resources directed towards research on bacterial nitrogen-fixing symbioses over the past two decades, considerably fewer resources have been directed towards investigations of cyanobactenal nitrogen-fixing symbioses, particularly those in volving plants In retrospect, as these commentaries show, this is even more surpnsing, given that cyanobacterial symbioses with plants, in particular, involve a diverse range of host species (Rai et al 2000), are of ancient ongin, occupy a wide range of habitats and, based on evidence presented in these commentaries, have considerable agrononuc potential Although an obvious goal of future in vestigations must be to increase our understanding of cyanobacterial symbioses, this needs to be placed into some kind of context with information on other symbioses, at molecular, biochemical and physiological levels, if we are to provide a compre hensive understanding of the evolutionary and functional significance of symbiotic partnerships Clearly, there are common threads underlying all symbioses For instance, host control of symbiont metabolism, recognition and signalling processes and the evolution of association-specific structures are likely to be features common to all symbioses. For cyanobacterial symbioses, in particular, our understanding of these processes, controls or struc tures is still in its infancy In most cases, the extent and ecological role of cyanobacterial nitrogen-fixing symbioses is un known, particularly in the oceans Recent research on free-living nitrogen-fixing cyanobacteria in marine habitats suggests that their contribution to nitrogen-cycling is of considerable significance New research has revealed a plethora of cyanobac terial associations, which could extend considerably the role of cyanobactena in nitrogenand carbon cycling in oceanic ecosystems, although their sig nificance is not known Elucidation of the ecological importance of these and other symbioses wil depend on the development of appropriate technologies for investigating carbon and nitrogen metabolism and for understanding the way in which a wide range of metabolic processes are integrated in a functional symbiosis Cyanobacteria are clearly ubiquitous and form intimate associ ations with a wide range of hosts-phytoplankton, plants, animals and fungi-that can coexist suc cessfully in diverse terrestrial, marine and fresh