Large size can have its advantages. Pound for pound, a large organism is often more energy-efficient than a small one. A larger organism may have an edge in competing for resources. Larger organisms may be less at risk from predation by other organisms. Therefore, it follows that even when life was a single-cell only affair, things would eventually get bigger. However, large size can also have its disadvantages. If we imagine (for the sake of argument) that the original single cells were roughly spherical, then increase in overall volume occurs much faster than overall surface area - exponentially faster, in fact. As the ratio of surface area to volume decreases, the efficiency of movement of nutrients and other other respiratory requirements into the cell and waste products out of the cell also decreases, until eventually a point is reached where it is no longer possible to move stuff in and out of the cell enough to meet the cell's requirements for survival. At this point, the organism essentially has only one of two options if it wishes (speaking metaphorically, of course) to keep growing. One is the option that our far-distant ancestors took - going multicellular, so that instead of being one large, inefficient sphere the organism can be many, more efficient spheres. The other option is arguably simpler - why stick to a sphere? Why not go filamentous? This week's highlight taxon belongs to one group of organisms that did just that - the bacterial family Streptosporangiaceae.
The filamentous option has actually been taken up by more clades of organisms than has true multicellularity. In an evolutionary sense, it's much easier - multicellular organisms require a host of measures to hold cells together and transport materials between them. Filamentous organisms, at the most basic level, simply need to grow along one geometrical axis faster than they grow along the others. Indeed, filamentous organisms rather blur the line between "multicellular" and "unicellular". Many of the larger filamentous organisms such as have cross-membranes dividing sections of hyphae, but they don't necessarily have to. Fungi and xenophyophores are both groups of organisms made up of masses of filaments, and is there any real reason other than comparative tradition for one to be regarded as multicellular and one as unicellular? (Does the concept of a "cell" even really apply when discussing hyphal organisms?) Bacteria are usually imagined as being unicellular, but many members of two bacterial groups, the cyanobacteria (blue-green algae) and the somewhat slime-mould-like Myxococcales, are multicellular for at least part of their life cycles. In contrast, the group of bacteria that made a go of the filamentous life-style was the Actinobacteria, the group to which the Streptosporangiaceae belong. Actinobacteria (or actinomycetes) include a diverse range of bacteria, both filamentous and not so (another subgroup of the Actinobacteria, the Corynebacterineae, has previously been covered here and here). Because of our shaky understanding of bacterial phylogenetics, it's difficult to say anything truly meaningful about the evolution of the filamentous habit in Actinobacteria, but it has most likely been lost and regained a number of times in the group. Some of the Actinobacteria have developed the hyphal habit to the extent that they are superficially ver similar to fungi, and indeed have been classified as such in the past.
The Streptosporangiaceae include some of the more mould-like actinobacteria. Like fungi, Streptosporangiaceae form branching, non-fragmenting hyphae. Most (but not all) members produce aerial mycelia, on the ends of which are produced the spores. Rather unusually among bacteria, the various genera can mostly be distinguished by morphological features related to the production of sporangia and arrangement of spores (Tamura et al., 2000), leading to many of them being give such names as Planomonospora or Microtetraspora. Planotetraspora, for instance, bears long, cylindrical sporangia containing four spores in a single row (Tamura & Sakane, 2004), while Acrocarpospora produces spherical or club-shaped sporangia containing coiled spore chains (Tamura et al., 2000). The distinctive appearance of Streptosporangiaceae also means that they are one of the few non-cyanobacterial prokaryote groups with an established fossil record, thanks to the description of Streptosporangiopsis from Cretaceous amber (Waggoner, 1994).
While widespread in soils around the world, the Streptosporangiaceae have often been regarded as rare compared to other groups. As methods of culturing them have improved, however, it has been suggested that Streptosporangiaceae are not so much rare as slow-growing. When soil samples are cultured in the laboratory, they are unable to compete with faster-growing taxa (Lazzarini et al., 2000), and their isolation requires the application of stressors that they can handle better than their competitors, such as heat-drying or chlorinating compounds. Despite the difficulty of culturing them, it has been suggested that doing so would be worthwhile - Streptosporangiaceae have proven a promising target in research into isolating new antibiotic compounds.
Lazzarini, A., L. Cavaletti, G. Toppo & F. Marinelli. 2000. Rare genera of actinomycetes as potential producers of new antibiotics. Antonie van Leeuwenhoek 78 (3-4): 399-405.
Tamura, T., & S. Sakane. 2004. Planotetraspora silvatica sp. nov. and emended description of the genus Planotetraspora. International Journal of Systematic and Evolutionary Microbiology 54 (6): 2053-2056.
Tamura, T., S. Suzuki & K. Hatano. 2000. Acrocarpospora gen. nov., a new genus of the order Actinomycetales. International Journal of Systematic and Evolutionary Microbiology 50 (3): 1163-1171.
Waggoner, B. M. 1994. Fossil microorganisms from Upper Cretaceous amber of Mississippi. Review of Palaeobotany and Palynology 80 (1-2): 75-84.