A colony-forming microorganism with probable affinities to the Chroococcales (cyanobacteria) from the Lower Devonian Rhynie chert

Highlights

• An Early Devonian colony-forming microorganism from the Rhynie chert is described.

• Square groups and sarcinoid packages of cells represent basic modular units in colony construction.

• Fossils are morphologically similar to extant cyanobacteria, especially Cyanosarcina and Myxosarcina.

• Discovery expands knowledge of primary producer diversity in the Rhynie paleoecosystem.

Abstract

The Rhynie chert has provided a wealth of information on Early Devonian non-marine aquatic and terrestrial life. However, our understanding of the microbial biodiversity in this hot spring paleoecosystem remains incomplete. In this paper we describe Rhyniosarcina devonica nov. gen. et sp., a microorganism from the Rhynie chert that forms cubical, spheroidal, or irregular colonies of up to several hundred densely packed cells (< 2.5–5.5 μm in diameter). Square groups of four cells and sarcinoid packages of eight cells represent the basic modular units in the construction of the colonies. Cell division is by binary fission in three planes in space; new colonies form by colony dissociation. Rhyniosarcina devonica is interpreted as a cyanobacterium with affinities to the Chroococcales based on morphological similarities to extant colony-forming chroococcalean cyanobacteria, especially members in the genus Cyanosarcina. This discovery expands our knowledge of the diversity of primary producers in the Rhynie chert.

Introduction

The discovery of the Lower Devonian Rhynie chert represents a critical point in paleobiology. The site was initially made famous by Kidston and Lang, 1917, Kidston and Lang, 1920a, Kidston and Lang, 1920b, Kidston and Lang, 1921a, Kidston and Lang, 1921b describing early land plants, algae, fungi, nematophytes, bacteria, and cyanobacteria. Since the initial report, additional details about the land plants have been documented (e.g., Edwards, 1986, Roth-Nebelsick et al., 2000, Daviero-Gomez et al., 2005, Kerp et al., 2013), and in several instances the complete life history has been reconstructed (Taylor et al., 2005, Kerp and Hass, 2004, Kerp and Hass, 2009). Moreover, several invertebrate animals (surveyed in Fayers and Trewin, 2003) and fungus-like peronosporomycetes (e.g., Krings et al., 2012, Krings et al., 2013) have been described that further expanded our understanding of the diversity within this ancient community.

As these studies have progressed, new areas of research have explored interactions that occurred among the Rhynie chert organisms. For example, it is now understood that some of the land plants were associated with mycorrhizal fungi (Remy et al., 1994, Taylor et al., 1995, Strullu-Derrien et al., 2014), while other fungi were parasites of land plants (Taylor et al., 1992b). Within the chert is also evidence of lichen-like symbioses (Taylor et. al., 1997), pathogens that altered plant growth and development (Taylor et al., 1992a, Krings et al., 2007b), and several different food webs (Habgood et al., 2003). Moreover, unicellular algae and cyanobacteria in the Rhynie chert suggest that diverse photoautotrophs functioned as primary producers at the base of the food chain (e.g., Edwards and Lyon, 1983, Dotzler et al., 2007, Kustatscher et al., 2014a, Kustatscher et al., 2014b).

Primary producers are instrumental to the functioning of modern aquatic ecosystems, and thus figure prominently in aquatic ecology research (Zehr, 2010). Although photoautotrophic microorganisms were no doubt also important in the Rhynie paleoecosystem and must have occurred in large quantities (see Golubic, 1980), these life forms have not received much attention. This lack of documented evidence is due primarily to the fact that the classification of microscopic algae and cyanobacteria largely relies on features that are difficult, if not impossible to document from fossils (Golubic and Seong-Joo, 1999, Dotzler et al., 2007). As a result, the precise systematic affinities of fossil forms cannot normally be resolved through comparison with modern analogues. Nevertheless, some Rhynie chert microorganisms display complements of structural features that, albeit not diagnostic, are consistent among specimens, and thus make it possible to recognize distinctiveness.

In this paper we describe a conspicuous, colony-forming microorganism from the Rhynie chert, and document stages in the growth and pattern of colony development based on data from several hundred specimens. The fossils are interpreted as a cyanobacterium, likely with affinities to the Chroococcales based on morphological similarities to several extant colony-forming chroococcalean cyanobacteria, especially members in the genus Cyanosarcina Kováčik. This discovery expands our knowledge of the diversity of primary producers in the Early Devonian Rhynie paleoecosystem.