Microbial mats are important components of perennially ice-covered Antarctic lakes in the McMurdo Dry Valleys, where they often comprise the dominant biomass in this cold, shaded environment. These lakes represent some of the most extreme lacustrine environments on Earth, including a persistent ice-cover, stable, stratified water columns, with strong salinity related density gradients. In these low-disturbance environments, the microbial consortia develop macroscopic emergent structures such as pinnacles and ridges. Such structures are speculated to confer advantageous survival traits and have also been found in the Precambrian fossil record as “conophyton” stromatolites – arguably some of the earliest evidence of life – and it has been suggested that a better understanding of the growth dynamics of modern “conophyton” will inform our understanding of what was required for these early fossils to be produced.

Despite decades of research, there are few studies of the structural basis of conophyton producing microbial mats in Antarctic lakes. To help address this gap, complex microbial mats along a transect established in Lake Fryxell, one of the McMurdo Dry Valley lakes, were examined; with the aim of documenting the distribution of different types of photosynthetic organisms and mat morphologies along environmental gradients such as light, conductivity, oxygen concentration and depth. Microbial mat samples were taken along the transect and analysed in New Zealand using confocal laser scanning microscopy, along with conventional pigment extraction techniques.

Correlations between mat morphology, pigment content and lake properties were found. The appearance of bacteriochlorophylls, characteristic of green sulfur bacteria within and below the oxycline confirm a shift from aerobic to anaerobic metabolism that was consistent with mats taking on a prostrate appearance. The cyanobacterial pigment phycoerythrin was only found in the hyperoxic, relatively well lit region of the transect, and was associated with the mats forming a distinctive macroscale morphology with dense fields of cm-scale cuspate pinnacles. Between these, a hypoxic region was characterised by a relatively flat mat within which were many cm-scale pits. Structural investigations were focussed on two distinct morphologies, pinnacle and honeycomb mat. Nearly all photosynthetic organisms were located in the upper 2 mm of both pinnacle and honeycomb mats, mainly comprising diatoms and cyanobacteria. Pinnacle mats were dominated by a narrow cyanobacterium, probably a species of Leptolyngbya, which were mostly oriented vertically, which placed trichomes parallel to the direction of pinnacle extension. The honeycomb mat contained fewer narrow trichomes, rather the bulk of photosynthetic organisms were diatoms from the genera Muellaria, Navicula and Diadesmis, together with broad-trichome cyanobacteria that formed a thin skin on the surface of the mat, though absent from the pits. The type of emergent structure that is formed appears to be related to species composition, and this in turn appears to be related to the growth conditions. A model was developed to explain how species-specific growth mechanisms are involved in emergent structure formation for honeycomb and pinnacle mats.