Unusual biogenic calcite structures in two shallow lakes, James Ross Island, Antarctica

J Elster,K Láska,J Komárek,J Haloda,R Vodrážka,L Nedbalová

doi:10.5194/bg-13-535-2016

Abstract

Abstract. The floors of two shallow endorheic lakes, located on volcanic surfaces on James Ross Island, are covered with calcareous organosedimentary structures. Their biological and chemical composition, lake water characteristics, and seasonal variability of the thermal regime are introduced. The lakes are frozen down to the bottom for 8–9 months a year and their water chemistry is characterised by low conductivity and neutral to slightly alkaline pH. The photosynthetic microbial mat is composed of filamentous cyanobacteria and microalgae that are considered to be Antarctic endemic species. The mucilaginous black biofilm is covered by green spots formed by a green microalga and the macroscopic structures are packed together with fine material. Thin sections consist of rock substrate, soft biofilm, calcite spicules and mineral grains originating from different sources. The morphology of the spicules is typical of calcium carbonate monocrystals having a layered structure and specific surface texture, which reflect growth and degradation processes. The spicules' chemical composition and structure correspond to pure calcite. The lakes' age, altitude, morphometry, geomorphological and hydrological stability, including low sedimentation rates, together with thermal regime predispose the existence of this community. We hypothesise that the precipitation of calcite is connected with the photosynthetic activity of the green microalgae that were not recorded in any other lake in the region. This study has shown that the unique community producing biogenic calcite spicules is quite different to any yet described.

Highlights

The floors of most Antarctic lakes are covered with photosynthetic microbial mats (Vincent and Laybourn-Parry, 2008)
The lakes are frozen down to the bottom for 8– 9 months a year and their water chemistry is characterised by low conductivity and neutral to slightly alkaline pH
This study has shown that the unique community producing biogenic calcite spicules is quite different to any yet described

Summary

Introduction

The floors of most Antarctic lakes are covered with photosynthetic microbial mats (Vincent and Laybourn-Parry, 2008). When growing in low-disturbance habitats, interactions between benthic microbial communities and their environments can produce complex emergent structures. Such structures are best developed in extreme environments, including benthic communities of deep, perennially ice-covered Antarctic lakes, where physical and chemical conditions and/or geographical isolation preclude the development of larger organisms that could otherwise disrupt organised microbial structures (Wharton, 1994; Andersen et al, 2011). In perennially ice-covered lakes, the seasonality of growth imposed by the summer–winter light–dark conditions can induce annual growth laminations (Hawes et al, 2001), reinforced by calcite precipitation during growth and sediment diagenesis (Wharton et al, 1982; Wharton, 1994; Sutherland and Hawes, 2009). A diversity of micro- to nanostructured CaCO3 associated with extracellular polymeric substances and prokaryotes was de-

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