Abstract
The Ikka Fjord (SW Greenland) harbors a unique microbial habitat in the form of several hundred submarine tufa columns composed of ikaite, a special hexahydrate form of calcium carbonate that precipitates when alkaline phosphate- and carbonate-enriched spring water seeping out of the sea floor meets cold seawater. While several unique heterotrophic microbes have been isolated from the tufa columns, the microbial activity, and the boundary conditions for microbial growth in ikaite have remained unexplored. We present the first detailed in situ characterization of the physico-chemical microenvironment and activity of oxygenic phototrophs thriving within the ikaite columns. In situ underwater microsensor measurements of pH, temperature, and irradiance in the porous ikaite crystal matrix, revealed an extreme microenvironment characterized by low temperatures, strong light attenuation, and gradients of pH changing from pH 9 at the outer column surface to above pH 10 over the first 1–2 cm of the ikaite. This outer layer of the freshly deposited ikaite matrix contained densely pigmented yellow and green zones harboring a diverse phototrophic community dominated by diatoms and cyanobacteria, respectively, as shown by amplicon sequencing. In situ O2 measurements, as well as underwater variable chlorophyll fluorescence measurements of photosynthetic activity, demonstrated high levels of oxygenic photosynthesis in this extreme gradient environment with strong irradiance-driven O2 dynamics ranging from anoxia to hyperoxic conditions in the ikaite matrix, albeit the local formation of gas bubbles buffered the day-night dynamics of O2 in the tufa columns. The microbial phototrophs in the ikaite matrix are embedded in exopolymers forming endolithic biofilms that may interact with mineral formation and cementing of ikaite crystals.
Highlights
The Ikka Fjord in southwestern Greenland harbors a unique assemblage of tufa columns found nowhere else on the planet (Buchardt et al, 1997; Seaman and Buchardt, 2006)
Temperature measurements recorded at three different depths in 2013, showed a dynamic temperature variation at the tufa column surface that correlated with tidal changes (Figure 4A)
The uppermost parts of ikaite tufa columns in Ikka Fjord, Greenland, harbor active populations of microalgae and cyanobacteria embedded in exopolymers and stratified into yellowish and green zones, respectively, in the outermost ∼2 cm of the porous ikaite matrix
Summary
The Ikka Fjord in southwestern Greenland harbors a unique assemblage of tufa columns found nowhere else on the planet (Buchardt et al, 1997; Seaman and Buchardt, 2006). Columns formed by the special carbonate mineral ikaite protrude from the seabed in sizes varying from a few centimeters to more than 20 m in height and several meters wide (Pauly, 1963). Using detailed side-scan sonar mapping, Seaman and Buchardt (2006) found 657 columns of >1 m height in Ikka Fjord, and they dated the oldest columns to have formed ∼10,000 years ago. While scientific exploration of these columns is much more recent, the first mention of their existence occurs in old Inuit legends, where the ikaite tufas are regarded as remnants of Norsemen standing on the fjord bottom after falling through the ice during a chase by the Inuit people (Rink, 1866). Further investigation of the columns was not initiated until 1995 and has since continued intermittently through a number of summer and winter fieldtrips encompassing geological, zoological, botanical, and microbiological studies
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