Modern stromatolite phototrophic communities: a comparative study of procaryote and eucaryote phototrophs using variable chlorophyll fluorescence

Stromatolites are laminated organosedimentary structures formed by microbial communities, principally cyanobacteria although eucaryote phototrophs may also be involved in the construction of modern stromatolites. In this study, productivity and photophysiology of communities from stromatolites (laminated) and thrombolites (nonlaminated) were analysed using fluorescence imaging. Sub‐samples of mats were excised at H ighborne C ay, B ahamas, and cross‐sectioned to simultaneously analyse surface, near‐surface (1–2 mm), and deeper (2–10 mm) communities. Rapid light curve parameters and nonphotochemical downregulation showed distinct differences between phototroph communities, consistent with the reported quasi‐succession of classic stromatolite mat types. Greater productivity was shown by cyanobacteria in Type 1 and Type 3 mats (first and final stage of the succession, S chizothrix gebeleinii and S olentia sp. respectively) and lower productivity within Type 2 mats (intermediate mat type). Eucaryote mat types, dominated by stalked ( S triatella sp. and L icmophora sp.) and tube‐dwelling (e.g. N itzschia and N avicula spp.) diatoms, showed greater productivity than cyanobacteria communities, with the exception of S triatella (low productivity) and an unidentified coccoid cyanobacterium (high productivity). Findings indicate comparative variability between photosynthetically active procaryote and eucaryote sub‐communities within stromatolites, with a pattern logically following the succession of ‘classic’ mat types, and lower than the productivity of eucaryote dominated ‘nonclassic’ mat types.