Epilithic Cyanobacterial Communities of a Marine Tropical Beach Rock (Heron Island, Great Barrier Reef): Diversity and Diazotrophy

Abstract

The diversity and nitrogenase activity of epilithic marine microbes in a Holocene beach rock (Heron Island, Great Barrier Reef, Australia) with a proposed biological calcification "microbialite" origin were examined. Partial 16S rRNA gene sequences from the dominant mat (a coherent and layered pink-pigmented community spread over the beach rock) and biofilms (nonstratified, differently pigmented microbial communities of small shallow depressions) were retrieved using denaturing gradient gel electrophoresis (DGGE), and a clone library was retrieved from the dominant mat. The 16S rRNA gene sequences and morphological analyses revealed heterogeneity in the cyanobacterial distribution patterns. The nonheterocystous filamentous genus Blennothrix sp., phylogenetically related to Lyngbya, dominated the mat together with unidentified nonheterocystous filaments of members of the Pseudanabaenaceae and the unicellular genus Chroococcidiopsis. The dominance and three-dimensional intertwined distribution of these organisms were confirmed by nonintrusive scanning microscopy. In contrast, the less pronounced biofilms were dominated by the heterocystous cyanobacterial genus Calothrix, two unicellular Entophysalis morphotypes, Lyngbya spp., and members of the Pseudanabaenaceae family. Cytophaga-Flavobacterium-Bacteroides and Alphaproteobacteria phylotypes were also retrieved from the beach rock. The microbial diversity of the dominant mat was accompanied by high nocturnal nitrogenase activities (as determined by in situ acetylene reduction assays). A new DGGE nifH gene optimization approach for cyanobacterial nitrogen fixers showed that the sequences retrieved from the dominant mat were related to nonheterocystous uncultured cyanobacterial phylotypes, only distantly related to sequences of nitrogen-fixing cultured cyanobacteria. These data stress the occurrence and importance of nonheterocystous epilithic cyanobacteria, and it is hypothesized that such epilithic cyanobacteria are the principal nitrogen fixers of the Heron Island beach rock.

Mapping phytoplankton blooms in deep subalpine lakes from Sentinel-2A and Landsat-8

“…In the central part of the Lake Maggiore, two specimens revealed the presence of several cyanobacteria mostly belonging to Oscillatoriales. The blooms were mainly due to Pseudanabaena spp., an unusual condition in Lake Maggiore, as this taxon commonly blooms in eutrophic water bodies (Mayer et al, 1997;Zwart et al, 2005); nonetheless, its characteristics are poorly investigated (Acinias et al, 2009) even widely distributed as it occurs in diverse aquatic as well as in benthic environments (Castenholz et al, 2001;Zwart et al, 2005;Diez et al, 2007). In the two samples collected in the central lake station, several species belonging to Oscillatoriales have contributed to the algal bloom, among which Pseudanabaena dominated, both in terms of density (68.6 and 57.0%, respectively) and biomass (47.2 and 52.6%).…”

Desiccation stress in two intertidal beachrock biofilms

“…DGGE based 16S rRNA analyses of microbial diversity revealed that the pink and the black biofilms were the 311 most genetically distinct of all the beachrock communities on Heron Island (Diez et al 2007). Of particular 312 interest, the black biofilm is dominated by heterocystous diazotrophs, whereas non-heterocystous cyanobacteria 313 dominate the pink biofilm (Diez et al 2007).…”

“…Like all intertidal 51 environments, the beachrock habitat is an environment of extreme conditions, being exposed to strong 52 insolation, extreme temperatures, periodic desiccation and concurrent hypersalinity stress in ponded regions of 53 the rock platform during low tide and air exposure. Such intertidal habitats are extremely diverse, as they 54 provide a steep environmental gradient over a small spatial scale (Davison and Pearson 1996), exhibiting 55 distinct transitions of diverse epilithic and endolithic microbial communities (Diez et al 2007). …”

“…2a). Additionally, the black biofilm is dominated by a layer of filamentous cyanobacteria 300 (Diez et al 2007). These long filaments provide a greater surface area and therefore greater potential for air 301 exchange, enhancing the rate with which desiccation and similarly, rehydration could occur.…”

“…Of particular 312 interest, the black biofilm is dominated by heterocystous diazotrophs, whereas non-heterocystous cyanobacteria 313 dominate the pink biofilm (Diez et al 2007). The potential difference in nitrogenase activity between the two 314 biofilms could help to explain the differences in the photosynthetic strategies they employ during desiccation.…”