Cyanobacterial diversity in natural and artificial microbial mats of Lake Fryxell (McMurdo Dry Valleys, Antarctica): a morphological and molecular approach

Abstract

Currently, there is no consensus concerning the geographic distribution and extent of endemism in Antarctic cyanobacteria. In this paper we describe the phenotypic and genotypic diversity of cyanobacteria in a field microbial mat sample from Lake Fryxell and in an artificial cold-adapted sample cultured in a benthic gradient chamber (BGC) by using an inoculum from the same mat. Light microscopy and molecular tools, including 16S rRNA gene clone libraries, denaturing gradient gel electrophoresis, and sequencing, were used. For the first time in the study of cyanobacterial diversity of environmental samples, internal transcribed spacer (ITS) sequences were retrieved and analyzed to complement the information obtained from the 16S rRNA gene. Microscopy allowed eight morphotypes to be identified, only one of which is likely to be an Antarctic endemic morphotype. Molecular analysis, however, revealed an entirely different pattern. A much higher number of phylotypes (15 phylotypes) was found, but no sequences from Nodularia and Hydrocoryne, as observed by microscopy, were retrieved. The 16S rRNA gene sequences determined in this study were distributed in 11 phylogenetic lineages, 3 of which were exclusively Antarctic and 2 of which were novel. Collectively, these Antarctic sequences together with all the other polar sequences were distributed in 22 lineages, 9 of which were exclusively Antarctic, including the 2 novel lineages observed in this study. The cultured BGC mat had lower diversity than the field mat. However, the two samples shared three morphotypes and three phylotypes. Moreover, the BGC mat allowed enrichment of one additional phylotype. ITS sequence analysis revealed a complex signal that was difficult to interpret. Finally, this study provided evidence of molecular diversity of cyanobacteria in Antarctica that is much greater than the diversity currently known based on traditional microscopic analysis. Furthermore, Antarctic endemic species were more abundant than was estimated on the basis of morphological features. Decisive arguments concerning the global geographic distribution of cyanobacteria should therefore incorporate data obtained with the molecular tools described here.

FIG. 1.

Diversity of cyanobacterial morphotypes identified in the artificial and natural microbial mats. (A) Nostoc sp.; (B) Hydrocoryne cf. spongiosa; (C) Nodularia cf. harveyana; (D) Leptolyngbya sp. 1; (E) Leptolyngbya sp. 2; (F) Phormidium cf. autumnale; (G) Oscillatoria cf. subproboscidea; (H) Schizothrix sp.

FIG.2.

Neighbor-joining tree based on partial 16S rRNA gene sequences corresponding to E. coli positions 405 to 780, with the exception of the sequences of the uncultured Antarctic cyanobacteria CLEAR-10, PENDANT-2, −4, and QSSC (E. coli positions 519 to 780). The tree includes the 49 sequences of clones and DGGE bands determined in the present study (boldface, italic, underlined type), 125 previously published sequences (Antarctic sequences are in boldface italic type, and Arctic sequences are in boldface roman type), and the E. coli sequence used as an outgroup. Bootstrap values equal to or greater than 70% are indicated at the nodes. The evolutionary distance between two sequences is obtained by adding the lengths of the horizontal branches connecting them and using the scale bar (0.1 mutation per position). Signatures are shown next to the cluster numbers. Abbreviations: Uncult., uncultured; cyanobact., cyanobacterium; Ant., Antarctic; str., strain.