microRNAs (miRNAs) are endogenous non-coding small RNAs (sRNA) that play important regulatory functions in growth, development, and environmental stress response of eukaryotes. Currently, miRNA-size sRNA (msRNAs) was discovered in several prokaryotes through deep sequencing. However, no data are available on whether msRNAs exist in cyanobacteria and whether they regulate biological tolerance to salt stress. In this study, three small RNA libraries were constructed from control (0.02 M NaCl), medium- (0.3 M NaCl), and high-salt treatments (0.5 M NaCl) of Spirulina platensis. After sequencing using a high-throughput Illumina Solexa system, nine msRNAs with msRNA* were identified, and 21 candidate msRNAs showed significantly differential expression under salt-stress conditions. Seven of the selected msRNAs presented the consistent expression trends when compared with their deep sequencing results as verified by quantitative real-time polymerase chain reaction (qRT-PCR), demonstrating that the expression analyses for msRNAs according to small RNA sequencing data were reliable. Through computational identification, 33 target genes were predicted for 12 msRNAs in S. platensis; Gene Ontology (GO) and KEGG enrichment analyses revealed that the putative target genes were grouped into the categories of proteolysis, protein homotrimerization, glycosylation, ubiquinone biosynthetic process, DNA restriction-modification system, and polysaccharide catabolic process. Using proteomic analysis, two target proteins (glyceraldehyde-3-phosphate dehydrogenase and forkhead-associated (FHA) domain-containing protein) were differentially expressed under salt-stress conditions, which might be regulated by msRNAs. Our study demonstrates that msRNAs exist in S. platensis, and these msRNAs may have an important role in salt-stress responses; however, their functional significance requires further investigation.

The removal efficiency of Cu(2+) by Spirulina platensis (strain FACHB-834), in viable and heat-inactivated forms, was investigated in the presence and absence of linear alkylbenzene sulfonate (LAS). When the initial Cu(2+) concentration was in the range of 0.5-1.5 mg · L(-1) , a slight increase in growth rate of FACHB-834 was observed. In contrast, when Cu(2+) or LAS concentrations were at or higher than 2.0 or 6.0 mg · L(-1) , respectively, the growth of FACHB-834 was inhibited and displayed yellowing and fragmentation of filaments. The presence of LAS improved Cu(2+) removal by ~20%, and accelerated attainment of Cu(2+) retention equilibrium. For the 2- mg · L(-1) Cu(2+) treatments, retention equilibrium occurred within 2 d and showed maximum Cu(2+) removal of 1.83 mg · L(-1) . In the presence of LAS, the ratio of extracellular bound Cu(2+) to intracellular Cu(2+) taken up by the cells was lower (1.05-2.26) than corresponding ratios (2.46-7.85) in the absence of LAS. The percentages of extracellular bound Cu(2+) to total Cu(2+) removal (both bound and taken up by cells) in the presence of LAS ranged from 51.2% to 69.3%, which was lower than their corresponding percentages (71.1%-88.7%) in the absence of LAS. LAS promoted biologically active transport of the extracellular bound form of Cu(2+) into the cell. In contrast, the addition of LAS did not increase the maximum removal efficiency of Cu(2+) (61.4% ± 5.6%) by heat-inactivated cells compared to that of living cells (59.6% ± 6.0%). These results provide a theoretical foundation for designing bioremediation strategies using FACHB-834 for use in surface waters contaminated by both heavy metals and LAS.