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Diatoms can be used as biosensors to assess aquatic environment quality, because they are widely distributed in almost all aquatic environments and show varied sensitivities toward heavy metal ions. The marine planktonic diatoms Phaeodactylum tricornutum (P. tricornutum) and Cylindrotheca fusiformis (C. fusiformis) are typical representatives of planktonic diatoms and benthic diatoms, respectively. C. fusiformis is very sensitive to changes in the concentration of heavy metal ions, and can be used as an indicator of the quality of the sedimental environment, while P. tricornutum can tolerate higher concentrations of heavy metal ions. To explore the potential difference in responses to heavy metal ions between planktonic and benthic diatoms, we compared the transcriptome of P. tricornutum and C. fusiformis under Cu2+ and Zn2+ treatment. The results indicated that P. tricornutum has several genes involved in ion transmembrane transport and ion homeostasis, which are significantly downregulated under Cu2+ and Zn2+ treatment. However, this enrichment of ion transmembrane transport- and ion homeostasis-related genes was not observed in C. fusiformis under Cu2+ and Zn2+ treatment. Additionally, genes related to heavy metal ion stress response such as peroxiredoxin, peroxidase, catalase, glutathione metabolism, phytochelatin, oxidative stress and disulfide reductase, were upregulated in P. tricornutum under Cu2+ and Zn2+ treatment, whereas most of them were downregulated in C. fusiformis under Cu2+ and Zn2+ treatment. This difference in gene expression may be responsible for the difference in sensitivity to heavy metals between P. tricornutum and C. fusiformis. [ABSTRACT FROM AUTHOR]

Background: Diatoms are well known for high photosynthetic efficiency and rapid growth rate, which are not only important oceanic primary producer, but also ideal feedstock for microalgae industrialization. Their high success is mainly due to the rapid response of photosynthesis to inorganic carbon fluctuations. Thus, an in-depth understanding of the photosynthetic carbon fixation mechanism of diatoms will be of great help to microalgae-based applications. This work directed toward the analysis of whether C4 photosynthetic pathway functions in the model marine diatom Phaeodactylum tricornutum, which possesses biophysical CO2-concentrating mechanism (CCM) as well as metabolic enzymes potentially involved in C4 photosynthetic pathway. Results: For P. tricornutum, differential proteome, enzyme activities and transcript abundance of carbon metabolism-related genes especially biophysical and biochemical CCM-related genes in response to different concentrations of CO2 were tracked in this study. The upregulated protein abundance of a carbonic anhydrases and a bicarbonate transporter suggested biophysical CCM activated under low CO2 (LC). The upregulation of a number of key C4-related enzymes in enzymatic activity, transcript and protein abundance under LC indicated the induction of a mitochondria-mediated CCM in P. tricornutum. Moreover, protein abundance of a number of glycolysis, tricarboxylic acid cycle, photorespiration and ornithine–urea cycle related proteins upregulated under LC, while numbers of proteins involved in the Calvin cycle and pentose phosphate pathway were downregulated. Under high CO2 (HC), protein abundance of most central carbon metabolism and photosynthesis-related proteins were upregulated. Conclusions: The proteomic and biochemical responses to different concentrations of CO2 suggested multiple carbon metabolism strategies exist in P. tricornutum. Namely, LC might induce a mitochondrial-mediated C4-like CCM and the improvement of glycolysis, tricarboxylic acid cycle, photorespiration and ornithine–urea cycle activity contribute to the energy supply and carbon and nitrogen recapture in P. tricornutum to cope with the CO2 limitation, while P. tricornutum responds to the HC environment by improving photosynthesis and central carbon metabolism activity. These findings can not only provide evidences for revealing the global picture of biophysical and biochemical CCM in P. tricornutum, but also provide target genes for further microalgal strain modification to improve carbon fixation and biomass yield in algal-based industry. [ABSTRACT FROM AUTHOR]

Sugarcane bagasse (SB), as a major by-product of sugarcane, is one of the most abundant organic matter and characterized by cheap and easily available carbon source in Hainan Island, China. The objective of this study was to isolate tropical cellulolytic bacteria from Hainan Island and demonstrate their prospects of utilization of SB as a low-cost carbon source to greatly reduce the cost of aquaculture. A total of 97 cellulolytic marine bacteria were isolated, of which, 58 cellulolytic marine bacteria displayed the hydrolysis capacity (HC) of more than 1, while 28 cellulolytic marine bacteria displayed more than 2. Of the 28 tropical cellulolytic bacterial strains with HC more than 2, Microbulbifer sp. CFW-C18 and Vibrio sp. MW-M19 exhibited excellent SB decomposition in a small-scale laboratory simulation of shrimp aquaculture, up to 75.31 and 74.35%, respectively, and both of them were safe for shrimps. Meanwhile, both of CFW-C18 and MW-M19 besides displaying low multiple antibiotic resistance (MAR) index, also increased the C/N ratio (CFW-C18: C/N ratio of 14.34; MW-M19: C/N ratio of 14.75) of the small-scale laboratory simulation of shrimp aquaculture by decreasing the nitrogen content after a supplement of SB for 15 days. More importantly, CFW-C18 and MW-M19 displayed a relatively low MAR index, 0.47 and 0.1, respectively, especially MW-M19, with the lowest MAR index (0.1), which was resistant to only three antibiotics, streptomycin, amikacin, and levofloxacin, indicating that this strain was safe and non-drug resistance for further use. Overall, tropical cellulolytic bacteria isolated from Hainan Island, especially CFW-C18 and MW-M19, will provide the proficient candidates as probiotics for further construction of the recirculating aquaculture system based on the supplement of low-cost external carbon source—SB. Tropical cellulolytic bacteria: potential utilization of sugarcane bagasse as low-cost carbon source in aquaculture. [ABSTRACT FROM AUTHOR]

Abstract Background Diatoms, which are important planktons widespread in various aquatic environments, are believed to play a vital role in primary production as well as silica cycling. The genomes of the pennate diatom Phaeodactylum tricornutum and the centric diatom Thalassiosira pseudonana have been sequenced, revealing some characteristics of the diatoms' mosaic genome as well as some features of their fatty acid metabolism and urea cycle, and indicating their unusual properties. To identify microRNAs (miRNAs) from P. tricornutum and to study their probable roles in nitrogen and silicon metabolism, we constructed and sequenced small RNA (sRNA) libraries from P. tricornutum under normal (PT1), nitrogen-limited (PT2) and silicon-limited (PT3) conditions. Results A total of 13 miRNAs were identified. They were probable P. tricornutum-specific novel miRNAs. These miRNAs were sequenced from P. tricornutum under normal, nitrogen-limited and/or silicon-limited conditions, and their potential targets were involved in various processes, such as signal transduction, protein amino acid phosphorylation, fatty acid biosynthetic process, regulation of transcription and so on. Conclusions Our results indicated that P. tricornutum contained novel miRNAs that have no identifiable homologs in other organisms and that they might play important regulator roles in P. tricornutum metabolism.

Microalgae are capable of accumulating biomass and high value-added products in mixotrophic culture. Carbon sources and light intensity have significant impacts on mixotrophic algal growth and metabolites accumulation. To investigate the underlying mechanism of interaction between metabolism of inorganic carbon and organic carbon, growth of Nannochloropsis oceanica with two different carbon sources (organic sodium acetate and inorganic sodium bicarbonate) and under three different light conditions (low, medium and high) was examined. A combination of sodium acetate and sodium bicarbonate significantly enhanced algal growth with higher acetate assimilation, when compared with single carbon sources. With increasing light intensity from low to high, the algal growth rates decreased in both autotrophic and mixotrophic cultures. Proteome and 13C metabolic flux results showed that acetate utilization via the glyoxylate (GOX) cycle was enhanced, and both the tricarboxylic acid (TCA) cycle and putative C4 pathway were stimulated in the presence of sodium acetate and sodium bicarbonate. These data suggest that sodium bicarbonate could facilitate acetate assimilation and enhance the putative C4 pathway, GOX cycle and TCA cycle in N. oceanica under mixotrophic culture conditions. [ABSTRACT FROM AUTHOR]

Iron is a limiting factor that controls the phytoplankton biomass in the modern ocean, and iron fertilization of the ocean could lead to blooms dominated by diatoms. Thus, iron plays an important role in controlling the distribution of diatoms. In this study, we measured the growth rate and photosynthetic activity of the model diatom Phaeodactylum tricornutum cultured under different iron concentrations and found that it grew more rapidly and had a much higher photosynthetic efficiency under higher iron concentrations. In order to explore the unique mechanism of the response of diatoms to iron, a proteomic analysis was carried out, and the results indicated that iron promotes the Calvin cycle of P. tricornutum. Diatoms can tolerate the pressure of iron limitation by replacing iron‐rich proteins with flavodoxin, and so on. Moreover, we found that the photosystem I (PSI) activity of iron‐limited algae that were treated by N’,N’,N’,N’‐tetramethyl‐p‐phenylenediamine (TMPD) was increased significantly. As TMPD plays the role of a cytochrome b6/f complex that transfers electrons from photosystem II to PSI, the cytochrome b6/f complex is the key to photosynthesis regulation. Iron could influence the growth of P. tricornutum by regulating its biosynthesis. All of the results suggest that iron might affect the growth of diatoms through the Calvin cycle and the cytochrome b6/f complex. [ABSTRACT FROM AUTHOR]