Responses Of Biochemical Composition And Molecular Biology Characteristics In Marine Phytoplankton To Different Nutrient Conditions

In this article, transcriptional changes of four functional genes—high affinitynitrate transporter gene (Nrt2), high affinity phosphate transporter gene (Pho),proliferating cell nuclear antigen gene (PCNA) and ribulose-1,5-bisphosphatecarboxylase/oxygenase large subunit gene (rbcL)—were studied in marine eukaryoticphytoplanktons when culture in different nutrient conditions by means of real-timefluorescence quantitative PCR. Relationships between the transcript levels and algaegrowth or nutrient conditions were analyzed. We aimed to elucidate the molecularmechanisms of phytoplankton response to environmental nutrient conditions. Themain results were as follows:Nrt2transcript levels mainly correlated with the nitrate concentrations in themedium. Higher nitrate concentrations repress the transcript levels of Nrt2and lowernitrate concentrations promote the transcript levels. Results of batch cultures showedthat the Nrt2transcript levels increased rapidly when the nitrate was exhausted andreached the highest values3days after that. Correlation analysis showed that Nrt2transcript levels correlated negatively with the nitrate in the medium. Further more,there was a linear relationship between them when the nitrate was below40μM.Lower phosphate conditions would reduce the Nrt2transcript levels. All these resultsindicated that Nrt2was a good indicator of phytoplankton N conditions.The Pho gene transcript levels correlated with the phosphorus status of algae. Pdeficiency would promote the Pho transcript levels remarkably, but P addition wouldrepress the transcript levels. The inorganic phosphate added group had the lowesttranscript levels. There was a significant negative relationship between Pho gene transcript levels and cellular phosphorus content of microalgae (P <0.01). Therefore,the Pho gene had the potential to be a good indicator of algal phosphorus status.Significant differences existed among photosynthetic rates when the algae werecultured in different nutrient conditions. The photosynthetic rate increased with theincrement of nitrate and phosphate concentrations. Simultaneously detected rbcL genetranscript levels showed that, it correlated significantly with nitrate concentrations,but not with phosphate. This indicated the different regulation mechanism between Nand P on the algal photosynthesis: N may influence the photosynthesis from thetranscriptional level, however, the effect of P may from the post transcriptional level.Extreme significant correlations were found between rbcL gene transcript levels andphotosynthetic rates (P <0.01). rbcL gene maybe a good molecular indicator ofphytoplankton primary productions.PCNA gene transcript levels changed in accordance with the growth of algae,and correlated significantly with the specific growth rates (P <0.01). The relationshipprovided theoretic basis of detecting in situ phytoplankton growth rate and maybeused in the forecast of harmful algae bloom developments.In addition, responses of nucleic acid and protein content in batch cultures of themarine dinoflagellate Prorocentrum donghaiense were studied in relation to differentnitrate and phosphate concentrations. Results showed that DNA content varied indifferent growth stages, and it was insensitive to nutrient status. RNA content wasstrongly associated with both N and P concentrations (P<0.01). Different growthstages could also change cellular RNA content. The RNA content decreased alongwith the extension of experimental time. RNA/DNA had significant correlation withthe N and P concentrations. There appeared to be a linear relationship between growthrate and RNA/DNA. Cellular total protein content was significantly affected by Nconcentrations in the culture. P deficiency could also reduce cellular protein content.In conclusion, RNA and RNA/DNA could be used as algae nutrient status indicators.