| Phosphorus(P)is an essential nutrient element required for the growth of phytoplankton.Dissolved inorganic phosphorus(DIP)is the preferred form of P for phytoplankton.However,DIP in the euphotic zone often falls below growth-limiting concentrations in many parts of the ocean.Thus,phytoplankton have evolved adaptive mechanisms to cope with the shortage of DIP,including the utilization of dissolved organic phosphorus(DOP)when DIP is limited.How DIP deficiency affects the progression of the cell division cycle and photosynthesis,and how well DOP can restore normal cell cycle in phytoplankton is unclear,however.Dinoflagellates are an important group of phytoplankton in marine ecosystems and a major contributor of marine primary production,many species of which also cause harmful algal blooms(HABs).Here we investigated the effects of DIP limitation and supply of adenosine 5'-triphosphate(ATP)as sole P-source on the growth and cell cycle in Prorocentrum donghaiense and the effects of DIP limitation on Amphidinium carterae through the observation of physiology,biochemical criterions,and molecular biology,two common harmful algae bloom dinoflagellates.The methology and results are following:1.In the study on impacts of P limitation and ATP on the growth and cell cyle of P.donghaiense,three P treatments were set up:DIP-replete group,ATP group(where ATP was both DOP and the unique P resource),and P-deprived group.P.donghaiense cells were cultured under axenic condition(treated with antibiotics).Cell growth curve,growth rate,cell size,DIP concentration in the media,and cell cycle progression with flow cytometry were measured during the experimental period.Results showed that part of the culture under DIP-replete condition progressed through a complete cell cycle(G1?S?G2M?G1)in 24 h with a typical diel rhythm of each phase,exhibiting a daily growth rate of 0.40 d-1 and average cell size 11.19±0.06 ?m in equivalent spherical diameter.Under phosphorus deficiency,population growth of P.donghaiense averaged at 0.09 d-1 and the cell cycle was blocked in G1 phase,while cell size averaged at 11.65± 0.05 ?m.When DIP was replaced by ATP at equivalent molar concentration,the cultures displayed similar growth rate,cell size,and cell cycle oscillation as in the DIP-replete treatment.The results suggest that phosphorus deficiency blocks the cell cycle due to the inability to complete DNA duplication or check-point protein phosphorylation while still allowing photosynthesis and metabolisms,and that DOP such as ATP can fully complement DIP in P.donghaiense.2.In the present study on the effects of P deficiency on the growth and cell cycle of Amphidinium carterae,we investigated the impact of P deficiency on the cell division cycle,the abundance of the carbon-fixing enzyme Rubisco,and other cellular characteristics in the Gymnodiniales peridinin-plastid species Amphidinium carterae.We found that under P-replete condition,the cell cycle actively progressed in the culture in a 24-h diel cycle with daily growth rates markedly higher than the P-deficient cultures,in which cells were arrested in the G1 phase and cell size significantly enlarged.The results suggest that,as in previously studied dinoflagellates,P deficiency likely disenables A.carterae to complete DNA duplication or check-point protein phosphorylation.We further found that under P-deficient condition,overall photosystem II quantum efficiency(Fv/Fm)and Rubisco abundance decreased but not significantly,while cellular contents of carbon,nitrogen,and proteins increased significantly.These observations indicated that under P-deficiency,this dinoflagellate was able to continue photosynthesis,carbon fixation,and nitrogen uptake and assimilation,such that proteins and photosynthetically fixed carbon could accumulate resulting in continued cell growth in the absence of division.This is likely an adaptive strategy thereby P-limited cells can be ready to resume the cell division cycle upon resupply of phosphorus. |
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