Riverine fluxes of different species of phosphorus in the Pearl River estuary

Phosphorus is the most limiting nutrient in coastal waters of China, particularly in the Pearl River (PR) estuary. Rivers have different P forms including dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP), and particulate organic phosphorus (POP). Their input to coastal seas has been overlooked. We hypothesize that DIP is a small fraction of total phosphorus (TP). We investigated these P forms and estimated their fluxes in PR eight outlets during 2015-2019. DIP on average is only a 30.90 % fraction of TP with PIP, POP and DOP accounting for 22.43, 31.56 and 15.37 %, respectively. The average annual fluxes of TP, DIP, DOP, PIP and POP were 12.58×, 3.34×, 1.68×, 3.19× and 4.26 × 106 mol/month, respectively, which are regulated by runoff and suspended particulate matter (SPM). The finding reveals the importance of other P forms for phytoplankton in the Pearl River estuary and their bio-availability deserves further study.

SPX-related genes regulate phosphorus homeostasis in the marine phytoplankton, Phaeodactylum tricornutum

Phosphorus (P) is an essential nutrient for marine phytoplankton. Maintaining intracellular P homeostasis against environmental P variability is critical for phytoplankton, but how they achieve this is poorly understood. Here we identify a SPX gene and investigate its role in Phaeodactylum tricornutum. SPX knockout led to significant increases in the expression of phosphate transporters, alkaline phosphatases (the P acquisition machinery) and phospholipid hydrolases (a mechanism to reduce P demand). These demonstrate that SPX is a negative regulator of both P uptake and P-stress responses. Furthermore, we show that SPX regulation of P uptake and metabolism involves a phosphate starvation response regulator (PHR) as an intermediate. Additionally, we find the SPX related genes exist and operate across the phytoplankton phylogenetic spectrum and in the global oceans, indicating its universal importance in marine phytoplankton. This study lays a foundation for better understanding phytoplankton adaptation to P variability in the future changing oceans.

Specific acclimations to phosphorus limitation in the marine diatom Phaeodactylum tricornutum

Phosphorus (P) is a crucial element and diatoms, unicellular phototrophic organisms, evolved efficient strategies to handle limiting phosphorus concentrations in the oceans. In the last decade, several groups investigated the model diatom Phaeodactylum tricornutum concerning phosphate homeostasis mechanisms. Here, we summarize the actual status of knowledge by linking the available data sets, thereby indicating experimental limits but also future research directions.

Mobilization and Cellular Distribution of Phosphate in the Diatom Phaeodactylum tricornutum

Unicellular organisms that live in marine environments must cope with considerable fluctuations in the availability of inorganic phosphate (P_i). Here, we investigated the extracellular P_i concentration-dependent expression, as well as the intracellular or extracellular localization, of phosphatases and phosphate transporters of the diatom Phaeodactylum tricornutum. We identified P_i-regulated plasma membrane-localized, ER-localized, and secreted phosphatases, in addition to plasma membrane-localized, vacuolar membrane-localized, and plastid-surrounding membrane-localized phosphate transporters that were also regulated in a P_i concentration-dependent manner. These studies not only add further knowledge to already existing transcriptomic data, but also highlight the capacity of the diatom to distribute P_i intracellularly and to mobilize P_i from extracellular and intracellular resources.

The Myb-like transcription factor phosphorus starvation response (PtPSR) controls conditional P acquisition and remodelling in marine microalgae

Phosphorus (P) is one of the limiting macronutrients for algal growth in marine environments. Microalgae have developed adaptation mechanisms to P limitation that involve remodelling of internal phosphate resources and accumulation of lipids. Here, we used in silico analyses to identify the P-stress regulator PtPSR (Phaeodactylum tricornutum phosphorus starvation response) in the diatom P. tricornutum. ptpsr mutant lines were generated using gene editing and characterised by various molecular, genetics and biochemical tools. PtPSR belongs to a clade of Myb transcription factors that are conserved in stramenopiles and distantly related to plant P-stress regulators. PtPSR bound specifically to a conserved cis-regulatory element found in the regulatory region of P-stress-induced genes. ptpsr knockout mutants showed reduction in cell growth under P limitation. P-stress responses were impaired in ptpsr mutants compared with wild-type, including reduced induction of P-stress response genes, near to complete loss of alkaline phosphatase activity and reduced phospholipid degradation. We conclude that PtPSR is a key transcription factor influencing P scavenging, phospholipid remodelling and cell growth in adaptation to P stress in diatoms.

Utilization of different dissolved organic phosphorus sources by Symbiodinium voratum in vitro

This study examines the physiological responses of the Symbiodiniumvoratum (clade E) to two types of phosphates having different chemical bonds-phosphoesters (C-O-P bonds) and phosphonates (C-P bonds) to explore Symbiodinium cell growth and the molecular perspective of the P utilization process. Alkaline phosphatase (AP), PhnX, PhoA and PhoX expression was profiled for different P conditions using the RT-qPCR method. In a sterile system, Symbiodinium could decompose phosphoesters, such as ATP and glucose 6-phosphate (G-6-P), into dissolved inorganic P (DIP) to supplement inorganic phosphorus but could not directly use phosphoesters for growth. The growth rate and photosynthetic efficiency of zooxanthellae in phosphoester-containing media did not significantly differ from those in the DIP group but were significantly inhibited in medium containing phosphonates such as N-(phosphonomethyl)glycine (glyphosate) and 2-aminoethylphosphonic acid (2-AEP), as well as in DIP-poor medium. The phosphonate group DIP concentration did not change remarkably, indicating that phosphonates can neither be directly used by zooxanthellae nor decomposed into DIP. Our RT-qPCR results support our views that the phosphoesters (C-O-P) had been hydrolyzed outside the cell before being absorbed into the Symbiodinium cell, and implies that PhnX, PhoA and PhoX are perhaps responsible for transporting DIP from medium into cells and for storage of DIP.