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Abstract
Posidonia oceanica, an endemic seagrass of the Mediterranean Sea harbors a high diversity of N2-fixing prokaryotes. One of these is Halothece sp., a unicellular N2-fixing cyanobacteria detected through nifH analysis from the epiphytes of P. oceanica. The most related strain in culture is Halothece sp. PCC 7418 and this was used as the test organism in this study. In the Mediterranean Sea, phosphorus (P) and iron (Fe) can be the major limiting nutrients for N2 fixation. However, information about the mechanisms of P-acquisition and the role of metals (i.e., Fe) in these processes for N2-fixing bacteria is scarce. From our genomic analyses of the test organism and other phylogenetically related N2-fixing strains, Halothece sp. PCC 7418 is one of the strains with the greatest number of gene copies (eight copies) of alkaline phosphatases (APases). Our structural analysis of PhoD (alkaline phosphatase type D) and PhoU (phosphate acquisition regulator) of Halothece sp. PCC 7418 showed the connection among metals (Ca2+ and Fe3+), and the P-acquisition mechanisms. Here, we measured the rates of alkaline phosphatase activity (APA) through MUF-P hydrolysis under different combinations of concentrations of inorganic P () and Fe in experiments under N2-fixing (low availability) and non-N2 fixing (high availability) conditions. Our results showed that APA rates were enhanced by the increase in Fe availability under low levels of , especially under N2-fixing conditions. Moreover, the increased -uptake was reflected in the increased of the P-cellular content of the cells under N2 fixation conditions. We also found a positive significant relationship between cellular P and cellular Fe content of the cells (r2 = 0.71, p < 0.05). Our results also indicated that Fe-uptake in Halothece sp. PCC 7418 was P and Fe-dependent. This study gives first insights of P-acquisition mechanisms in the N2-fixing cyanobacteria (Halothece sp.) found in P. oceanica and highlights the role of Fe in these processes.
Highlights
Posidonia oceanica is an endemic seagrass in the Mediterranean Sea, forming extensive meadows with valuable established key ecological services: high primary productivity, as a carbon sink, as a habitat and nursery for a variety of micro- and macroorganisms, as sediment stabilizers, as buffers for ocean acidification, and as an important site for biogeochemical processes (Gutiérrez et al, 2012; Campagne et al, 2015; Agawin et al, 2016)
A huge variety of diazotrophic cyanobacteria have been detected based on the sequence analysis of nifH gene on the leaves of P. oceanica (Agawin et al, 2016, 2017)
Pasteur Culture Collection of Cyanobacteria (PCC) 7418 is composed of genes whose protein products are involved in different functions: autokinase activity of PhoR and phosphate transport (PhoU); high-affinity phosphate transport (PstS, PstC, PstA, and PstB), in a two-component regulatory system (PhoR-PhoB); extracellular enzymes capable of obtaining PO34- from organic phosphates (Alkaline Phosphatases, alkaline phosphatases (APases)); and polyphosphate metabolism (PpK, PpX, and PpA) (Santos-Beneit, 2015)
Summary
Posidonia oceanica is an endemic seagrass in the Mediterranean Sea, forming extensive meadows with valuable established key ecological services: high primary productivity, as a carbon sink, as a habitat and nursery for a variety of micro- and macroorganisms, as sediment stabilizers, as buffers for ocean acidification, and as an important site for biogeochemical processes (e.g., nitrogen cycles) (Gutiérrez et al, 2012; Campagne et al, 2015; Agawin et al, 2016). Changes affecting the N:P ratios in their environment by limiting concentration of N or P, could change their N:P tissue composition and may have consequences in their adaptation and survival and possibly the N2 fixation activities of diazotrophic cyanobacteria (Sañudo-Wilhelmy et al, 2001; Sohm et al, 2011). These versatile microorganisms may have several adaptive mechanisms to changes in their dynamic marine environment (e.g., nutrient availability) (Tandeau de Marsac and Houmard, 1993; Schwarz and Forchhammer, 2005; Herrero and Flores, 2008)
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