Abstract
The hydrolytic activity of different types of polyphosphates, and the induction of soluble pyrophosphatase (sPPase; EC 3.6.1.1) activity have been assessed in cell extracts of nutrient limited green seaweed Ulva pseudorotundata Cormaci, Furnari & Alongi subjected to different phosphorus regimes. Following a long period of nutrient limitation, the addition of different types of (poly)phosphates to artificial seawater enhanced growth rates on fresh weight and area, but not on dry weight bases. Chlorophyll and internal P content were affected by P supply. In contrast, internal soluble reactive P was kept low and was little affected by P additions. Soluble protein content increased in all treatments, as ammonium was added to prevent N limitation. The C:N:P atomic ratio revealed great changes depending on the nutrient regime along the experiment. Cell extracts of U. pseudorotundata were capable of hydrolyzing polyphosphates of different chain lengths (pyro, tripoly, trimeta, and polyphosphates) at high rates. The sPPase activity was kept very low in P limited plants. Following N and different kind of P additions, sPPase activity was kept low in the control, but slightly stimulated after 3 days when expressed on a protein basis. The highest activities were found at the end of the experiment under pyro and polyphosphate additions (7 days). The importance of alternative P sources to phosphate and the potential role of internal soluble pyrophosphatases in macroalgae are discussed.
Highlights
Nitrogen (N) is considered to be the main limiting nutrient in the ocean [1], phosphorus (P) can limit primary production of algae in some coastal areas [2]
Ulva pseudorotundata was maintained for a long period under nutrient limitation and, subsequently, grown under different P sources, but with NH4 + addition to avoid N-limitation
Seaweed growth rates were affected by P treatments, but their values depended largely on the plant attributes used in the calculations (Figure 1a)
Summary
Nitrogen (N) is considered to be the main limiting nutrient in the ocean [1], phosphorus (P) can limit primary production of algae in some coastal areas [2]. The major P form for algae is inorganic ortho-phosphate (Pi), whose uptake in seaweeds occurs through specific transport systems [3,4,5]. Pi is the main form of P used by algae, their growth can be supported by more complex inorganic P compounds [7]. Pyrophosphates (PPi) and polyphosphates (polyP) could be important sources of P [8]. These compounds are not molybdenum sensitive, not being quantified in standard analytical measurements of soluble reactive phosphorus (SRP) [9,10]
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