Phosphatase activities of a microepiphytic community during a bloom of Ostreopsis cf. ovata in the northern Adriatic Sea
Graphical abstract
Introduction
In the last few decades, large scale changes in the summer microphytobenthos assemblages of Mediterranean coastal areas have occurred due to significant proportional increases of toxic benthic dinoflagellates of the genus Ostreopsis (Accoroni and Totti, 2016). Among the ten described species of Ostreopsis, three have been recorded in the Mediterranean Sea: O. cf. ovata, O. cf. siamensis (Penna et al., 2012) and O. fattorussoi (Accoroni et al., 2016a), the first of which is the most abundant and widely distributed (Battocchi et al., 2010). Toxin production by Ostreopsis spp. is known to cause a range of human health problems and cause suffering or mass mortalities of various marine organisms (Del Favero et al., 2012, Gorbi et al., 2013).
Ostreopsis spp. generally grow attached to a substrate and, when in high abundance, produce a large amount of mucilage in which the cells aggregate (Totti et al., 2010). This brownish mat has a complex structure formed by a network of long fibers, formed by trichocysts extruded through thecal pores and by an amorphous matrix of acidic polysaccharides (Escalera et al., 2014). The production of large quantities of Exo-Polymeric Substances (EPS) by Ostreopsis has been shown to reduce epiphyte species diversity affecting the composition of microphytobenthos community living within this extracellular matrix (Accoroni et al., 2016b).
Given the fast spread of these undesirable blooms in temperate areas, attempts to understand their ecology have increased considerably in the last years. Findings so far have shown that the main factors causing blooms are hydrodynamics, water temperature and nutrients (Accoroni and Totti, 2016). Originally, coastal primary production was conventionally thought to be nitrogen limited, but later evidence shows that it can often be restricted by phosphorus (Elser et al., 2007, Hoppe, 2003). Identification of phosphorus (P) limitation however, has generally been defined by concentrations of dissolved inorganic P (DIP) as it was typically considered the only bioavailable P source. However, many studies have now demonstrated that a wide range of aquatic phototrophs can effectively use dissolved organic P (DOP) as a source of P (e.g. Cembella et al., 1984, Orchard et al., 2010, Whitton et al., 2005), including dinoflagellates (Huang et al., 2005, Oh et al., 2002). And since DOP concentrations can often far exceed those of DIP in coastal and open ocean surface waters (Dyhrman and Ruttenberg, 2006, Suzumura et al., 1998), its inclusion in coastal ecology studies is vital.
The lack of information on aquatic particulate and dissolved organic P looks like a particularly salient point when considering the evidence of possible mixotrophic behavior in Ostreopsis spp. (Burkholder et al., 2008, Faust and Morton, 1995).
Although not shown for Ostreopsis, for other dinoflagellates, there have been reports of elevated phosphatase activities as a result of P-limitation, indicating possible surface-bound or extracellular processing of organics to obtain P (Dyhrman and Ruttenberg, 2006, Sakshaug et al., 1984). There are a group of phosphatases that catalyse the hydrolysis of simple dissolved organic P compounds, releasing orthophosphate and an organic moiety. Some phosphatases are synthesized and secreted externally to the cytoplasmic membrane and in most cases this has been shown to be a response to a P requirement (e.g. Bieleski, 1974, Whitton et al., 2005). The release of orthophosphate from organic P compounds will increase the available phosphate in the immediate environment of the organism. This process is particularly important in environments where organic phosphates are a major component of aquatic P in P-limited environments (Hoppe, 2003, Turner et al., 2001, Whitton et al., 2005) such as coastal zones (Hoppe, 2003).
The aim of this study is to determine the potential utilization of DOP by microepiphytic-mats during an Ostreopsis bloom by measuring phosphatase rates across a growing season and by locating the activity using specific colorimetric stains. To have an idea of the potential of these organisms to hydrolyse diverse organic P forms, phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities were both investigated. Both these enzymes are specific for the ester bonds (monester and diester bonds) and not for the organic compounds. Samples of microepiphytic community during the bloom of Ostreopsis were collected from the beginning to the end of the bloom, and were used to determine cell abundance and to measure PMEase and PDEase activities. Water samples were taken to determine ambient nutrient concentrations (inorganic, dissolved organic and total fractions). Measurements of a range of chemico-physical variables were made on each sampling occasion to help determine controlling factors of phosphatase activity other than phosphorus. The Passetto station, Conero Riviera, was chosen as a study site as it suffers from annually recurring blooms of Ostreopsis and is located in the northern Adriatic Sea which is considered strongly P-limited (Marini et al., 2008).
Section snippets
Study area
The Passetto station, Conero Riviera (N Adriatic Sea, 43°37′09″N, 13°31′54″E) is a semi-enclosed area, sheltered by a natural reef and located on a shore at the base of 30 m vertical sandstone cliffs. This shallow site (mean depth 2 m) is characterized by a mostly rocky benthic surface. There is moderate-to-high anthropogenic impact during the summer, mainly due to small holiday holdings that have been excavated into the cliff face, that often discharge wastewater directly into the sea water.
Chemical-physical parameters
The surface water temperature ranged from a maximum of 27.7 °C at the beginning of the study (16–30 July) and steadily decreased to a minimum of 15.4 °C by the end of the study (9 November). During the sampling period, the pH of the water continually increased from pH 7.8 to 8.4. Dissolved oxygen concentrations fluctuated around 100% saturation with one super-saturation event on 11 September (134%). The trend of salinity was unimodal, increasing early in the season, and then peaking by 16
Discussion
The microepiphytic community in the Passetto station was subject to a quite severe P limitation, with markedly high water N:P ratios and frequently below detection level concentrations of inorganic P. Assessing the environment using TIN:FRP (inorganic ratio) shows primary productivity to be highly P limited (mean value 359). In comparison, the organic ratio FON:FOP, although still indicating P limitation, was much lower (mean value 203). This difference in ratios was due to the comparatively
Conclusions
Ostreopsis displayed high rates of both PMEase and PDEase, highlighting a potential to rapidly use a wide range of organic P sources. In nutrient poor environments, having a large suite of hydrolytic enzymes within and closely associated with cells would allow Ostreopsis to quickly take advantage of any high concentration nutrient pulses. Many microorganisms have been shown to produce copious EPS at the onset of nutrient limitation, although, the ecophysiological implications of this have not
Acknowledgments
The authors wish to thank Eleonora Antonucci and Nicolé Caputo for field and laboratory assistance. This publication has been produced with the financial assistance of the European Union under the ENPI CBC Mediterranean Sea Basin Programme (M3-HABs project).
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