Mariachiara Naldi

CHANGES IN NITROGEN POOLS IN ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA) UNDER NITRATE AND AMMONIUM ENRICHMENT

The accumulation of nitrogen in different cellular pools by the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) was studied in a laboratory experiment. After 8 or 9 days of nitrogen enrichment, nitrate, ammonium, free amino acid (FAA), protein, chlorophyll (chl), phycoerythrin (PE), and insoluble nitrogen pools were extracted and analyzed, and their relative contribution to total nitrogen (TN) was assessed. In U. fenestrata, the nitrate and ammonium enrichments resulted in a significant increase of TN from 2.41% dry weight (dw) to 4.19% and 4.71% dw, respectively. All the extracted N pools increased significantly. In G. pacifica, TN increased more under ammonium enrichment than under nitrate enrichment. In both macroalgae, proteins and FAA were the most important N storage pools. Protein‐N ranged from 700 to 2300 μmol N·g dw−1 (43%–66% of TN) and contributed the most to TN increase (41%–89%). The FAA pool was always larger in G. pacifica than in U. fenestrata. In both species, the FAA pool accounted for 4%–17% of TN (70–600 μmol N·g dw−1). In U. fenestrata, nitrate can represent a temporary storage pool: it accumulated up to 200 μmol N·g dw−1 (7% of TN) and contributed more than FAA to overall increase in cellular nitrogen. In contrast, G. pacifica had a small nitrate pool. The PE pool in G. pacifica increased with TN but was never more than 9% of total protein‐N or 6% of TN, and it was less important than FAA as a storage pool. All TN was recovered in the extracted and insoluble N pools at the end of the experiment in U. fenestrata. In G. pacifica, the extracted and insoluble N pools accounted on average for 83%–90% of TN.

Growth of the seaweed Ulva rigida C. Agardh in relation to biomass densities, internal nutrient pools and external nutrient supply in the Sacca di Goro lagoon (Northern Italy)

Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply. Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida. Two types of field experiments were conducted by incubating Ulva thalli inside large cages. In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage.We recorded specific growth rates (NGR) ranging from 0.025 to 0.081 d−1 for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300–500 g AFDW m−3). These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column. Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column. Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow.In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight). The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration. The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing NY ratio (49.4 to 14.6). The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva. However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m−3, the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds.

Nitrate uptake and storage in the seaweed Ulva rigida C. Agardh in relation to nitrate availability and thallus nitrate content in a eutrophic coastal lagoon (Sacca di Goro, Po River Delta, Italy)

The seasonal cycle of biomass and tissue composition of Ulva rigida C. Agardh, in relation to nitrogen availability in the water column, was studied in 1991–1992 in the Sacca di Goro, a highly eutrophic lagoon in the Po River Delta (Italy). Nitrate uptake rates and storage capacity were also determined in laboratory experiments. The seasonal growth of U. rigida was related to the seasonal trend of nitrogen concentration in the water column. U. rigida biomass increased exponentially during spring and attained peaks of about 300–400 g dry mass (DM) m−2 in June. As biomass increased, U. rigida depleted nitrate in the water column. Thallus nitrate reserves also declined from 100 μmol N (g DM)−1 to almost undetectable levels, and total thallus nitrogen declined from 4% to 2.5% DM and 1.25% DM in 1991 and 1992, respectively. During summer, U. rigida decomposition increased, and organic nitrogen concentrations in the water column increased. The uptake experiments demonstrated an inverse relationship between thallus nitrate content and nitrate uptake rates. A modified Michaelis–Menten equation that accounts for thallus nitrate fit the uptake data well. U. rigida can accumulate up to about 400–500 μmol nitrate (g DM)−1 in cellular reserves. U. rigida in the Sacca di Goro has higher Km and lower Vmax/Km ratios for nitrate uptake than other chlorophycean species, indicating a low efficiency of uptake at low nitrate concentrations. This low uptake efficiency, and the ability to exploit N availability by storing cellular nitrate pools in excess of immediate growth needs, may represent a physiological response to an eutrophic environment where nitrate is in large supply for most of the year.

Nitrogen cycling networks of coastal ecosystems: influence of trophic status and primary producer form

Abstract We have used ecological network analysis to compare nitrogen cycles from five well-researched coastal ecosystems. These included a representative ricefield and two lagoons (Tancada and Encanysada lagoons) in the Ebro River delta, Spain; a region of the Sacca di Goro, a lagoon at the mouth of the Po River, Italy; and a drowned river estuary in North Carolina, USA, the Neuse River estuary. We constructed networks for the various systems and ranked them by trophic status (i.e., degree of eutrophication) using four indices. We then considered the importance of (1) trophic status, (2) growth form of dominant primary producer and (3) water residence time to the intensity and pattern of recycling and to the manner in which the systems can “filter” N. Three indices of flux (rate of import, primary producivity and total systems throughput) gave similar rankings of trophic status among ecosystems with the Italian and U.S. systems being most eutrophic, ricefields next, and then the two Spanish lagoons. Patterns of N export and of cycling within the systems were most closely related to the growth form of dominant primary producers. Phytoplankton, with their rapid turnover rate, foster rapid recycling within the water column and continuous transfer to sediments and export. Submersed and emergent aquatic vegetation and macroalgae create lags and pulses within systems by sequestering N during growth and releasing it during senescence, death and decomposition. Trends in cycling among systems relative to trophic status or water residence appear largely secondary to primary producer growth form.

15N measurements of ammonium and nitrate uptake by Ulva fenestrata (Chlorophyta) and Gracilaria pacifica (Rhodophyta): Comparison of net nutrient disappearance, release of ammonium and nitrate, and 15N accumulation in algal tissue1

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4+ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw−1·h−1 in U. fenestrata and 32 μmol N·g dw−1·h−1 in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4+ release rate of 11 μmol N·g dw−1·h−1. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4+ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4+ disappearance and 15N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4+ concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15NH4+ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3− was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3− pools) or physiological/metabolic changes required before the uptake of NO3−. During the subsequent days, NO3− uptake rates were similar for the two macroalgae but much lower than NH4+ uptake rates (1.97–3.19 μmol N·g dw−1·h−1). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added 15NO3− was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4+ uptake experiments conducted with N‐rich macroalgae and that determination of 15N atom % enrichment of the dissolved NH4+ is recommended to avoid poor isotope recovery and underestimation of uptake rates.

Nutrient and iron limitation to Ulva blooms in a eutrophic coastal lagoon (Sacca di Goro, Italy)

Growth patterns and bloom formation of the green seaweed Ulva rigida were analysed in the eutrophic Sacca di Goro lagoon (Po River Delta, Italy). Variations of standing biomasses and elemental composition of Ulva were analysed through an annual cycle with respect to nitrogen, phosphorus and iron. Growth rates, nutrient and iron uptake and nitrate storage by macroalgal thalli were also assessed with field experiments during the formation of a spring bloom. The control of Ulva growth and the bloom formation depended on multiple factors, especially on nitrogen availability and iron deficiency. In the nitrate rich waters of the Sacca di Goro lagoon, nitrate accumulation in Ulva thalli was inversely related with Fe uptake, indicating an influence of Fe limitation on N acquisition. Since length and magnitude of nitrate luxury uptake are inversely related to the size of the intracellular nitrate pools, in nitrate rich waters the fast growing Ulva may face risk of N-limitation not only when exposed to low N concentrations or at high biomass levels, but also when exposed to pulsed dissolved nitrate concentrations at low iron availability. The potential Fe limitation could be affected by processes controlled by geochemical reactions and by macroalgal growth and decomposition. Both Fe oxidation during the active macroalgal growth and the formation of insoluble FeS and FeS2 during bloom collapse can result in a drastic decrease of soluble iron. Thus, a potential limitation of Fe to macroalgae can occur, determining positive feedbacks and potentially controlling the extent of bloom development and persistence.

Assessing the Potential Impact of Clam Rearing in Dystrophic Lagoons: An Integrated Oxygen Balance

In this work we propose an integrated model to simulate the oxygen balance of a eutrophic lagoon exploited for mollusks farming. The balance is determined by maeroalgal primary production and respiration rates plus the oxygen demand by clams and sediment. The aim is to evaluate the impact of intensive clam rearing on the vulnerability of the lagoon ecosystem to anoxic crises. The model is based on field data collected in the Sacca di Goro lagoon (Po River Delta) and has a stochastic formulation accounting for environmental unpredictability. The results show that clams have a considerable impact on the ecosystem, i.e. densities of 500 clams m−2 can cause hypoxic events (DO < 2mgO2 L−1) in June and September, whilst densities over 1000 clamsm-2 (one half the maximum observed seeding densities) can determine a state of chronic hypoxia during the whole summer period, with minimum DO values lower than 1 mgO2 L−1. The model provides a valuable tool for assessing the sustainability of different rearing policies.

Influence of Clam Farming on Macroalgal Growth: A Microcosm Experiment

In this work we tested whether macroalgal growth can be stimulated by clams filtration activity and nutrient excretion. Thalli of Ulva rigida were grown suspended in semi-opened microcosms containing sand, in presence and absence of the clam Tapes philippinarum. Macroalgal growth and nitrogen pools were measured on subsamples collected during the experiment whilst oxygen and inorganic nitrogen fluxes were measured via short light and dark microcosm incubations. The presence of the clams stimulated significantly primary production of both macroalgae and microphytobenthos; in the light. gross O2 fluxes up to 52.5 ± 6.9 and 99.2±4.6 mmol m−2 h−1 were measured respectively in the chambers without and with clams. Daily inorganic nitrogen fluxes were mostly negative with a peak (−49.4 mmol N m−2 d−1) measured in the clam chambers. U. rigida cultured with clams had a higher growth rate and maintained a higher nitrogen content than U. rigida in bare sediment chambers.

Construction and Analysis of Static, Structured Models of Nitrogen Cycling in Coastal Ecosystems

Microbial processes often dominate biogeochemical cycling within ecosystems. Therefore, microbial ecology interfaces with ecosystem ecology as appropriately as plant or animal ecology does. However, it is difficult for many microbiologists to interpret ecosystem-level phenomena. Often field measurements are made separately on individual components of an ecosystem. Thus one measures a single process, such as phytoplankton primary productivity, or a single assemblage of organisms, such as benthic (i.e., referring to sediments) microalgae. The interactions of components are inferred from observations of patterns of several variables over time or space. Or one conducts experiments using a limited number of variables where inferences involve only direct interactions of a few components. It is more difficult to examine indirect influences of one component on another or to simultaneously evaluate numerous interactions. We have found that constructing and analyzing simple mathematical models can be useful in greater synthesis of microbiological and ecological information (Christian et al. 1986; Christian and Wetzel 1991). Modeling provides a mechanism by which one can extend inferences to include numerous components and both direct and indirect interactions simultaneously.

Benthic nitrogen metabolism in a macrophyte meadow (Vallisneria spiralis L.) under increasing sedimentary organic matter loads

Organic enrichment may deeply affect benthic nitrogen (N) cycling in macrophyte meadows, either promoting N loss or its recycling. This depends upon the plasticity of plants and of the associated microbial communities, as those surrounding the rhizosphere. Rates of denitrification, dissolved inorganic N fluxes and N uptake were measured in sediments vegetated by the submerged macrophyte Vallisneria spiralis L. under increasing organic matter loads. The aim was to investigate how the combined N assimilation and denitrification, which subtract N via temporary retention and permanent removal, respectively, do vary along the gradient. Results showed that V. spiralis meadows act as regulators of benthic N cycling even in organic enriched sediments, with negative feedbacks for eutrophication. A moderate organic load stimulates N uptake and denitrification coupled to nitrification in the rhizosphere. This is due to a combination of weakened competition between macrophytes and N cycling bacteria and enhanced radial oxygen loss by roots. An elevated organic enrichment affects N uptake due to hostile conditions in pore water and plant stress and impairs N mineralisation and its removal via denitrification coupled to nitrification. However, the loss of plant performance is almost completely compensated by increased denitrification of water column nitrate, resulting in a shift between the relative relevance of temporary and permanent N removal processes.