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Marine Ecology Progress Series

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MEPS 288:103-114 (2005)  -  doi:10.3354/meps288103

Nitrate reductase activity in macroalgae and its vertical distribution in macroalgal epiphytes of seagrasses

E. B. Young1,3,*, P. S. Lavery2, B. van Elven2, M. J. Dring1, J. A. Berges1,3

1Marine Systems Group, School of Biology and Biochemistry, Queen’s University of Belfast, Belfast BT9 7BL, UK
2Centre for Ecosystem Management and School of Natural Sciences, Edith Cowan University, Joondalup, Western Australia 6027, Australia
3Present address: Department of Biological Sciences, University of Wisconsin-Milwaukee, PO Box 413, Milwaukee, Wisconsin 53201, USA

ABSTRACT: Macroalgal epiphytes within seagrass meadows make a significant contribution to total primary production by assimilating water column N and transferring organic N to sediments. Assimilation of NO3 requires nitrate reductase (NR, EC; NR activity represents the capacity for NO3 assimilation. An optimised in vitro assay for determining NR activity in algal extracts was applied to a wide range of macroalgae and detected NR activity in all 22 species tested with activity 2 to 290 nmolNO3 min–1 g–1 frozen thallus. With liquid-N2 freezing immediately after sample collection, this method was practical for estimating NR activity in field samples. Vertical distribution of NR activity in macroalgal epiphytes was compared in contrasting Posidonia sinuosa and Amphibolis antarctica seagrass meadows. Epiphytes on P. sinuosa had higher mass-specific NR activity than those on A. antarctica. In P. sinuosa canopies, NR activity increased with distance from the sediment surface and was negatively correlated with [NH4+] in the water but uncorrelated with [NO3]. This supported the hypothesis that NH4+ released from the sediment suppresses NR in epiphytic algae. In contrast, the vertical variation in NR activity in macroalgae on A. antarctica was not statistically significant although there was a weak correlation with [NO3], which increased with distance from the sediment. Estimated capacities for NO3 assimilation in macroalgae epiphytic on seagrasses during summer (24 and 46 mmolN m–2 d–1 for P. sinuosa and A. antarctica, respectively) were more than twice the estimated N assimilation rates in similar seagrasses. When the estimates were based on annual average epiphyte loads for seagrass meadows in other locations, they were comparable to those of seagrasses. We conclude that epiphytic algae represent a potentially important sink for water-column nitrate within seagrass meadows.

KEY WORDS: Nitrate reductase · Macroalgae · Seagrass epiphytes · Dissolved inorganic nitrogen · Ammonium · Nitrate · Irradiance

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