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

• Published in: Marine ecology. Progress series (Halstenbek) 2005-03, Vol.288, p.103-114
• Main Authors: Young, E. B., Lavery, P. S., van Elven, B., Dring, M. J., Berges, J. A.
• Format: Article
• Language: English
• Subjects: Algae; Antarctic regions; Epiphytes; Macroalgae; Meadows; Nitrates; Nitrogen; Quaternary ammonium compounds; Sediments; Stems
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps288103

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 1.6.6.1); NR activity represents the capacity for NO₃⁻ assimilation. An optimisedin vitroassay 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 nmol NO₃⁻ min–1g–1frozen 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 contrastingPosidonia sinuosaandAmphibolis antarcticaseagrass meadows. Epiphytes onP. sinuosahad higher mass-specific NR activity than those onA. antarctica. InP. sinuosacanopies, 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 onA. antarcticawas 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 mmol N m–2d–1forP. sinuosaandA. 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.