Vol. 14: 121–128, 2012 AQUATIC BIOLOGY Published online January 4 doi: 10.3354/ab00382 Aquat Biol Uptake and transport of nitrogen derived from sessile epifauna in the giant kelp Macrocystis pyrifera Christopher D. Hepburn1,3, Russell D. Frew2, Catriona L. Hurd1 Department of Botany1, and Department of Chemistry2, University of Otago, PO Box 56, Dunedin 9054, New Zealand 3Present address: Department of Marine Science, University of Otago, PO Box 56, Dunedin 9054, New Zealand ABSTRACT: Sessile epifauna that excrete ammonium as a waste product are often closely asso- ciated with surfaces of macroalgae that are able to absorb and use ammonium for growth. This close association means that nitrogen provisioning from epifauna to host macroalgae is probable, but barriers to uptake, formed by epifaunal colonies, and the physiological status of colonised macroalgal tissue may limit the amount of ammonium absorbed by colonised macroal- gae. Evidence for the movement of nitrogen from sessile epifauna into and around the giant kelp Macrocystis pyrifera was gathered using a 15N-enriched stable isotope tracer. Experiments in which 15N-labelled phytoplankton was fed to the epifaunal bryozoan Membranipora mem- branacea did not to find conclusive evidence for the flow of nitrogen released by bryozoan colonies into colonised M. pyrifera tissue. The low degree of transmission of 15N observed may be due to the barrier to nitrogen uptake that is formed by M. membranacea tissue and the low affinity for ammonium uptake by M. pyrifera blades during winter. M. membranacea is unlikely to provide significant benefits to host macroalgae via nitrogen provision in many situa- tions, due to its sheet-forming habit and its absence during summer low-nitrogen periods. In situ incubations of M. pyrifera blades in 15N-enriched ammonium were used to determine if ammo- nium provided by epifaunal colonies could be taken up by mature blades tissue and exported to the stipe for long-distance transport to actively growing meristems. Uptake and export of labelled nitrogen from basal blades into the stipe was measured, and the potential for long- distance transport of nitrogen from older blades was confirmed. Active transport of nitrogen from older macro algal tissue colonised by epifauna such as hydroids could be an important mechanism allowing kelp with specialised long-distance solute transport systems to tolerate nitrogen limitation. KEY WORDS: Ammonium excretion · Bryozoans · Epifauna · Kelp · Long-distance transport · Membranipora membranacea · Macrocystis pyrifera · Nitrogen · Stable isotope tracers · 15N Resale or republication not permitted without written consent of the publisher INTRODUCTION nitrogen limitation via their excretion of ammonium (Gerard & Mann 1979, Probyn & Chapman 1983, Nitrogen is the major limiting nutrient for macro - Hurd et al. 1994, Taylor & Rees 1998, Hepburn & algal primary productivity (Hanisak 1983) and there- Hurd 2005, Fram et al. 2008). The cheilostomate fore is an important factor in the functioning of bryo zoan Membranipora membranacea is a key epi- coastal ecosystems that rely on this productivity faunal species on macroalgal surfaces in both north- (Duggins et al. 1989). Mobile and sessile fauna that ern and southern hemispheres (Dixon et al. 1981, live on macroalgal surfaces may reduce the effects of Seed & O’Conner 1981, Harvell 1986, Berman et al. *Email: [email protected] © Inter-Research 2012 · www.int-res.com 122 Aquat Biol 14: 121–128, 2012 1992, Lambert et al. 1992, Schwaninger 1999, Hep- able to distant meristems at frond apices. Growth burn & Hurd 2005, Saunders & Metaxas 2008). and nitrogen status of apical sections of M. pyrifera Through the capture and digestion of food items (e.g. fronds have been demonstrated to increase with phytoplankton, detritus, dissolved organic matter; increasing cover of hydroids found on basal blades Ryland 1970, Winston 1977, De Burgh & Fankboner that were many metres distant from growing apices 1978), suspension feeding by M. membranacea (Hepburn & Hurd 2005). Transport of nitrogen colonies may make nitrogen bound within particu- derived from epifauna growing on basal blades to late matter in the water column available to its kelp distant meristems is possible through specialised substratum in the form of ammonium. cells found in the stipes of kelp (Raven 2003). These Membranipora membranacea is able to absorb cells resemble the sieve tube elements of higher exudates from macroalgal surfaces as a supplement plants, and are able to transport carbon, mainly as to food it captures through suspension feeding (De mannitol, and probably nitrogen (as amino acids and Burgh & Fankboner 1978, Manríquez & Cancino protein) rapidly along the frond (Parker 1963, 1965, 1996). In turn, ammonium diffuses rapidly from M. Lobban 1978, Schmitz & Srivastava 1979a, Manley membranacea due to its high solubility and low 1983). Due to environmental (Gerard 1984) and molecular size. It has been proposed that some physiological reasons (Kain 1982) it is estimated that waste ammonium from M. membranacea is excreted the majority of M. pyrifera production occurs in the through its base, which is in direct contact with the canopy region (Towle & Pearse 1973, Gerard 1984) seaweed surface (Hurd et al. 1994). This pathway has and that basal blades that are senescent have limited been proposed as a basis for a mutualistic relation- roles in light harvesting. It is possible that basal ship between this bryozoan and the giant kelp Macro - blades that are heavily colonised by sessile epifauna cystis pyrifera (Hurd et al. 1994) and could provide could have a significant role in providing nitrogen to enhanced productivity of important kelp forest habi- the apical and basal meristems of M. pyrifera if nitro- tats over the wide geographic distribution of both gen uptake and transport capacity remains in mature partners (e.g. Schwaninger 1999, Steneck et al. 2002). blade tissue, despite apparent senescence and disin- The hypothesis that Membranipora membranacea tegration (Hepburn & Hurd 2005). provides nitrogen to underlying Macrocystis pyrifera blade tissue was founded on the observation that M. pyrifera blade tissue beneath bryozoan colonies MATERIALS AND METHODS had higher nitrogen content when compared to surrounding bryozoan-free blade tissue (Hurd et al. Collection of Macrocystis pyrifera blade tissue for 1994). However, flow of nitrogen from M. mem- feeding experiments took place at Harington Point branacea into colonised blade tissue has never been at the entrance to Otago Harbour, South Island, demonstrated. Feeding experiments utilising phyto- New Zealand (45° 47’ 03.5’’ S, 170° 43’ 22.7’’ E). In situ plankton enriched with the stable isotope 15N as nitrogen transport experiments were conducted at a food items were designed to determine if significant kelp bed at Omimi Cove (45° 41’ 29.0’’ S, 170° 37’0.0’’ E), amounts of ammonium released by M. membranacea a sheltered bay 13 km northwest of Harington Point. colonies during the feeding process (e.g. rupture of This area was chosen due to the lack of strong tidal cells) and the production of waste products following currents that would have made carrying out this digestion were taken up by underlying M. pyrifera experiment at Harington Point difficult. blade tissue. The addition of 15N labelled phyto- plankton was predicted to enrich M. pyrifera blade tissue colonised by M. membranacea with 15N but not Laboratory feeding experiments bryozoan-free blade tissue as the process of feeding and digestion of phytoplankton by the bryo zoan Dunaliella tertiolecta was selected as food for colony would be required to release labelled nitro- Membranipora membranacea in feeding experiments gen stored within phytoplankton cells to make it as it is an excellent food source for bryozoan colonies available for uptake by blade tissue. (Winston 1977). D. tertiolecta (innoculant from Porto- This study also aimed to identify if nitrogen ex- bello Marine Laboratory, University of Otago) was creted by sessile epifauna, such as bryo zoans and cultured in 1 l Erlenmeyer flasks in a culture medium other important epifaunal groups (e.g. hydroids) that made up with seawater filtered using Whatman are most common at the base of Macrocystis pyrifera GF/C filters (1 μm) and sterilized in an autoclave, and fronds (Hepburn & Hurd 2005) can be made avail- f/2 medium (Guillard & Ryther 1962) with 20 μM Hepburn et al.: N uptake and transport in Macrocystis 123 15 10.4% N-enriched (NH4)2SO4 (Novachem) as the Two treatment groups and 2 control groups of nitrogen source. paired bryozoan-colonised and bryozoan-free blade Culture greater than 3 wk old were used for feed- tissue were used (n = 5). One ml of concentrated ing experiments as it was assumed that most 15N- phytoplankton food was added to each treatment labelled ammonium would by then have been group, while no phytoplankton was added to control absorbed into phytoplankton biomass. Three litres of groups. All colonies were observed to be actively 15N-enriched phytoplankton cultures were cen- feeding during the incubation. Both control and trifuged in 500 ml centrifuge tubes at 1000 rpm for treatment flasks were left in the incubator on orbital 5 min after which the supernatant was decanted shaker tables set at 120 rpm for 8 h. Blade sections off. The remaining phytoplankton mixture was cen- were then removed and washed in purified water. trifuged again for 5 min at 1000 rpm before the super- Bryozoan colonies were scraped off the blade tissue natant was decanted once more. This left 40 ml of discs and dried in plastic tubes at 80°C. Blade sec- highly concentrated phytoplankton for use in feeding tions were washed in purified water and all bryozoan experiments. Observations of phytoplankton through tissue was removed before drying. Blade and bryo - a microscope before and after centrifugation showed zoan tissue were ground in a mortar and pestle no evidence of a reduction in the numbers of motile before storage in sealed tubes prior to analysis for phytoplankton as a result of this process.