Marine Biology61,151-158 (1981) MARINE BIOLOGY 9 Springer-Verlag1981 Nitrogen Fixation by Blue-Green Algae Associated with the Siphonous Green Seaweed Codium decorticatum: Effects on Ammonium Uptake G. Rosenberg 1 and H. W. Paerl 2 t Duke University Marine Laboratory; Beaufort, NC 28516, USA 2 Institute of Marine Science, University of North Carolina; Chapel Hill, Morehead City, NC 28557, USA Abstract Nitrogen fixation (acetylene reduction) at rates of up to 1.2 gg N2 g dry wt -1 h -1 was measured for the siphonous green seaweed Codium decorticatum. No nitrogenase activity was detected in C. isthmocladum. The nitrogenase activity was light sensitive and was inhibited by the addition of DCMU and triphenyl tetra- zolium chloride. Additions of glucose did not stimulate nitrogen fixation. Blue-green algae (C.alothriz sp., Anabaena sp., and Phormidium sp.) were implicated as the organisms responsible for the nitrogenase activity. They occurred in a reduced microzone within the C decorticatum thallus where nitrogen fixation was optimized. Nitrogen fixation did not affect the kinetic constants for ammonium uptake in C decorlicatum (Ks = 12.0 #M, Vma x = 13A /~mol NH3 g dry wt -1 h -1) determined using the perturbation method. Never- theless, C decorticatum thalli which fixed nitrogen had internal dissolved nitrogen concentrations which were over 1.4 times higher than in non-fixing thalli. This suggests that if C decorticatum does derive part of its nitrogen requirement from the blue-green algae which it harbors, the transfer does not involve competition between this process and the uptake of ambient ammonium. Introduction Nitrogenase activity in the marine environment has repeatedly been found in association with sea grasses (Goering and Parker, 1972; Capone and Taylor, 1977). More recently, nitrogen fixation (acetylene reduction) has been reported to be associated with macroalgae in the Bahamas (Capone et al., 1977) and in species of the siphonous green seaweed Codium in New England, USA (Head and Carpenter, 1975) and New Zealand (Dromgoole et al., 1978). The nitrogenase activity in Codium spp. has been attributed both to nitrogen- fixing bacteria (e.g. Azotobacter sp.) and heterocystous blue-green algae (e.g. Calothrix sp.) which occur as epibionts on the seaweed. Ecological interest has centered on the possible role of these nitrogen-fixing microorganisms in the nitrogen budget of the marine plants on which they occur. In the case of Codium spp., the measured rates of nitrogen fixation are considerable, amounting to about 7% of the daily Codium spp. nitrogen requirement. However, there is no positive evidence to date that the host plants do in fact derive significant amounts of nitrogen in this fashion. Several species of Codium occur in the vicinity of Beaufort, North Carolina, USA. C decorticatum (Woodward) Howe and C isthmocladum Vickers are found in the intertidal and shallow subtidal. A third species, C carolinianum Searles, has only been found in deeper waters on the continental shelf. The estuarine waters of the Beaufort region are characterized by extremely low levels of biologically available nitrogen. Thayer (1971) found average annual nitrate levels of 0.35/~g-at N. 1-1 (range: 0 to 2.65 ~g- at N. 1-1). The corresponding figures for nitrite were 0.03 /~g-at N. 1-1 (range: 0 to 0.50/~g-at N. 1-1) while ammonia averaged 0.44 gg-at N. 1-1 (range: 0 to 4.30/~g- at N. 1-1 ). Over this synoptic study, total available nitro- gen averaged only 0.82/~g-at N. 1-1 , which is quite low in comparison to other temperate estuarine areas, such as Chesapeake Bay, USA (Patten et al., 1963)and Long Island Sound, USA (Harris, 1959). For this reason, diurnal variations in biologically available nitrogen, particularly ammonia, are comparable to those occurring seasonally (G. Rosenberg and W. Litaker, unpublished data). Nitrate enrichment resulted in enhanced growth of the brown seaweedDictyotadichotoma (G. Rosenberg, unpublished data). Accordingly, it appears likely that nitrogen is limiting seaweed growth in the waters of the Beaufort area. 0025-3162/81/0061/0151/$ 01.60