Stimulation of nitrogen fixation in refractory organic sediments by Caulerpa taxifolia (Chlorophyta)

Estimates of N2 fixation by substrata associated with the rhizophytic alga Caulerpa taxifolia were obtained using the acetylene reduction method. On average, growth of C. taxifolia enhanced rates of acetylene reduction in underlying dead sea grass sediments by a factor of 28, although the degree of enhancement was variable. The average rate of ethylene production was 3.96 nmol cm23 h21 (range 5 2.5–6.2 nmol cm23 h21). There was no apparent stimulation of N2 fixation in substrata collected close to a wastewater outlet. C. taxifolia appears to enhance N2 fixation by releasing photosynthetic product into the rhizosphere, mimicking the behavior of saltwater vascular plants. The excreted organic C activates the fermenting bacterial community, which in turn makes substrates available to the sulfate reducers. The associated microbial reactions create strong reducing conditions that favor N2 fixation, of which many sulfate reducers are capable. Nitrogen fixation can serve to reduce the N deficit that inhibits bacterial decomposition of refractory sea grass waste, thereby enhancing organic matter turnover and nutrient supply to the alga’s rhizoids. This process likely assists C. taxifolia to proliferate upon refractory organic sediments in low-nutrient seawater. The green alga Caulerpa produces a pseudo-root system that functions to anchor the plant, as well as to take up nutrients (Williams 1984; Chisholm et al. 1996). Nutrient uptake from the substratum potentially enables the alga to proliferate in oligotrophic seawater on eutrophicated substrata (Chisholm et al. 1996). Biogeochemical studies in the northwest Mediterranean have indicated a link between the proliferation of Caulerpa taxifolia Vahl. C. (Agardh) and the availability of nutrients discharged in urban wastewater or resulting from the decay of sea grass vegetation (Chisholm et al. 1997). Association with wastewater pollution is not surprising because algal production has often been shown to increase with nutrient discharge. Association with decomposition of sea grass vegetation is less straightforward because sea grass tissues generally have low N content relative to C, which limits bacterial nutrient turnover (Goldman et al. 1987; Paerl 1990). Field and laboratory observations demonstrate that sediments containing dead sea grass matter frequently turn black in color after penetration by the rhizoids of C. taxifolia (Fig. 1). The black coloration is due to bacterial reduction of sulfate to sulfide, indicating strong reducing conditions that favor N2 fixation. Nitrogen fixation could alleviate barriers to organic decomposition (Paerl 1990; Tibbles et al. 1994), thus enhancing nutrient supply to the alga’s rhizoids. C. taxifolia 1 To whom correspondence should be addressed. Present address: 149 Eyre Street, North Ward, Queensland 4810, Australia ([email protected]).