(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 un- derlying 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 cm 23 h 21 (range 5 2.5-6.2 nmol cm 23 h 21 ). 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 avail- able to the sulfate reducers. The associated microbial reactions create strong reducing conditions that favor N 2 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.