Nutritional requirements of the submerged angiosperm Ruppia maritima in algae-free culture

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Abstract
R. maritima has the potential to become a model laboratory organism for studies with submerged aquatic vascular plants. Algae-free R. maritima grew well in a defined medium without sediment. Growth was a linear response to photon flux density over the range of light tested (up to 450 .mu.E [einstein] m-2 s-1). Vitamins may be a necessary addition in artificial seawater. Trace metals caused little or no increase in growth during short-term (3 wk) growth studies, but appear to be required for long-term cultivation. Fe also caused no increase in growth, at the concentrations tested, but plants were greener in 1.46 .mu.M Fe. A nitrate concentration of 110 .mu.M and a phosphate concentration of 2.3 .mu.M were sufficient for maximum growth. However, 4.5 .mu.M phosphate eliminated occasional CaCO3 precipitation in stock cultures. Critical tissue N content was between 2.5 and 3.0% and critical P content between 0.25 and 0.35%. A comparison with field data suggests that R. maritima was deficient in N and P during much of the summer. Final dry weight and number of leafy shoots per plant correlated positively (r2 = 0.92). A census of leafy shoots could thus be a useful, nondestructive technique for following the growth of individual plants. This study is the 1st record of algae-free cultures of R. maritima.