'Roots' in mixotrophic algae

Macroalgae are thought to depend on absorption of nutrients from the water column because, unlike vascular plants, they lack root systems with which to exploit nutrient resources in substrata. Here we show that the giant marine coenocyte, Caulerpa taxifolia, possesses a "root system" containing endocellular bacteria which can take up inorganic phosphorus and organic nitrogen from substrata and translocate nutrient products to the photoassimilatory organs. The uptake of carbon, nitrogen and phosphorus from substrata can explain the alga's ability to proliferate in oligogrophic water. Caulerpa taxifolia produces stolons, fronds and gravipositive rhizoids which mimic the organs of a vascular plant. Mixed bacteria coat the outer surfaces of the rhizoids, whereas large numbers of bacterial rods (10 super(4)-10 super(5)/mm super(3)) exist in the cytoplasm. The 16S ribosomal RNA sequences of these organisms place bacterium A, a halophile, close to unnamed species of Rhodopseudomonas in the Agrobacterium-Rhizobium group of branch- alpha proteobacteria, and bacterium B in the fluorescent Pseudomonas group of branch- gamma proteobacteria.