Characteristic bacteria associated with surfaces of coralline algae: a hypothesis for bacterial induction of marine invertebrate larvae

Abstract

Crustose coralline algae (CCA) induce high rates of settlement and metamorphosis in a diversity of marine invertebrate larvae. It is often assumed that inducers associated with CCA are algal in origin, but an alternative hypothesis is that they originate from bacteria on the algal surface. Because many species manifest specificity in settling on CCA, a necessary condition of the hypothesis of bacterial origin is that bacterial assemblages on CCA are distinctive. Bacteria isolated from surfaces of 2 South African species of CCA (Sporolithon sp. and Clathromorphum sp.), 2 seawater samples collected adjacent to the CCA, and ground-glass slides incubated next to the corallines were examined for 34 morphological and physiological characters. Bacteria from the 2 CCA were distinct from those from other microhabitats and from each other, whereas isolates from the 2 seawater samples were similar, and those from glass slides were more similar to seawater populations than to coralline-associated bacteria. Bacteria from corallines were distinguished by narrower temperature ranges for growth, and in general, inability to (1) utilize glucose, (2) hydrolyse complex carbohydrates and urea, (3) utilize simple amino acids, and (4) reduce nitrate. These characteristics and the distributions of bacteria on crust surfaces suggest that epiphytic bacteria may utilise compounds from the algae for growth. Bacteria on Sporolithon sp. grew mostly from within recently damaged cells, whereas on Clathromorphum sp. they proliferated on a mucus-like matrix on its surface For some strains the association of bacteria with host alga may be specialised. These results are consistent with the hypothesis of bacterial origin of inducers associated with CCA, but they do not provide a critical test and thus do not resolve the question of algal or bacterial origin of inducers. In examining both hypotheses, we suggest methods to critically discern among these 2 alternatives. There was no trace of free GABA (y-aminobutyric acid) or L-DOPA (L-beta-3,4- dihydroxyphenylalanine) in cell-free extracts of either species of South African coralline algae or in bacteria isolated from the corallines and the other microhabitats. This finding supports other work suggesting that these compounds have no ecological relevance as inducers.