Evaluation of coral pathogen growth rates after exposure to atmospheric African dust samples

Abstract

Laboratory experiments were conducted to assess if exposure to atmospheric African dust stimulates or inhibits the growth of four putative bacterial coral pathogens. Atmospheric dust was collected from a dust-source region (Mali, West Africa) and from Saharan Air Layer masses over downwind sites in the Caribbean [Trinidad and Tobago and St. Croix, U.S. Virgin Islands (USVI)]. Extracts of dust samples were used to dose laboratory-grown cultures of four putative coral pathogens: Aurantimonas coralicida (white plague type II), Serratia marcescens (white pox), Vibrio coralliilyticus, and V. shiloi (bacteria-induced bleaching). Growth of A. coralicida and V. shiloi was slightly stimulated by dust extracts from Mali and USVI, respectively, but unaffected by extracts from the other dust sources. Lag time to the start of log-growth phase was significantly shortened for A. coralicida when dosed with dust extracts from Mali and USVI. Growth of S. marcescens and V. coralliilyticus was neither stimulated nor inhibited by any of the dust extracts. This study demonstrates that constituents from atmospheric dust can alter growth of recognized coral disease pathogens under laboratory conditions. Introduction African dust from the Sahara and Sahel of West Africa has been hypothesized to contribute to coral diseases through several mechanisms: transport of coral pathogens to downwind sites (Shinn and others, 2000); impairment of coral immune function or disease resistance from exposure to dust-associated synthetic organic contaminants, such as pesticides, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls (Garrison and others, 2003); stimulation of microbial growth or alteration of coral resistance to pathogens from exposure to iron, trace metals, and nutrients associated with Saharan dust (Garrison and others, 2003); and induction of virulence factor expression in resident and (or) introduced microbes from exposure to iron, trace metals, and (or) nutrients in dust (Hayes and others, 2001; Garrison and others, 2003). Environmental factors have been reported to induce pathogenicity and to stimulate growth of coral pathogens. An increase in temperature (≥ 25 °C) has been shown to induce pathogenicity of the coral pathogens Vibrio coralliilyticus and V. shiloi (Toren and others, 1998; Kushmaro and others, 2001; Ben-Haim and others, 2003a, b). Remily and Richardson (2006) reported that higher temperatures (25–35 °C) increased the range of pH tolerance of Aurantimonas coralicida and suggested that response could facilitate colonization of the coral mucopolysaccharide layer, which has a relatively low pH. Inorganic-nutrient (nitrate, ammonium, and phosphorus) dosing increased aspergillosis (infection by Aspergillus sydowii) severity, measured as tissue loss (Bruno and others, 2003). Thus, it is feasible that constituents of African dust may induce pathogenicity or virulence factors, and (or) stimulate growth of opportunistic pathogens regardless of their origin. The objective of the pilot study reported here was to determine if the addition of African dust to laboratory-grown cultures of putative coral disease isolates stimulated or inhibited growth. The linear portions of the growth curves were compared to determine if the dust significantly increased, decreased, or had no effect on growth rates.