Evaluation of Coral Pathogen Growth Rates After 

Exposure to Atmospheric African Dust Samples

By John T. Lisle, Virginia H. Garrison, and Michael A. Gray

Open-File Report 2014–1017

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Cover photographs: From the Virgin Islands contrasting a clear day with a day on which a dust event occurred 

(Photo credit: Ginger Garrison, USGS).

Suggested citation: 

Lisle, J.T., Garrison, V.H., and Gray, M.A., 2014, Evaluation of Coral Pathogen Growth Rates After Exposure to 

Atmospheric African Dust Samples: U.S. Geological Survey Open-File Report 2014–1017, 12 p., 

http://dx.doi.org/10.3133/ofr20141017

ISSN 2331-1258 (online)
iii

Acknowledgments 

In-kind support from the following is greatly appreciated: in Mali, U.S. Embassy–Bamako, 

Ministry of Communication, Centre Nacionale Recherche Scientifique et Technologique, and ORTM 

Emetteur Kati; in the USVI, Dr. and Mrs. Richard Van Heckmann; and in Tobago, the Tobago House of 

Assembly. 

The authors thank: the USGS Earth Surface Dynamics, Coastal and Marine Geology, and the 

Terrestrial, Aquatic, and Marine Ecosystems programs for funding the project; and Paul Lamothe 

(USGS, Denver, Colo.) for providing element analysis and Michael Majewski (USGS, Sacramento, 

Calif.) for heroics in the field in Tobago. 
iv
v

Contents

Acknowledgments ...................................................................................................................................................iii

Abstract .................................................................................................................................................................... 1

Introduction............................................................................................................................................................... 1

Methods.................................................................................................................................................................... 2

Sample Collection ................................................................................................................................................ 2

Growth Medium .................................................................................................................................................... 2

Filter Preparation and Dust Extraction ................................................................................................................. 3

Bacterial Isolates ................................................................................................................................................ 3

Growth Experiments ............................................................................................................................................ 3

Elemental Analysis ............................................................................................................................................... 3

Data Analysis ........................................................................................................................................................... 4

Results and Discussion ............................................................................................................................................ 4

Recommendations.................................................................................................................................................... 6

References Cited.................................................................................................................................................... 11

Figures

1. The effect of dust addition on the growth of Aurantimonas coralicida by site. ............................................ 7

2. The effect of dust addition on the growth of Vibrio shiloi by site.................................................................. 8

3. The effect of dust addition on the growth of Serratia marcescens by site................................................... 9

4. The effect of dust addition on the growth of Vibrio coralliilyticus by site.................................................... 10

Tables

1. Sample sites and collection data................................................................................................................. 2

2. Bacterial species used in this study............................................................................................................. 3

3. Linear regression models for log-phase segment of growth curves............................................................ 5
vi

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.394 inch (in.)

millimeter (mm) 0.039 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.621 mile (mi)

meter (m) 1.094 yard (yd)

Area

square centimeter (cm2

) 0.001 square foot (ft2)

square meter (m2

) 10.76 square foot (ft2)

square centimeter (cm2

) 0.155 square inch (ft2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.264 gallon (gal)

cubic meter (m3

) gallon (gal)

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3)

Mass

gram (g) 0.035 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Pressure

kilopascal (kPa) 0.010 atmosphere, standard (atm)

kilopascal (kPa) 0.1450 pound per square inch (lb/in2

)

Density

gram per cubic centimeter (g/cm3

) 62.4220 pound per cubic foot (lb/ft3)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F–32)/1.8

264.2 
1

Evaluation of Coral Pathogen Growth Rates After 

Exposure to Atmospheric African Dust Samples

By John T. Lisle, Virginia H. Garrison, and Michael A. Gray

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. 
2

Methods

Sample Collection 

Air samples were collected from Kati, Mali, West Africa—a known dust source location (Gillies and others, 1996; McTainsh and others, 1997), and from two downwind sites: in the northwestern 

Caribbean [St. Croix, U.S. Virgin Islands (USVI)] and in the southeastern Caribbean (Tobago, Republic 

of Trinidad and Tobago; table 1). These downwind sites are historically impacted by dust originating 

from the Sahara Desert and Sahel region of Africa (Garrison and others, 2003). The origin of the collected dust was confirmed using model forecasts (Navy Aerosol Analysis and Prediction System, Global 

Aerosol Model of the Naval Research Laboratory, Monterey, California), satellite images, local atmospheric conditions, and iron (Fe) content and rare-earth element ratios [lanthanum (La), scandium (Sc), 

and thorium (Th); Muhs and others, 2007)] of the dust samples.

Table 1. Sample sites and collection data.

[masl, meters above sea level; m3, cubic meters; mg, milligrams]

Sample site Date Latitude Longitude Site elevation Sample volume Aerosol mass

masl m3 mg

Kati, Mali April 4–6, 2008 12°41.28' N 08°01.65' W 555 1,852 431.0

St. Croix, USVI August 15–18, 2008 17°44.73' N 64°35.15' W 27 1,437 203.7

Tobago June 9–11, 2008 11°19.67' N 60°32.50' W 329 1,236 111.2

Atmospheric dust was collected on 20.3 centimeters (cm) × 25.4 cm quartz-fiber filters (Whatman Grade QM-A) that had been prebaked at 600 °C for 8 hours (h), cooled, and weighed. A high-volume blower motor, isolated from the filters, was used to draw air through the filters at 25–40 cubic meters (m3

) h–1 for approximately 48 h. At the completion of each sampling period, filters were removed, 

sealed in sterile polyethylene bags, and stored at -20 °C. Subsequently, all filters were freeze-dried to 

remove moisture and until a constant weight was achieved and then stored at -20 °C. Field blanks were 

collected for each site.

Growth Medium

Modified seawater-yeast extract medium [MSWYE; 1.0 gram (g) peptone, 1.0 g yeast extract, 

23.4 g sodium chloride (NaCl), 2.34 g magnesium sulfate (MgSO4

), 0.75 g potassium chloride (KCl) per 

liter (L)] (Oliver and Colwell, 1973) was selected as the growth medium for all experiments. All laboratory glassware and plasticware used in the growth experiments were cleaned as follows: deionized water 

rinses (3×); 24-h soak in 2N hydrochloric acid (HCl; trace-metals analysis grade); reagent-grade water 

rinses (3×); 24-h soak in 3N nitric acid (HNO3

; trace-metals analysis grade); reagent-grade water rinses 

(3×); and 24-h soak in reagent-grade water. 
3

Filter Preparation and Dust Extraction 

Prior to the initiation of the growth experiments, all filters were sterilized at 170 °C for 4 h to 

inactivate microorganisms (Harvard Biosafety, 2009). Subsequently, sample and control filters were 

placed in separate containers that contained 1.0 L of sterile MSWYE broth, incubated [25 °C, 20 minutes (min)] to stimulate sporulation, and autoclaved [20.0 min at 121 °C, 20 pounds per square inch 

(psi)]. Each sealed container was then placed on a rocking platform for 72 h to extract soluble constituents from the dust particles. Hereafter, this solution is referred to as the dust-extract solution. Control 

filters were handled identically, producing a “MSWYE-filter” extract to control for filter and medium 

effects.

Bacterial Isolates 

Four bacterial isolates that have been implicated in one or more coral diseases and that could be 

grown in the laboratory with MSWYE were selected for testing their growth response following exposure to African dust (table 2). Despite repeated attempts, a fifth coral pathogen, Thalassomonas loyana

(implicated in causing a white plague-like disease of corals in the Red Sea; Thompson and others, 

2006), could not be grown.

Table 2. Bacterial species used in this study.

Bacterial species Coral disease Strain (source) Reference

Aurantimonas coralicida White plaque (type II) 14790 (DSMZ) Denner and others, 2003

Serratia marcescens White pox BAA-632 (ATCC) Patterson and others, 2002

Vibrio coralliilyticus Bleaching BAA-450 (ATCC) Ben-Haim and others, 2003b

Vibrio shiloi Bleaching 13779 (DSMZ) Kushmaro and others, 2001

Growth Experiments 

The experimental design consisted of dosing a culture of each bacterial isolate with an African 

dust extract in MSWYE medium and corresponding MSWYE-filter control (no dust extract added). 

Each experiment was performed in triplicate. For each experiment, 20.0 milliliters (mL) of the respective dust-extract solutions were transferred to 20-millimeter (mm) × 150-mm glass tubes. A volume 

(1.0 mL) of an overnight culture of the respective bacterial strains that had been grown in MSWYE at 

25 °C with shaking [150 revolutions per minute (rpm)] was used for inoculation. All tubes were gently 

mixed, and the absorbance value (Ab0) was immediately recorded using a spectrophotometer (590 nanometers). All experiments incubated the cultures at 25 °C in a shaking incubator (150 rpm). Absorbance 

values (Abt

) were recorded hourly for the first 10 h, then twice daily for the following two days. 

Elemental Analysis 

Dust samples and field blanks were analyzed for La, Sc, Th, and Fe using inductively coupledplasma mass spectrometry (Briggs and Meier, 2002), instrumental neutron activation analysis, and (or) 

graphite-furnace atomic-absorption spectrometry. 
4

Data Analysis 

All absorbance data were normalized to the time-zero absorbance values as a ratio (i.e., 

Abt

/Ab0). The normalized data were log-transformed and subsequently used to generate the respective 

growth curves for the complete incubation periods. The data used for the linear-regression models were 

selected from the linear segment of the total growth curve. The linear segments of the growth curves 

were then used to compare the growth rates (slopes) of dust-extract treated and corresponding filter 

control using the General Linear Model (GLM) for an analysis of covariance (Fry, 1993). The GLM was 

modified to test paired datasets used in the linear-regression models using the Tukey HSD method for 

pair-wise comparisons. The initial test was to determine if the slopes of the regression lines were parallel (i.e., equal) and, if so, if the intercepts were equal. If the slopes and intercepts of the treatment and 

control were statistically equal, the regression lines were considered coincident, representing no significant differences between bacterial growth rates in cultures exposed and not exposed to African dust 

extracts. All statistical analyses were conducted using Minitab (ver. 15; Minitab, Inc., State College, 

Pennsylvania).

Results and Discussion

There were minor, though statistically insignificant, stimulatory effects on pathogen growth 

when comparing the linear-regression data from the dust-extract solutions to that from the control cultures for A. coralicida (p=0.074) and V. shiloi (p=0.077), when dosed with dust extracts from Mali and 

USVI, respectively (table 3; figures 1–2). There were no significant differences between the growth rates 

of V. coralliilyticus or S. marcescens in control and dust-dosed cultures for all of the dust sources (table 

3; figures 3–4). 

Caveats that should be considered when interpreting the data from this study include the differences in dust sample masses collected at each site; the differences in particle sizes (and resulting 

bioaccessibility of bioactive elements) among sites; and the difficulty in extracting dust from the filter 

(quartz fiber) matrix. There were significant differences among the amounts of dust collected at Mali 

[0.23 milligrams (mg) m–3], USVI (0.14 mg m–3), and Tobago (0.09 mg m–3; table 1), a function of 

particle concentrations in the air masses sampled. Despite differences in dust mass collected at the three 

sites, elemental-analysis data confirmed an African origin for all samples (La–Sc–Th ratios for samples 

and local soils). Concentrations of dust in Mali are orders of magnitude greater than in downwind sites, 

such as the Caribbean, 5,000 kilometers (km) distant. Atmospheric-dust concentrations in Mali can 

exceed 13,000 microgram (μg) m–3 with mean background concentrations of 575 μg m–3 (Gillies and others, 1996). In contrast, atmospheric dust concentrations of 200–600 μg m–3 in the mid-Atlantic and Barbados (Prospero and others, 1981; Talbot and others, 1986), 100–200 μg m–3 in the Virgin Islands, and 

mean concentrations of 3–20 μg m–3 in the Caribbean region have been reported (Prospero and Nees, 

1986; Perry and others, 1997). Sample solutions were not normalized to milligrams per liter among the 

sites because the intent was to test for effects from real-world dust concentrations impacting a site—

trace element components as well as mass influx.

As the mass of dust particles decreases with increasing distance and time from the dust-source 

region, so does particle size. As the Saharan Air Layer exits the dust-source region, particles are lost via 

gravitational settling based on aerodynamic diameter and mass, such that primarily fine particles remain 

after approximately 5,000 km of atmospheric transport across the Atlantic Ocean to the Caribbean. The 

deposition of larger dust particles earlier in the transport process may provide a greater dose to marine 

systems closest to the African coast; however, the chemical composition of the dose could differ with 
5

Table 3. Linear regression models for log-phase segment of growth curves.

Bacterial species Dust source Experiment type Linear-regression model R2 Slope comparison (p-value) Intercept comparison (p-value)

Aurontimonas coralicidia Mali Dust y = -0.274 + 0.099x 0.905 0.074 0.002

No Dust y = 0.046 + 0.074x 0.983

Tabago Dust y = -0.392 + 0.099x 0.956 0.174 0.760

No Dust y = -0.422 + 0.116x 0.977

USVI Dust y = 0.294 + 0.104x 0.987 0.141 < 0.001

No Dust y = -0.081 + 0.092x 0.982

Serratia marcescences Mali Dust y = 0.133 + 0.197x 0.915 0.326 0.359

No Dust y = 0.037 + 0.235x 0.975

Tabago Dust y = 0.055 + 0.157x 0.957 0.251 0.232

No Dust y = -0.068 + 0.198x 0.957

USVI Dust y = 0.094 + 0.178x 0.933 0.901 0.831

No Dust y = 0.069 + 0.174x 0.953

Vibrio coralliilyticus Mali Dust y = 0.090 + 0.201x 0.877 0.203 0.811

No Dust y = 0.059 + 0.287x 0.945

Tabago Dust y = 0.059 + 0.179x 0.920 0.299 0.413

No Dust y = -0.025 + 0.232x 0.928

USVI Dust y = 0.122 + 0.292x 0.838 0.180 0.873

No Dust y = 0.370 + 0.117x 0.956

Vibrio shiloi Mali Dust y = 0.020 + 0.223x 0.955 0.434 0.746

No Dust y = 0.046 + 0.258x 0.945

Tabago Dust y = 0.395 + 0.115x 0.931 0.108 0.234

No Dust y = 0.037 + 0.269x 0.955

USVI Dust y = 0.123 + 0.277x 0.822 0.077 0.446

No Dust y = 0.417 + 0.106x 0.970
6

distance from the source region. The mineral and trace-element composition differs among particle-size 

fractions due to erosional processes, differences in parent mineral characteristics, and chemical and 

physical processes during long-distance atmospheric transport (Moreno and others, 2006; Castillo and 

others, 2008). Of greater biological significance, the biomobility and bioaccessibility of elements and 

cofactors have been shown to differ among atmospheric dust collected in Mali, Tobago, and the USVI, 

with bioactive elements generally more bioaccessible following long-distance transport to Tobago and 

USVI (Morman and others, 2013). Enhanced biomobility and bioaccessibility are likely a result of 

higher surface-area-to-volume ratios of fine particles that could enhance surface reactions and changes 

in the surface chemistry as a result of the extreme physical conditions of high-altitude long-distance 

transport, such as freeze-thaw cycles, exposure to UV, microbial activities, and atmospheric chemical reactions that could alter the surface components of the particles (Morman and others, 2013). The greater 

the distance from the dust-source region, the greater the duration of exposure to those conditions. 

Removal of the dust particles from the quartz fiber matrix of the filters was problematic, and 

some dust was observed within filters following the 72-h extraction period. Whether dust particles were 

selectively retained as a function of size is unknown. In future studies, testing different filter matrices 

for particle-release is recommended prior to filter selection.

This study is the first to assess the influence that the constituents in African dust have on the 

growth of bacterial isolates that have been implicated in coral diseases. The data from these experiments 

indicate the growth of the coral disease pathogens, A. coralicida and V. shiloi, may be stimulated by African dust extracts. However, the absence of a growth response from the other bacterial strains following 

exposure to dust-extract solutions is not necessarily proof that African dust is nonstimulatory. Though 

this is a possibility, another explanation is that the dose-response thresholds for the other bacterial 

strains are higher than that of A. coralicida and V. shiloi and that the constituents in the dust extracts did 

not exceed those thresholds. The simple increase or reduction in biomass production and (or) response 

time to metabolizing available nutrients is not necessarily directly associated with the ability of a bacterial species to induce a disease response in its host. Either response would provide a selective advantage 

under natural conditions, providing an opportunity to dominate a niche on or in the host and initiate disease pathology. These preliminary findings indicate that further laboratory studies are needed to quantify 

and elucidate the effects of African dust on coral pathogens. 

Recommendations

1. Culture conditions should be optimized to model those predicted in situ; for example, oligotrophic media or defined minimal media, temperatures in the range of those recorded at sea surface, 

and extended incubation times. 

2. A culturability assay should be included from which a standard curve of absorbance versus 

colony forming units (CFU) can be constructed.

3. An assay should be included that measures changes in respiration rates; for example, 14C-labeled 

substrates that generate 14CO2

, should be incorporated. This would be useful because stimulatory 

effects of dust may produce an increase in respiration without an increase in bacterial productivity. 
7

Aurantimonas coralicida

Aurantimonas coralicida

6050403020100

6050403020100

6050403020100

-0.2

Aurantimonas coralicida

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

2.5

2.0

1.5

1.0

0.5

0.0

Absorbance [log 10(Ab/Ab 0)] Absorbance [log 10(Ab/Ab 0)]

Absorbance [log 10(Ab/Ab 0)]

Virgin Islands

Time (h)

Tobago

Time (h)

Mali

Time (h)

Figure 1. The effect of dust addition on the growth of Aurantimonas coralicida by site. The black 

symbols represent data for the filter control (i.e., no dust addition) culture and the white symbols represent data from the experimental (i.e., dust addition) culture. Those symbols with a colored center 

are the data values used in the calculations of the linear regression models.
8

Vibrio shiloi

Vibrio shiloi Vibrio shiloi

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1050 15 20 25 30

1050 15 20 25 30

1050 15 20 25 30

-0.2

Absorbance [log 10(Ab/Ab 0)] Absorbance [log 10(Ab/Ab 0)]

Absorbance [log 10(Ab/Ab 0)]

Virgin Islands

Time (h)

Tobago

Time (h)

Mali

Time (h)

Figure 2. The effect of dust addition on the growth of Vibrio shiloi by site. The black symbols represent data for the filter control (i.e., no dust addition) culture and the white symbols represent data 

from the experimental (i.e., dust addition) culture. Those symbols with a colored center are the data 

values used in the calculations of the linear regression models.
9

Serratia marcescens

Serratia marcescens Serratia marcescens

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1050 15 20 25 30

1050 15 20 25 30

1050 15 20 25 30

Absorbance [log 10(Ab/Ab 0)] Absorbance [log 10(Ab/Ab 0)]

Absorbance [log 10(Ab/Ab 0)]

Virgin Islands

Time (h)

Tobago

Time (h)

Mali

Time (h)

Figure 3. The effect of dust addition on the growth of Serratia marcescens by site. The black symbols represent data for the filter control (i.e., no dust addition) culture and the white symbols represent data from the experimental (i.e., dust addition) culture. Those symbols with a colored center are 

the data values used in the calculations of the linear regression models.
10

Vibrio coralliilyticus

Vibrio coralliilyticus Vibrio coralliilyticus

1050 15 20 25 30

1050 15 20 25 30

1050 15 20 25 30

-0.2

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Absorbance [log 10(Ab/Ab 0)] Absorbance [log 10(Ab/Ab 0)]

Absorbance [log 10(Ab/Ab 0)]

Virgin Islands

Time (h)

Tobago

Time (h)

Mali

Time (h)

Figure 4. The effect of dust addition on the growth of Vibrio coralliilyticus by site. The black symbols 

represent data for the filter control (i.e., no dust addition) culture and the white symbols represent 

data from the experimental (i.e., dust addition) culture. Those symbols with a colored center are the 

data values used in the calculations of the linear regression models.
11

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