The earth's skies are a conduit for dust lifted high into the atmosphere by winds from the plant's deserts. Desert dusts travel thousands of kilometers before being deposited back to the earth's surface, including oceans. The majority of dust in the atmosphere comes from the Sahara and Sahel regions of Africa and is transported in large pulses across the Atlantic to South America in winter and spring and to the Caribbean and southeast US in the summer. Desert dusts are a rich source of nutrients (e.g., phosphorus and nitrogen) as well as essential trace elements (e.g., iron). The nutrients provided fertilize downwind ecosystems where dust eventually settles, and can be especially important in subtropical and tropical ocean waters where they occur in very limited supply. Dust has been shown to stimulate marine algae growth, but our team suspected that growth of certain marine bacteria may also be stimulated by dust and may even outcompete algae when dust first arrives. This project was aimed at testing this idea and focused in detail on a marine bacterial group, Vibrio, as a model organism. 

Vibrio are common marine bacteria, especially in tropical and subtropical waters. While they are typically found at low abundances, they are opportunistic and can often outcompete other bacteria when new sources of nutrients and iron are introduced. As 'first responders' to new nutrients, they can grow into large blooms for a short period of time. This ability to bloom is important for at least two reasons: 1) their rapid growth can affect availability of dust nutrients and trace metals to other marine organisms, and 2) high concentrations can affect both ecosystem and human health given that Vibrio are pathogenic to variety of marine organisms (including corals) and humans. 

Our team included microbiologists, oceanographers, and chemists who conducted lab-based studies and field work in the Florida Keys National Marine Sanctuary to address two main objectives. The first was to identify how and when the bacterial community in seawater changed with the introduction of Saharan dust and second to identify the components of dust that induced those changes. 

We studied dust events in the Florida Keys, Barbados, and the tropical Mid-Atlantic and found that levels of Vibrio consistently increased within 24 hours of dust arrival. Abundances increased by 2->100 fold between pre-dust and 24-h post dust water samples. During a 26-day field study conducted offshore in the Florida Keys, we found that the bacterial community was dynamic but largely consistent until dust arrival. Over three different dust events, we found the bacterial community first shifted toward Vibrio (<24 hours) before a pattern of successive 'blooms' of other bacteria and finally small cyanobacteria. These studies point to Vibrio having an important but short-lived role in processing dust associated nutrients, iron, or other factors that can then be used by other members of the marine community. We also observed that the early bloom in Vibrio appeared even in the benthic community (corals). This has broader implications, given that Vibrio can be important coral pathogens. 

To understand the specific components of dust that gave rise to this microbial community shift and increase in Vibrio during natural dust events, we conducted a series of controlled microcosm studies. Seawater from the Florida Keys was amended with inorganic iron, nitrate, ammonium, phosphate, and carbon and sampled at short intervals for >24 hours. We found that these amendments could not replicate what we observed during dust events in the field. Additional tests compared dust that had been leached (to obtain the soluble component of dust thought to contain the most bioavailable fractions of iron and nutrients) to whole dust (not leached so that particulates remained). The only treatment that effectively replicated the natural dust events was treatment with whole dust, where community composition changed in favor of Vibrio within 16 hours. Analyses of dust confirmed that the Vibrio were not introduced from the dust itself, but rather the delivery of particulate dust was essential for Vibrio growth. 

Education, training, and outreach were an important part of this study. Our team trained undergraduate, MS, and PhD students who have used elements from these studies toward theses and dissertations. Students included women and persons from under-represented groups in marine science. This work provided the basis for classroom lesson plans for local teachers as part of the SciREN-GA (https://sciren.org/about-sciren/our-teams/georgia-team/) organization led by one of our PhD students. Our work also reached a much broader audience through a BBC documentary "Dust Storms" that aired in March 2017, which highlighted our work as part of a broader examination of the effects of dust storms across the globe.  

Last Modified: 07/03/2019
Modified by: Erin K Lipp