Seagrass ecosystems reduce exposure
to bacterial pathogens of humans,
fishes, and invertebrates
Joleah B. Lamb,1 Jeroen A. J. M. van de Water,2,3 David G. Bourne,2,4 Craig Altier,5
Margaux Y. Hein,4 Evan A. Fiorenza,1 Nur Abu,6 Jamaluddin Jompa,6 C. Drew Harvell1
Plants are important in urban environments for removing pathogens and improving water
quality. Seagrass meadows are the most widespread coastal ecosystem on the planet.
Although these plants are known to be associated with natural biocide production, they
have not been evaluated for their ability to remove microbiological contamination. Using
amplicon sequencing of the 16S ribosomal RNA gene, we found that when seagrass
meadows are present, there was a 50% reduction in the relative abundance of potential
bacterial pathogens capable of causing disease in humans and marine organisms.
Moreover, field surveys of more than 8000 reef-building corals located adjacent to
seagrass meadows showed twofold reductions in disease levels compared to corals at
paired sites without adjacent seagrass meadows. These results highlight the importance
of seagrass ecosystems to the health of humans and other organisms.
Natural filtration systems are widely used to remove human pathogenic microorga- nisms from terrestrial effluent. Successful applications range from sewage treatment works to constructed wetlands (1), mangrove forests (2), and restored bivalve reefs (3).
Despite being the most widespread coastal ecosystem on the planet (4), seagrass meadows have
not been evaluated as a system for pathogen removal or disease mitigation. Plants have been
used effectively in many pollution treatment systems (5) because they inactivate pathogens by
exposure to natural biocides, biofilm interactions,
nutrient removal, and the alteration of soil or
water chemistry. In vitro, phytochemicals isolated from seagrass tissues have been shown to
kill or inhibit numerous bacterial pathogens that
affect humans, fishes, and invertebrates (6).
Here we collected data from a natural field
setting (7) to assess the influence of biodiverse
seagrass meadows on marine microbial pathogens
and disease. We selected four islands located
on the midshelf of the Spermonde Archipelago,
Indonesia (fig. S1). Each island consists of bio-
genic sediments from the surrounding reef plat-
forms (8) that enclose areas of intertidal and
shallow subtidal flats with expansive areas of
mixed-species seagrass meadows. Human popu-
lation densities on the islands range between 96
and 325 people per hectare (9). The islands lack
basic sanitation systems, and surface soils are
thin and poorly retain wastewater (10). To en-
sure suitable comparisons, we selected a set of
paired sites (7) where seagrass meadows were
either present or absent on the intertidal flat
surrounding each island (fig. S2 and table S1).
As a standard indicator for enteric wastewater
pollution and source of multiple infections in
humans and marine organisms (11), we first used
Enterococcus assays to test whether the presence
of seagrass meadows influenced the level of a
single bacterial pathogen (7). Our shore seawater
samples, collected from all four paired island sites
[median colony forming units (CFU) 100 ml−1 =
1123, n = 33 samples for each status (seagrass
present versus absent)], exceeded the U.S. Envi-
ronmental Protection Agency (EPA)–recommended
human health risk exposure levels of Enterococcus
in recreational water by 10-fold (12) (Fig. 1).
Seawater collected in open water between the
study islands (7) had low levels of Enterococcus
(range = 0 to 12 CFU, median = 4 CFU, n = 40
samples), indicating that wastewater pollution
was diffuse and likely originated from individ-
ual islands. Levels of Enterococcus in seawater
were reduced when seagrass meadows were
present compared with paired sites without sea-
grass meadows (generalized linear mixed model,
estimate ± SEstatus × location = –0.786 ± 0.202, P <
0.001) (Fig. 1 and table S2). On the intertidal
flats, Enterococcus levels in seawater were three-
fold lower when seagrass meadows were pres-
ent (n = 82 samples) compared with the paired
sites (n = 72 samples). Similarly, seawater col-
lected above the coral reef adjacent to seagrass
meadows (n = 86 samples) had twofold lower
levels of Enterococcus than paired sites (n = 82
samples) (generalized linear mixed model, Fig.
1 and table S3).
Enterococcus levels often correlate with other
human bacterial pathogens found in wastewater
1Department of Ecology and Evolutionary Biology, Cornell
University, Ithaca, NY, USA. 2Australian Institute of Marine
Science, Townsville, Queensland, Australia. 3Department of
Marine Biology, Centre Scientifique de Monaco, Monaco.
4College of Science and Engineering, James Cook University,
Townsville, Queensland, Australia. 5Department of Population
Medicine and Diagnostic Sciences, College of Veterinary
Medicine, Cornell University, Ithaca, NY, USA. 6Faculty of
Marine Science and Fisheries, Hasanuddin University,
Makassar, Sulawesi, Indonesia.
*Corresponding author. Email: firstname.lastname@example.org
Fig. 1. Seagrass meadows influence the occurrence of enteric indicator bacteria. Comparisons of
mean (±SE) CFU of enterococci in 100 ml of seawater. Values above the red dashed line exceed the EPA-recommended human health risk exposure levels for a single water sample in recreational waters (12).
Differences tested using a generalized linear mixed model followed by pairwise comparisons and Bonferroni
correction, where ***P < 0.001 (table S3). Means calculated from paired sites among four study islands. Petri
dish exemplifies cultured enterococci.