Many of us are too young to remember a world without plastics. They’ve brought many conveniences to our lives, but the disposable nature of plastics has also created a consumer culture filled with social and ecological issues. At least 8 million tons of plastics enter the oceans annually and have generated a “Plastisphere” that is disrupting native flora and habitats. The organisms that colonize buoyant marine plastic debris can be transported across the oceans and, in some cases, become invasive species of fragile ecosystems.
Trillions of plastic debris fragments are afloat at sea, creating a previously-unknown habitat for microbial colonization.
Introduced more than 50 years ago, plastic substrates are a novel microbial habitat in the world’s oceans. (“Plastics” are polymeric materials that can be molded or shaped, usually by the application of heat and pressure. “Substrates” are used in a converting process such as printing or coating to generally describe the base material onto which images will be printed.) Plastics become rapidly colonized by microbes when released into marine environments.
What Are The Implications Of Plastic Marine Debris?
The plastisphere consists of a complex community comprised of bacterial, archaeal, and eukaryotic microorganisms as well as microscopic animals.
Biofilm formation in the marine environment – a collective of one or more types of microorganisms that can grow on many different surfaces – is a complex process, involving many variables. Biofilms usually start with the adhesion of bacterial cells which modify the surface physicochemical properties, thus influencing the adhesion of successive colonizers such as algae, cyanobacteria, and protists.
Buoyant plastics in the photic layers of the oceans are colonized by photosynthetic primary producers that empower the Plastisphere through the production of labile photosynthate. The availability of this labile source of carbon and energy will facilitate the development of large biofilms that specialize in the use of photosynthate, outcompeting any potential plastic — or plastic additive — biodegrading organism, according to studies in Environmental Science & Technology. These unnatural additions to sea surface waters and the large quantity of cells and biomass carried by plastic debris has the potential to impact biodiversity, ecological functions, and biogeochemical cycles within the ocean.
FAU Oceanographic Research About The Plastisphere
Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute and Harriet L. Wilkes Honors College, in collaboration with Utrecht University, Netherlands, the University of Amsterdam, and The Royal Netherlands Institute for Sea Research (NIOZ), examined cell abundances, size, cellular carbon mass and how photosynthetic cells differ on polymeric and glass substrates over time. This study is fundamentally different from others due to the relatively non-biased visualization methods used to arrive at a quantitative number for biomass, which is the first estimate of its kind.
A research summary describes how the team investigated nanoparticle generation from plastic such as polystyrene, which is known to disintegrate into nanoparticles in sunlight and ultraviolet radiation, and how this might disrupt microalgae. Examination of time-course elements to incubation schemes demonstrated that buoyant plastics remaining in the euphotic layers of the ocean will be colonized by thriving phototrophic microbes that feed the Plastisphere. As soon as the particle sinks out of the euphotic layer and photosynthate is no longer produced, the biofilm will self-consume, reducing its size and, hence, regaining buoyancy.
Results of the study, published in the ISME Journal, a monthly publication of the International Society for Microbial Ecology, reveal that by measuring the average microbial biomass carrying capacity of different plastic polymers and, by extension, plastic marine debris in the global ocean, conservative estimates suggest that about 1% of microbial cells in the ocean surface microlayer inhabit plastic debris globally. This mass of cells would not exist if plastic debris was not in the ocean, and therefore, represents a disruption of the proportions of native flora in that habitat.
Using confocal laser scanning microscopy with sophisticated imaging software, researchers directly obtained data ranging from cell counts, size and the characterization of microbial morphotypes, to complete three-dimensional constructs. They tested a range of chemically distinct substrates that included polypropylene, polystyrene, polyethylene, and glass. Polypropylene is used by the automotive industry, for consumer goods such as packaging, industrial applications and the furniture market; polystyrene is used to make clear products like food packing or laboratory equipment; and polyethylene is the most widely used plastic in the world ranging from products such as clear food wrap to shopping bags to detergent bottles.
Data from the confocal laser scanning microscopy showed that early biofilms displayed a high proportion of diatoms (unicellular eukaryotic microalgae that have cell walls made of glass). These diatoms could play a key role in the sinking of plastic debris. Unexpectedly, plastic substrates appeared to reduce the growth of photosynthetic cells after 8 weeks compared to glass.
Diatoms at high coverage have the ability to make plastics sink due to their dense glass cell walls. Cellular DNA shows up as blue due to the stain the intercalates in between the DNA base pairs. The blue dotted lines are filamentous bacteria consisting of chains of cells, each blue dot represents a single cell with a bundle of DNA at the center. The scale bar is 20 microns in length.
“The quantification of cell numbers and microbial biomass on plastic marine debris is crucial for understanding the implications of plastic marine debris on oceanic ecosystems,” said Shiye Zhao, PhD, first author and a post-doctoral fellow at FAU’s Harbor Branch. “Future efforts should focus on how this biomass fluctuates with season and latitude and its potential to perturb the flux of nutrients in the upper layers of the ocean.”
The researchers estimated that plastic marine debris (PMD) surface area indicates a potentially large and durable plastisphere for microbial colonization. Trace nutrients were concentrated on solid surfaces in the water column, becoming more bioavailable and stimulating bacterial respiration. Thus, PMD in the oligo-trophic subtropical gyres could provide a nutritional benefit for sessile microbes, altering the open ocean community and microbial loop by stimulating attached communities and gross production. It would also simultaneously strip nutrients from the water column, inhibiting the growth of free-living cells.
“In the open ocean, nutrients are limiting. Just like we need to put fertilizer on a garden, microorganisms in the ocean are limited by nitrogen, iron, or phosphorous depending upon where they are — except in the open ocean, there is typically no fertilizer, so something has to die for another organism to live,” said Tracy Mincer, PhD, lead author and an assistant professor of biology/bio-geochemistry at FAU’s Harbor Branch and Wilkes Honors College. “With the advantage of a surface, which concentrates nutrients, organisms colonizing plastics in the ocean are taking up those limiting nutrients that normally would have been consumed or out-competed by free-living microbes. So essentially, these microbes on plastics are taking habitat space away and represent the beginning of a regime shift for these habitats.”
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