The aggregation of phytoplankton leads to the settling of particulate organic carbon in the form of marine snow, making it an important process in marine biogeochemical cycles. Diatoms >20 µm in size are considered to contribute appreciably to sinking particle fluxes due to aggregation and the production of transparent exopolymeric particles (TEP), the matrix for marine snow aggregates; however, it is not known whether nano-sized (2–20 µm) diatoms are able to aggregate and produce TEP. Here, we tested the aggregation and production of TEP by the nano-diatom Minutocellus polymorphus and investigated if interactions with bacteria influence aggregation by comparing axenic M. polymorphus cultures with co-cultures of the diatom with bacterial taxa known to colonize marine snow particles. We found that M. polymorphus form sinking aggregates and produce TEP comparably to other phytoplankton groups and that aggregation and TEP production were influenced depending on the species of bacteria added. Aggregation was enhanced in the presence of Marinobacter adhaerens HP15, but not in the presence of Pseudoalteromonas carrageenovora or Vibrio thalassae. Cell aggregation mediated by interactions with specific bacterial species are possible mechanisms behind the export of nano-sized diatoms, such as M. polymorphus, especially in oligotrophic open ocean regions where small phytoplankton dominate.
Effects of Phytoplankton Growth Phase on Settling Properties of Marine Aggregates
Marine snow aggregates often dominate carbon export from the surface layer to the deep ocean. Therefore, understanding the formation and properties of aggregates is essential to the study of the biological pump. Previous studies have observed a relationship between phytoplankton growth phase and the production of transparent exopolymer particles (TEP), the sticky particles secreted by phytoplankton that act as the glue during aggregate formation. In this experimental study, we aim to determine the effect of phytoplankton growth phase on properties related to aggregate settling. Cultures of the diatom Thalassiosira weissflogii were grown to four different growth phases and incubated in rotating cylindrical tanks to form aggregates. Aggregate excess density and delayed settling time through a sharp density gradient were quantified for the aggregates that were formed, and relative TEP concentration was measured for cultures before aggregate formation. Compared to the first growth phase, later phytoplankton growth phases were found to have higher relative TEP concentration and aggregates with lower excess densities and longer delayed settling times. These findings may suggest that, although particle concentrations are higher at later stages of phytoplankton blooms, aggregates may be less dense and sink slower, thus affecting carbon export.
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- Award ID(s):
- 1654276
- NSF-PAR ID:
- 10167433
- Date Published:
- Journal Name:
- Journal of Marine Science and Engineering
- Volume:
- 7
- Issue:
- 8
- ISSN:
- 2077-1312
- Page Range / eLocation ID:
- 265
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)Marine picocyanobacteria are ubiquitous primary producers across the world’s oceans, and play a key role in the global carbon cycle. Recent evidence stemming from in situ investigations have shown that picocyanobacteria are able to sink out of the euphotic zone to depth, which has traditionally been associated with larger, mineral ballasted cells. The mechanisms behind the sinking of picocyanobacteria remain a point of contention, given that they are too small to sink on their own. To gain a mechanistic understanding of the potential role of picocyanobacteria in carbon export, we tested their ability to form “suspended” (5–60 μm) and “visible” (ca. > 0.1 mm) aggregates, as well as their production of transparent exopolymer particles (TEP)—which are a key component in the formation of marine aggregates. Additionally, we investigated if interactions with heterotrophic bacteria play a role in TEP production and aggregation in Prochlorococcus and Synechococcus by comparing xenic and axenic cultures. We observed TEP production and aggregation in batch cultures of axenic Synechococcus, but not in axenic Prochlorococcus. Heterotrophic bacteria enhanced TEP production as well as suspended and visible aggregate formation in Prochlorococcus, while in Synechococcus, aggregation was enhanced with no changes in TEP. Aggregation experiments using a natural plankton community dominated by picocyanobacteria resulted in aggregation only in the presence of the ballasting mineral kaolinite, and only when Synechococcus were in their highest seasonal abundance. Our results point to a different export potential between the two picocyanobacteria, which may be mediated by interactions with heterotrophic bacteria and presence of ballasting minerals. Further studies are needed to clarify the mechanistic role of bacteria in TEP production and aggregation of these picocyanobacteria.more » « less
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Exopolymer particles are found throughout the ocean and play a significant biogeochemical role in carbon cycling. Transparent exopolymer particles (
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/jmse7080265
