Aggregation of marine picophytoplankton

Aggregation of marine picophytoplankton

The oceanic biological carbon pump (BCP) is the biologically-mediated way by which the oceans transform dissolved inorganic carbon into particulate and dissolved organic carbon, as well as subsequently export to depth. The traditional view of the BCP is the export of organic carbon by the settling of zooplankton fecal pellets and aggregates of large, mineral-ballasted phytoplankton cells — such as diatoms and coccolithophores; however, recent in-situ evidence have shown that picophytoplankton (0.2-2μm in size) may also contribute significantly to the export of particulate organic matter.

In this research, the Neuer lab will investigate the specific mechanisms that lie behind the contribution of these pico-sized cells — which have been previously thought to be too small to sink. Under controlled laboratory experiments, the Neuer group has elucidated the ability of the ubiquitous marine picocyanobacteria Synechococcus to form aggregates and sink at velocities comparable to those of marine snow, especially with the addition of clay minerals (Deng et al. 2015), in a project with efforts to investigate the potential mechanisms of carbon export in the early, Proterozoic ocean. Transparent exopolymeric particles (TEP) were shown to form the matrix of these aggregates (Deng et al. 2016), and in agreement with research conducted with other phytoplankton groups, interactions with heterotrophic bacteria further enhanced this aggregation and sinking.

In collaboration with the MENTOR effort at ASU, led by Associate Professor Dr. Hinsby Cadillo-Quiroz, the Neuer lab will expand upon the previous research on Synechococcus to the ubiquitous Prochlorococcus as well as eukaryotic groups of the picoplankton. In the lab, xenic and axenic pairs of picophytoplankton cultures will be screened for their ability to form aggregates and produce TEP. In addition, with the use of traditional, as well as high-throughput techniques, Cadillo-Quiroz’s group will isolate heterotrophic bacteria from aggregates formed in the laboratory and from marine snow collected in-situ — with the goal to elucidate potential interactions with associated prokaryotes that may enhance picophytoplankton aggregation and TEP production. This exciting project will contribute to the understanding of the BCP in oligotrophic ocean regions, where these micrometer-sized cells dominate.

 

Graduate student Bianca Cruz holding a flask of cultured phytoplankton.

Graduate student Bianca Cruz holding a flask containing cultured picophytoplankton.

 

Title image: Aggregates in roller tanks from xenic cultures of Micromonas pusilla (A) and Prochlorococcus marinus MED4 (B) in a 0.5mg/L kaolinite clay treatment.

References:

Deng W., B.N. Cruz and S. Neuer. 2016. Effects of nutrient limitation on cell growth, TEP production and aggregate formation of marine Synechococcus. Aquatic Microbial Ecology, 78, 39-49, doi.org/10.3354/ame01803

Deng W., L. Monks and S. Neuer. 2015. Effects of clay minerals on the aggregation and subsequent settling of marine Synechococcus. Limnology and Oceanography, 60, 805-816, DOI: 10.1002/lno.10059

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Posted on

July 8, 2017