Astrobiology and cyanobacterial aggregation

In today’s ocean, animal grazers produce fecal pellets and aggregates that dominate carbon export, or the transport of organic matter from the surface ocean to the deep sea. During most of Earth’s history, however, marine life was limited to unicellular prokaryotes. These microorganisms are too small to sink as single cells, which presents a problem: how did carbon sink from the surface ocean to the deep if bacteria and their progenitors couldn’t sink on their own? The oxygenation of early Earth’s atmosphere could not have occurred without the sedimentation and burial of these single cells because carbon gobbled up in the surface ocean is simply released back into the atmosphere by respiration, interfering with oxygenation. The leading hypothesis states that these prokaryotic cells can form aggregates heavy enough to sink gravitationally to depth. Aggregate formation is driven by the production of sticky exopolymeric substances (EPS). Because of relation to carbon flux—which has implications for climate change—and to life on ancient Earth, this is a current topic of interest in ocean biogeochemistry and astrobiology.

As part of the Astrobiology Team at ASU, the Neuer lab elucidated what drives and affects aggregate formation by using a strain of Synechococcus, a marine cyanobacterium, as a model organism for early photosynthetic marine bacteria. Graduate students Wei Deng and Amy Hansen and undergraduate students Kim Mohabir and Logan Monks have combined various techniques to tackle this issue. With epifluorescence microscopy and particle counter analysis, cell growth and the number and size of aggregates can be tracked over culture ages. Alcian Blue test is used to quantify the transparent exopolymer particles (TEP, the main component that holds aggregates together), together with other biochemical measurements of Chl-a, DOC and POC etc. to evaluate the aggregation of Synechococcus. Cultures are also transferred into roller tanks to enhance the aggregation like natural marine snow, with subsequent determination of different aggregate characteristics (number, size, sinking speed etc.). Current and future experiments involve testing the influence of clay minerals, nutrient limitation and acidification on Synechococcus aggregation, as well as making implications for the carbon export from experimental settings that mimic the early oceanic environment.

 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