Archive for harmful algal bloom

The ecology of the biological carbon pump

Posted by mmaheigan 
· Tuesday, October 15th, 2019 

Plankton in the surface ocean convert CO2 into organic biomass thereby fueling marine food webs. Part of this organic biomass sinks down into the deep ocean, where the surface-derived organic carbon, or respired CO2, is locked in for decades to millennia. Without the biological carbon pump, atmospheric CO2 would be ~200 ppm higher than it is today. We know that ecological processes in the surface ocean plankton communities have a paramount importance on the efficiency of the biological carbon pump. Unfortunately, however, the mechanisms how ecology determines sinking fluxes are poorly understood.

A recent study in Global Biogeochemical Cycles used large-scale in situ mesocosms to explore how the ecological interplay within plankton communities affects the downward flux of organic material. Organic biomass tends to sink faster when produced by smaller organisms because the sinking material they generate forms dense aggregates. Conversely, larger organisms produce relatively porous particles that sink more slowly.

Figure: Flow chart illustrating how plankton community structure affects the properties of sinking organic particles and ultimately the strength and efficiency of the biological carbon pump. The thick arrows at the bottom indicate that flux attenuation depends on the properties of particulate matter formed in the surface ocean. For example, slow-sinking porous aggregates containing large amounts of easily degradable organic substances will decay faster (right side) than dense aggregates of more refractory organic matter (left side).

The key finding of this study was the unexpectedly large influence that plankton community composition has on the degradation rate of sinking organic biomass. In fact, degradation rates changed maximally 15-fold over the course of the study while sinking speed changed only 3-fold. Degradation rate of sinking material, measured in oxygen consumption assays, was quite variable and tended to be higher for more easily degradable fresh organic matter. The rate was lower during harmful algal blooms, which produce toxic substances that inhibit organisms that feed on aggregates thereby reducing degradation rates. These findings are an important step forward as they show that our predictive understanding of the biological carbon pump could be improved substantially when linking degradation rates of sinking material with ecological processes in surface ocean plankton communities.

Authors:
L. T. Bach (University of Tasmania)
P. Stange, J. Taucher, E. P. Achterberg, M. Esposito, U. Riebesell (GEOMAR)
M. Algueró‐Muñiz (Alfred-Wegener-Institut Helmholtz)
H. Horn (NIOZ and Utrecht University)

Phytoplankton can actively diversify their migration strategy in response to turbulent cues

Posted by mmaheigan 
· Thursday, August 17th, 2017 

Turbulence is known to be a primary determinant of plankton fitness and succession. However, open questions remain about whether phytoplankton can actively respond to turbulence and, if so, how rapidly they can adapt to it. Recent experiments have revealed that phytoplankton can behaviorally respond to turbulent cues with a rapid change in shape, and this response occurs over a few minutes. This challenges a fundamental paradigm in oceanography that phytoplankton are passively at the mercy of turbulence.

Phytoplankton are photosynthetic microorganisms that form the base of most aquatic food webs, impact global biogeochemical cycles, and produce half of the world’s oxygen. Many species of phytoplankton are motile and migrate in response to gravity and light levels: Upward toward light during the day to photosynthesize and downward at night toward higher nutrient concentrations. Disruption of this diurnal migratory strategy is an important contributor to the succession between motile and non-motile species when conditions become more turbulent. However, this classical view neglects the possibility that motile species can actively respond in an effort to avoid layers of strong turbulence. A recent study by Sengupta, Carrara and Stocker, published in Nature has shown that some raphidophyte and dinoflagellate phytoplankton can actively diversify their migratory strategy in response to hydrodynamic cues characteristic of overturning by the smallest turbulent eddies in the ocean. Laboratory experiments in which cells experienced repeated overturning with timescales and statistics representative of ocean turbulence revealed that over timescales as short as ten minutes, an upward-swimming population split into two subpopulations, one swimming upward and one swimming downward. Quantitative morphological analysis of the harmful algal bloom-forming raphidophyte Heterosigma akashiwo revealed that this behavior was accompanied by a change in cell shape, wherein the cells that changed their swimming direction did so by going from an asymmetric pear shape to a more symmetric egg shape. A model of cell mechanics showed that the magnitude of this shift was minute, yet sufficient to invert the cells’ preferential swimming direction. The results highlight the advanced level of control that phytoplankton have on their migratory behavior.

Understanding how fluctuations in the oceans’ turbulence landscape impacts phytoplankton is of fundamental importance, especially for predicting species succession and community structure given projected climate-driven changes in temperature, winds, and upper ocean structure.

An upward-swimming phytoplankton population splits into upward- and downward-swimming sub-populations when exposed to turbulent eddies, due to a subtle change in cell shape. Illustration by: A. Sengupta, G. Gorick, F. Carrara and R. Stocker

 

This work was co-funded by a Human Frontier Science Program Cross Disciplinary Fellowship (LT000993/2014-C to A.S.), a Swiss National Science Foundation Early Postdoc Mobility Fellowship (to F.C.), and a Gordon and Betty Moore Marine Microbial Initiative Investigator Award (GBMF 3783 to R.S.)

 

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