Cyanobacterial carbon concentrating mechanisms facilitate sustained CO<sub>2</sub> depletion in eutrophic lakes
Phytoplankton blooms are increasing in frequency, intensity, and duration in aquatic ecosystems worldwide. In many eutrophic lakes, these high levels of primary productivity correspond to periods of CO<sub>2</sub> depletion in surface waters. Cyanobacteria and other groups of phytopla...
| Main Authors: | , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
|
| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/14/2865/2017/bg-14-2865-2017.pdf |
| Summary: | Phytoplankton blooms are increasing in frequency, intensity, and
duration in aquatic ecosystems worldwide. In many eutrophic lakes, these high
levels of primary productivity correspond to periods of CO<sub>2</sub> depletion in
surface waters. Cyanobacteria and other groups of phytoplankton have the
ability to actively transport bicarbonate (HCO<sub>3</sub><sup>−</sup>) across their cell
membrane when CO<sub>2</sub> concentrations are limiting, possibly giving them a
competitive advantage over algae not using carbon concentrating mechanisms
(CCMs). To investigate whether CCMs can maintain phytoplankton bloom biomass
under CO<sub>2</sub> depletion, we measured the <i>δ</i><sup>13</sup>C signatures of dissolved
inorganic carbon (<i>δ</i><sup>13</sup>C<sub>DIC</sub>) and phytoplankton
particulate organic carbon (<i>δ</i><sup>13</sup>C<sub>phyto</sub>) in 16
mesotrophic to hypereutrophic lakes during the ice-free season of 2012. We
used mass–balance relationships to determine the dominant inorganic carbon
species used by phytoplankton under CO<sub>2</sub> stress. We found a significant
positive relationship between phytoplankton biomass and phytoplankton
<i>δ</i><sup>13</sup>C signatures as well as a significant nonlinear negative
relationship between water column <i>ρ</i>CO<sub>2</sub> and isotopic composition of
phytoplankton, indicating a shift from diffusive uptake to active uptake by
phytoplankton of CO<sub>2</sub> or HCO<sub>3</sub><sup>−</sup> during blooms. Calculated
photosynthetic fractionation factors indicated that this shift occurs
specifically when surface water CO<sub>2</sub> drops below atmospheric equilibrium.
Our results indicate that active HCO<sub>3</sub><sup>−</sup> uptake via CCMs may be an important
mechanism in maintaining phytoplankton blooms when CO<sub>2</sub> is depleted. Further
increases in anthropogenic pressure, eutrophication, and cyanobacteria blooms
are therefore expected to contribute to increased bicarbonate uptake to
sustain primary production. |
|---|---|
| ISSN: | 1726-4170 1726-4189 |