CO₂ limited conditions favor cyanobacteria in a hypereutrophic lake: An empirical and theoretical stable isotope study

Abstract

Harmful algal blooms (HABs) are a global problem, exacerbated by rising temperatures, cultural eutrophication, urbanization, and agricultural development. During these HABs, phytoplankton consumption of CO2 may result in conditions of C limitation, where algal taxa best adapted for these conditions will be at a competitive advantage. Many cyanobacteria are capable of alleviating CO2 limitation by a variety of strategies, including the active assimilation of HCO3−. In this study, we utilized a high‐resolution, month‐long time series of stable C isotopes and high‐performance liquid chromatograph‐based algal taxonomy in the hypereutrophic Lake Taihu, China, to investigate whether cyanobacteria are indeed advantaged by CO2 limiting conditions. We employed a model of phytoplankton C acquisition to support the inferences derived from direct measurements. Diurnal cycles of production and respiration caused δ13CDIC to vary between −4‰ and −9‰, while δ13CPOC varied between −29.6‰ and −19.6‰. Measured and modeled phytoplankton fractionation of DIC were positively correlated with pCO2 and negatively correlated with cyanobacterial abundance, suggesting that CO2 limitation preferentially favored increased cyanobacterial biomass, relative to other taxa. We propose that the ability of many cyanobacteria to access otherwise limiting pools of inorganic C is intrinsically linked with their capacity to cope with CO2 limiting conditions, and may be a key factor in their dominance during HABs.

Sources and sinks of dissolved inorganic carbon in an urban tropical coastal bay revealed by δ13C-DIC signals

“…Under conditions of low pCO 2 , the δ 13 C-DIC fractionation decreases during the afternoon, reflecting the decline in carbon assimilation efficiency. This decrease of fractionation under CO 2 limitation was showed experimentally for marine diatoms (Burkhardt et al, 1999), and it is consistent with results found in a hypertrophic Chinese lake under conditions of CO 2 sub saturation (Van Dam et al, 2018). In addition, under low availability of dissolved CO 2 , the phytoplankton can consumes bicarbonate (HCO 3 − ) (Burns and Beardall, 1987), and this could contribute to decrease the isotopic fractionation factor since that the isotope ratio of HCO 3 − is about 8‰ more positive than that of dissolved CO 2 (Fogel and Cifuentes, 1993).…”

“…This suggests that the discrimination against 13 C is higher when the availability of dissolved CO 2 is higher. Previous studies showed similar results, both in culture experiments (Fogel and Cifuentes, 1993), and in situ at a Chinese hypereutrophic lake (Van Dam et al, 2018). The δ 13 C-DIC fractionation by phytoplankton in Guanabara Bay follows a Rayleigh distillation (Fig.…”

Carbon and nitrogen recycling during cyanoHABs in dreissenid-invaded and non-invaded US midwestern lakes and reservoirs

“…Labels are removed in the bottom panel for clarity. Cyanobacteria are typically a large fraction of total biomass in eutrophic lakes (Van Dam et al, 2018) and were abundant in our samples (Figs. 1, 2; Fig.…”

“…Here, we assume photosynthesis from water column algal and cyanobacterial carbon fixation is the dominating factor imparting the d 13 C values of organic carbon. We acknowledge that this is a simplification as biotic and abiotic factors including the carbon concentrating reaction, growth rate, photochemical degradation, and pCO 2 can influence d 13 C values of organic carbon in eutrophic lakes (Morales-Williams et al, 2017;Van Dam et al, 2018). However, given the abundance of cyanobacteria during cyanoHABs including in our data set ( Fig.…”

“…S1), it is reasonable to assume that photosynthesis in the water column is the major factor imparting the d 13 C values of organic carbon. Indeed, in Lake Taihu, pCO 2 was below equilibrium with the atmosphere at the peak of the cyanoHAB (Van Dam et al, 2018).…”