Recent theoretical work (Vos et al. 2004) predicts that inducible defences prevent strong population fluctuations under high levels of nutrient enrichment. Here we evaluate this model prediction and show that inducible defences in algae stabilize the dynamics of experimentally assembled bi-and tritrophic planktonic food chains. At high phosphorus levels, we observed strong population fluctuations in all food chains with undefended algae. These fluctuations did not occur when algae had inducible defences. At low phosphorus levels, we observed deterministic consumer extinctions, as predicted by stoichiometric theory. Our study thus shows that both biotically and abiotically induced changes in algal food quality affect the stability and persistence of planktonic food chains.
w ww ww w. .f fr ro on nt ti ie er rs si in ne ec co ol lo og gy y. .o or rg g I n 1934, Alfred C Redfield reported that the ratios between the elements carbon, nitrogen, and phosphorus (C, N, and P, respectively) in marine phytoplankton were remarkably constant (Redfield 1934). His famous C:N:P ratio of 106:16:1 (by atom) has since become known as the "Redfield ratio" (Falkowski and Davis 2004). Redfield further noted that the N:P ratios of phytoplankton resembled the nitrate:phosphate ratio found in the deep waters of the oceans (Redfield 1934). Thus, the elemental composition of marine plankton reflected that of their environment, and vice versa. However, even the seemingly constant pelagic environment is currently affected by changes in the Earth's atmosphere. The 1934 atmosphere contained ~300 parts per million (ppm) of carbon dioxide (CO 2 ), which has risen to the present-day value of 385 ppm, a level that far exceeds that of the natural range of the past 650 000 years. Atmospheric CO 2 levels are expected to rise further, to ~750 ppm by the year 2100 (Solomon et al. 2007). The rapid increase in atmospheric CO 2 and other greenhouse gases is accompanied by global warming. Average global temperatures have risen by 0.6˚C since 1934, and an additional 3˚C increase is expected to occur over the course of the 21st century (Solomon et al. 2007). These changes in global climate will affect many chemical and physical processes in aquatic ecosystems, with possible implications for the elemental composition of plankton communities.Ecological stoichiometry is a rapidly expanding research field investigating how the elemental composition of organisms affects ecological processes (Sterner and Elser 2002; Panel 1). Inspired by Redfield and the recent advances in ecological stoichiometry, this review explores the potential impacts of climate change on the C and nutrient availability in aquatic ecosystems, its consequences for the C:N:P stoichiometry of plankton communities, and its implications for the structure of aquatic food webs. Rising CO 2 and ocean acidificationThe current rise in atmospheric CO 2 levels is having a major impact on the C chemistry of the oceans (Doney et al. 2009). In fact, it is estimated that almost 50% of the anthropogenic CO 2 input into the atmosphere since the advent of the Industrial Revolution has been absorbed by the oceans (Sabine et al. 2004). Compared to the large pool of bicarbonate (HCO 3 -), dissolved CO 2 constitutes only a minor fraction of the total concentration of dissolved inorganic carbon (DIC) in the oceans. Yet, rising concentrations of atmospheric CO 2 increase the concen- In particular, atmospheric CO 2 concentrations will rise to unprecedented levels by the end of this century, while global warming will enhance stratification of aquatic ecosystems and may thereby diminish the supply of nutrients into the surface layer. These processes enrich phytoplankton with carbon, but suppress nutrient availability. Phytoplankton with a high carbon-to-nutrient content provide poor-q...
Several studies have described that cyanobacteria use blue light less efficiently for photosynthesis than most eukaryotic phototrophs, but comprehensive studies of this phenomenon are lacking. Here, we study the effect of blue (450 nm), orange (625 nm), and red (660 nm) light on growth of the model cyanobacterium Synechocystis sp. PCC 6803, the green alga Chlorella sorokiniana and other cyanobacteria containing phycocyanin or phycoerythrin. Our results demonstrate that specific growth rates of the cyanobacteria were similar in orange and red light, but much lower in blue light. Conversely, specific growth rates of the green alga C. sorokiniana were similar in blue and red light, but lower in orange light. Oxygen production rates of Synechocystis sp. PCC 6803 were five-fold lower in blue than in orange and red light at low light intensities but approached the same saturation level in all three colors at high light intensities. Measurements of 77 K fluorescence emission demonstrated a lower ratio of photosystem I to photosystem II (PSI:PSII ratio) and relatively more phycobilisomes associated with PSII (state 1) in blue light than in orange and red light. These results support the hypothesis that blue light, which is not absorbed by phycobilisomes, creates an imbalance between the two photosystems of cyanobacteria with an energy excess at PSI and a deficiency at the PSII-side of the photosynthetic electron transfer chain. Our results help to explain why phycobilisome-containing cyanobacteria use blue light less efficiently than species with chlorophyll-based light-harvesting antennae such as Prochlorococcus, green algae and terrestrial plants.Electronic supplementary materialThe online version of this article (10.1007/s11120-018-0561-5) contains supplementary material, which is available to authorized users.
Abstract. Resource edibility is a crucial factor in ecological theory on the relative importance of bottom-up and top-down control. Current theory explains trophic structure in terms of the relative abundance and succession of edible and inedible species across gradients of primary productivity. We argue that this explanation is incomplete owing to its focus on inedibility and the assumption that plants and herbivores have fixed defense levels. Consumer-induced defenses are an important source of variation in the vulnerability of prey and are prevalent in natural communities. Such induced defenses decrease per capita consumption rates of consumers but hardly ever result in complete inedibility. When defenses are inducible a prey population may consist of both undefended and defended individuals. Here we use food chain models with realistic parameter values to show that variation in consumption rates on different prey types causes a gradual instead of stepwise increase in the biomass of all trophic levels in response to enrichment. Such all-level responses have been observed in both aquatic and terrestrial ecosystems and in microbial food chains in the laboratory. We stress that, in addition to the known food web effects of interspecific variation in edibility, intraspecific variation in edibility is another form of within-trophic-level heterogeneity that also has such effects. We conclude that inducible defenses increase the relative importance of bottom-up control.
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