Ewan J.A. Minter

Combined effects of ammonia and microcystin on survival, growth, antioxidant responses, and lipid peroxidation of bighead carp Hypophthalmythys nobilis larvae.

Hazardous materials, such as ammonia and microcystin, are released into lakes during cyanobacterial bloom degradation and may severely impact aquatic organisms. To assess the combined effects of ammonia and microcystin on survival, growth, and oxidative stress of larval fish, 14-day-old larvae of bighead carp Hypophthalmythys nobilis were exposed to solutions with different combined concentrations of ammonia (0, 0.06, 0.264mgL(-1)) and microcystin (0, 2, 10, 30μgL(-1)) for 10 days. Microcystin significantly decreased body length, while ammonia significantly increased body weight, specific growth rate, and condition factor, but there was no significant interaction between ammonia and microcystin on them. Superoxide dismutase, catalase, and malondialdehyde significantly changed with microcystin concentration, whereas glutathione was not affected by microcystin. Ammonia significantly affected the antioxidant system. There were significant interactions between ammonia and microcystin on superoxide dismutase and malondialdehyde. Our data clearly demonstrate that ammonia and microcystin adversely affect bighead carp larvae.

The interactive effects of microcystin and nitrite on life-history parameters of the cladoceran Daphnia obtusa

Elevated nitrite and microcystin concentrations co-occur during degradation of Microcystis blooms and are toxic to aquatic organisms. We studied the relative and combined effects of these on Daphnia obtusa life-history. Nitrite and microcystin-LR treatments were: 0, 1, 3 mg L(-1) and 0, 10, 100, 300 μg L(-1), respectively. Experiments were factorial with 12 treatment combinations. Incubations were 15 d and recorded: moult number; time to first batch of eggs; time to first clutch; size at first batch of eggs; size at first clutch; number of clutches per female; number of offspring per clutch; total offspring per female. Interactive effects of the toxins occurred for time to first batch of eggs and time to first clutch. The remaining traits were negatively affected by nitrite: a significant decrease occurred in number of offspring per clutch and total number of offspring per mother (both decreased by ∼ 50%); total clutches per mother; number of moults; mother size at first clutch; and first appearance of eggs (primarily at the highest nitrite concentration). We support the literature, recognising nitrite is toxic, and although Microcystis is toxic to zooplankton, the main threat is not from dissolved microcystin but from degradative products such as nitrite.

Host control and nutrient trading in a photosynthetic symbiosis

Photosymbiosis is one of the most important evolutionary trajectories, resulting in the chloroplast and the subsequent development of all complex photosynthetic organisms. The ciliate Paramecium bursaria and the alga Chlorella have a well established and well studied light dependent endosymbiotic relationship. Despite its prominence, there remain many unanswered questions regarding the exact mechanisms of the photosymbiosis. Of particular interest is how a host maintains and manages its symbiont load in response to the allocation of nutrients between itself and its symbionts. Here we construct a detailed mathematical model, parameterised from the literature, that explicitly incorporates nutrient trading within a deterministic model of both partners. The model demonstrates how the symbiotic relationship can manifest as parasitism of the host by the symbionts, mutualism, wherein both partners benefit, or exploitation of the symbionts by the hosts. We show that the precise nature of the photosymbiosis is determined by both environmental conditions (how much light is available for photosynthesis) and the level of control a host has over its symbiont load. Our model provides a framework within which it is possible to pose detailed questions regarding the evolutionary behaviour of this important example of an established light dependent endosymbiosis; we focus on one question in particular, namely the evolution of host control, and show using an adaptive dynamics approach that a moderate level of host control may evolve provided the associated costs are not prohibitive.

A rapid and cost-effective quantitative microsatellite genotyping protocol to estimate intraspecific competition in protist microcosm experiments

Summary 1. High levels of intraspecific variation are commonly observed in natural microbial populations, yet the consequences of this variation for ecological and evolutionary processes remain poorly understood. Protists are excellent experimental models for investigating fundamental and applied questions in ecology and evolution, but studying intraspecific variation remains a challenge due to a lack of molecular resources to aid in quantifying and distinguishing strains during experiments. 2. Here we present a molecular method, quantitative microsatellite genotyping, to accurately quantify strain-specific frequencies from microcosm experiments of the marine flagellate Oxyrrhis marina, both between many pairs of strains and between strains in a multistrain mixture. 3. We find that for pairs of strains, the method is effective for relative frequencies as low as 0� 02 and with around 99% accuracy. The method is able to quantify four strains reasonably well, though less accurate than for pairs (range 92–97% accuracy). 4. This makes accessible a cheap and easy-to-implement method for quantifying strain (or allele) frequencies and is suitable for use in a broad range of single-celled eukaryotes (protists) where copy number should correlate well with number of individuals (i.e. cells). This opens up the possibility of examining the role of intraspecific variation using experimental protist microcosms.

Negative frequency-dependent selection is intensified at higher population densities in protist populations

Natural populations of free-living protists often exhibit high-levels of intraspecific diversity, yet this is puzzling as classic evolutionary theory predicts dominance by genotypes with high fitness, particularly in large populations where selection is efficient. Here, we test whether negative frequency-dependent selection (NFDS) plays a role in the maintenance of diversity in the marine flagellate Oxyrrhis marina using competition experiments between multiple pairs of strains. We observed strain-specific responses to frequency and density, but an overall signature of NFDS that was intensified at higher population densities. Because our strains were not selected a priori on the basis of particular traits expected to exhibit NFDS, these data represent a relatively unbiased estimate of the role for NFDS in maintaining diversity in protist populations. These findings could help to explain how bloom-forming plankton, which periodically achieve exceptionally high population densities, maintain substantial intraspecific diversity.

Acute effects of ammonia on antioxidative response and gill Na+/K+ ATPase activity of juvenile Australian red claw crayfish (Cherax quadricarinatus)

The acute toxicity of ammonia to juvenile Cherax quadricarinatus was evaluated. The 24-, 48-, 72-, and 96-h LC50 values of total ammonia nitrogen (TAN) were 51.02 (44.58–58.43), 24.99 (19.17–32.89), 17.83 (13.50–37.22), and 15.38 (8.50–27.94) mg L−1 while the LC50s of un-ionized ammonia nitrogen (NH3–N) were 1.19 (1.04–1.37), 0.58 (0.45–0.77), 0.42 (0.32–0.87), and 0.36 (0.20–0.65) mg L−1, respectively. The safe concentration of ammonia (NH3) for juvenile C. quadricarinatus was TAN = 1.8 mg L−1 (NH3–N = 0.04 mg L−1, pH 7.6, 26°C). LC50 of NH3 decreased significantly with time and fitted a hyperbolic decay model well. Based on the LC50 values, a 96-h NH3 exposure was then conducted, using four test concentrations (4, 8, 12, and 16 mg L−1 TAN) plus a control group. Hepatopancreas catalase (CAT) and superoxide dismutase (SOD) activities fluctuated with time and NH3 concentration and had a similar trend at 24 and 48 h. Both exhibited a peak after 24 h of exposure to 8 mg L−1 TAN and did not change significantly at 48 h. At 96 h, SOD activity significantly decreased with increasing NH3 concentration while CAT activity remained unchanged. Malondialdehyde (MDA) level in hepatopancreas did not change in any groups at 24 h. At 48 and 96 h, it increased as NH3 concentration increased. There was no significant interaction between time and NH3 concentration on SOD, CAT, nor MDA. After 96 h, notable depressions of the gill Na+/K+ ATPase activity were observed in high NH3 concentration groups. The results suggest that the observed lethal toxicity of NH3 could be related to a gradual dysfunction of antioxidant systems and Na+/K+ ATPase activity leading to antioxidant system and osmoregulatory depression. The results will be helpful to aquaculture farmers in preventing depression of productivity caused by elevated NH3.

Variation and asymmetry in host-symbiont dependence in a microbial symbiosis

BackgroundSymbiosis is a major source of evolutionary innovation and, by allowing species to exploit new ecological niches, underpins the functioning of ecosystems. The transition from free-living to obligate symbiosis requires the alignment of the partners’ fitness interests and the evolution of mutual dependence. While symbiotic taxa are known to vary widely in the extent of host-symbiont dependence, rather less is known about variation within symbiotic associations.ResultsUsing experiments with the microbial symbiosis between the protist Paramecium bursaria and the alga Chlorella, we show variation between pairings in host-symbiont dependence, encompassing facultative associations, mutual dependence and host dependence upon the symbiont. Facultative associations, that is where both the host and the symbiont were capable of free-living growth, displayed higher symbiotic growth rates and higher per host symbiont loads than those with greater degrees of dependence.ConclusionsThese data show that the Paramecium-Chlorella interaction exists at the boundary between facultative and obligate symbiosis, and further suggest that the host is more likely to evolve dependence than the algal symbiont.