The balance between photosynthesis and respiration explains the niche differentiation between Crocosphaera and Cyanothece

Crocosphaera and Cyanothece are both unicellular, nitrogen-fixing cyanobacteria that prefer different environments. Whereas Crocosphaera mainly lives in nutrient-deplete, open oceans, Cyanothece is more common in coastal, nutrient-rich regions. Despite their physiological similarities, the factors separating their niches remain elusive. Here we performed physiological experiments on clone cultures and expand upon a simple ecological model to show that their different niches can be sufficiently explained by the observed differences in their photosynthetic capacities and rates of carbon (C) consumption. Our exper- iments revealed that Cyanothece has overall higher photosynthesis and respiration rates than Crocosphaera. A simple growth model of these microorganisms suggests that C storage and consumption are previously under-appreciated factors when evaluating the occupation of niches by different marine nitrogen fixers

Photosynthesis in Chromera velia Represents a Simple System with High Efficiency

Chromera velia (Alveolata) is a close relative to apicomplexan parasites with a functional photosynthetic plastid. Even though C. velia has a primitive complement of pigments (lacks chlorophyll c) and uses an ancient type II form of RuBISCO, we found that its photosynthesis is very efficient with the ability to acclimate to a wide range of irradiances. C. velia maintain similar maximal photosynthetic rates when grown under continual light-limited (low light) or light-saturated (high light) conditions. This flexible acclimation to continuous light is provided by an increase of the chlorophyll content and photosystem II connectivity under light limited conditions and by an increase in the content of protective carotenoids together with stimulation of effective non-photochemical quenching under high light. C. velia is able to significantly increase photosynthetic rates when grown under a light-dark cycle with sinusoidal changes in light intensity. Photosynthetic activities were nonlinearly related to light intensity, with maximum performance measured at mid-morning. C. velia efficiently acclimates to changing irradiance by stimulation of photorespiration and non-photochemical quenching, thus avoiding any measurable photoinhibition. We suggest that the very high CO(2) assimilation rates under sinusoidal light regime are allowed by activation of the oxygen consuming process (possibly chlororespiration) that maintains high efficiency of RuBISCO (type II). Despite the overall simplicity of the C. velia photosynthetic system, it operates with great efficiency

Electron & Biomass Dynamics of Cyanothece Under Interacting Nitrogen & Carbon Limitations

Marine diazotrophs are a diverse group with key roles in biogeochemical fluxes linked to primary productivity. The unicellular, diazotrophic cyanobacterium Cyanothece is widely found in coastal, subtropical oceans. We analyze the consequences of diazotrophy on growth efficiency, compared to NO3–-supported growth in Cyanothece, to understand how cells cope with N2-fixation when they also have to face carbon limitation, which may transiently affect populations in coastal environments or during blooms of phytoplankton communities. When grown in obligate diazotrophy, cells face the double burden of a more ATP-demanding N-acquisition mode and additional metabolic losses imposed by the transient storage of reducing potential as carbohydrate, compared to a hypothetical N2 assimilation directly driven by photosynthetic electron transport. Further, this energetic burden imposed by N2-fixation could not be alleviated, despite the high irradiance level within the cultures, because photosynthesis was limited by the availability of dissolved inorganic carbon (DIC), and possibly by a constrained capacity for carbon storage. DIC limitation exacerbates the costs on growth imposed by nitrogen fixation. Therefore, the competitive efficiency of diazotrophs could be hindered in areas with insufficient renewal of dissolved gases and/or with intense phytoplankton biomass that both decrease available light energy and draw the DIC level down

Non-Photochemical Quenching in Cryptophyte Alga Rhodomonas salina Is Located in Chlorophyll a/c Antennae

Photosynthesis uses light as a source of energy but its excess can result in production of harmful oxygen radicals. To avoid any resulting damage, phototrophic organisms can employ a process known as non-photochemical quenching (NPQ), where excess light energy is safely dissipated as heat. The mechanism(s) of NPQ vary among different phototrophs. Here, we describe a new type of NPQ in the organism Rhodomonas salina, an alga belonging to the cryptophytes, part of the chromalveolate supergroup. Cryptophytes are exceptional among photosynthetic chromalveolates as they use both chlorophyll a/c proteins and phycobiliproteins for light harvesting. All our data demonstrates that NPQ in cryptophytes differs significantly from other chromalveolates – e.g. diatoms and it is also unique in comparison to NPQ in green algae and in higher plants: (1) there is no light induced xanthophyll cycle; (2) NPQ resembles the fast and flexible energetic quenching (qE) of higher plants, including its fast recovery; (3) a direct antennae protonation is involved in NPQ, similar to that found in higher plants. Further, fluorescence spectroscopy and biochemical characterization of isolated photosynthetic complexes suggest that NPQ in R. salina occurs in the chlorophyll a/c antennae but not in phycobiliproteins. All these results demonstrate that NPQ in cryptophytes represents a novel class of effective and flexible non-photochemical quenching

Chlorophyll a fluorescence quenching in <i>R. salina</i>.

<p>Cells were dark adapted for 20 minutes before and after irradiation. NPQ was induced by 100 s of orange actinic light (622 nm, 600 µmol m⁻² s⁻¹; white bar). Fluorescence induction curve (black line) represents a typical curve. The extent of NPQ (grey symbols, top part of the figure) was calculated as quenching of maximal fluorescence (F_M′-F_M)/F_M′ for every saturating flash (n = 3); the maximal fluorescence measured after light period (F_M″) reflects a fast recovery part of the F_M quenching. The value of maximal PSII efficiency calculated in dark (F_V/F_M) and on light (Genty parameter - φ_PSII) was 0.79 and 0.1 respectively.</p

Effect of various inhibitors on NPQ in <i>R. salina</i>.

<p>Cells were dark adapted for 20 minutes and then the NPQ was induced by 100 s exposure to orange light (622 nm, 600 µmol m⁻² s⁻¹; see white bar). <b>A</b>) Effect of ΔpH uncouplers nigericin and NH₄Cl. The maximal efficiency of PSII photochemistry (F_V/F_M) in the presence of uncouplers was 0.75 for control, 0.62 for nigericin and 0.68 for NH₄Cl. <b>B</b>) Effect of inhibitors of linear and cyclic electron transport DCMU, antimycin and rotenone. All data represent typical curves aligned to the same F_o level.</p

Effect of different DCCD (N,N′-dicyclohexyl-carbodiimide) concentrations on the maximal efficiency of PSII photochemistry (F_V/F_M) and on NPQ.

<p>F_V/F_M values were calculated for dark adapted sample, NPQ was detected after 120 s irradiation by orange light (620 nm, 600 µmol m⁻² s⁻¹).</p

Light dependence of NPQ and the efficiency of PSII (Genty parameter) in <i>R. salina</i>.

<p>Fresh sample was used for each measurement and values were recorded always after 40 s of irradiation by orange light (622 nm). Data represent average and standard deviation for n = 3.</p