Eco-physiological responses of nitrogen-fixing cyanobacteria to light

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (mu(max), alpha, I (k)) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth (I (k) < 9.09 mu mol photon m(-2) s(-1)) and high efficiency (alpha < 0.11 day(-1) mu mol photon(-1) m(2) s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low alpha (0.009 day(-1) mu mol photon(-1) m(2) s) and higher I (k) (35.5 mu mol photon m(-2) s(-1)). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 mu mol photon m(-2) s(-1) with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances a parts per thousand currency sign17 mu mol photon m(-2) s(-1), thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 mu mol photon m(-2) s(-1)) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated.