Examining the life history of the cyanobacterium Cylindrospermopsis raciborskii in Indiana reservoirs using applied and molecular techniques

Clay Harrison Britton, Purdue University

Abstract

Cyanobacteria, also known as blue-green algae, are an integral part of the global ecosystem. Despite their ecological benefits, cyanobacteria can negatively impact the environment. They can also produce secondary metabolites that have potentially harmful effects on human health. Historically, management concerns have focused on the bloom-forming and toxin-producing “Big Three”: Anabaena sp., Aphanizomenon sp., and Microcystis sp. Within the past decade focus has shifted from these three to a fourth problematic cyanobacterium, Cylindrospermopsis raciborskii. Although numerous reports have described the growth characteristics and genetic variation of C. raciborskii populations from Australia and Europe, there are relatively few studies of North American populations. Most of these studies have been summer surveys or lab-based experiments on cultured isolates. Due to C. raciborskii ’s putative tropical origins, researchers were surprised when it was identified in bloom concentrations (i.e., >1.0x105 cells/mL) in Ball Lake, Indiana in 2001. Cylindrospermopsis raciborskii is now known to be prevalent in numerous temperate, North American systems. The purpose of this research was to examine the life history of Indiana populations of C. raciborskii through the use of applied and molecular techniques. A 28-month survey of Lake Lemon, Indiana followed seasonal and spatial fluctuations of C. raciborskii. We found that the cyanobacterium is able to persist year-to-year in a temperate climate. Water temperatures > 25°C were observed for at least three months of each year, leading to peak cell concentrations higher than 1.0x10 5 cells/mL each August. We also determined that akinetes (overwintering cells) were produced from May to November; their production presumably insures re-population of the lake each spring. Also, depth distribution data indicate that C. raciborskii is able to utilize the entire photic zone. An environmental chamber study allowed us to compare temperate (Lake Lemon) and semitropical (two Florida) isolates for growth rate, recovery ability, and akinete production under a light and temperature regime that mimicked the growing conditions of Lake Lemon during the summer, fall, winter, and spring seasons. Growth rates of all three isolates were comparable under good growing conditions, and they all recovered from 2-wk exposures to 5°C water temperatures. However, the temperate isolate recovered from these winter conditions at a faster rate than the subtropical isolates, suggesting that ecotypes of C. raciborskii may account for the adaptation of this species to northern waters. We also attempted to determine the ancestral, geographic origin of the Indiana C. raciborskii populations using phylogenetics, but our evidence does not indicate a “recent” migration of C. raciborskii. Instead, the data suggest that the cyanobacterium has a longer history in temperate, North America than initial reports indicate. To better understand the amount of genetic variation at the local population level and estimate the evolutionary potential of Indiana isolates (which might allow us to determine the likelihood of C. raciborskii ’s continued prevalence), we analyzed DNA sequences from six loci. We found evidence of high genetic variation among isolates within single populations, as well as between populations. We also discovered that each locus appears to be changing at different rates, suggesting that some loci are experiencing different selection pressures. Overall, this variation indicates that C. raciborskii is a highly adaptive organism. Understanding the life history of C. raciborskii populations will allow for the creation of appropriate management and monitoring programs for this dynamic cyanobacterium.

Degree

Ph.D.

Advisors

Lembi, Purdue University.

Subject Area

Microbiology

COinS