Evidence for psychrophiles outnumbering psychrotolerant marine bacteria in the springtime coastal Arctic

During a springtime study of pelagic microbial activities in the coastal Alaskan Arctic (Chukchi Sea, June 1998), a selective serial dilution technique was used to estimate the relative abundance of cold-loving (psychrophilic) and cold-tolerant (psychrotolerant or psychrotrophic) pelagic bacteria at four stations representing different stages of an algal bloom. Psychrophiles were numerically dominant, regardless of the organic nutrient concentration of their source waters (51–101 mmol L21 total organic carbon; 2–17 mmol L21 total organic nitrogen). At all stations, lower temperature incubations generated significantly higher most probable number (MPN) estimates. Further, these psychrophiles were unable to acclimate to 208C over the course of a 3-month incubation. Our pelagic experiments are the first of their kind reported from the Western Arctic, where surface waters are dominated by the Pacific Ocean inflow, and suggest that psychrophiles can dominate even in waters only recently subjected to perennially cold conditions, independent of nutrient status or bloom condition. Active microorganisms living in low-temperature environments are comprised primarily of two groups: psychrophilic and psychrotolerant (sometimes called psychrotrophic) organisms. According to a widely accepted definition proposed by Morita (1975), psychrophiles have optimum growth at #158C, with a maximum growth temperature ,208C and a minimum growth temperature #08C (see recent reviews by Russell 1990; Gounot 1991; Karl 1993 and reference therein, especially earlier reviews of Baross and Morita 1978 and Morita 1966, 1975). Despite living at these low temperatures, psychrophiles often have metabolic rates comparable with those displayed by microorganisms adapted to more moderate temperatures. Psychrotolerant organisms are typically considered able to grow near freezing but with optimum growth .208C (Russell 1990), although some argue for a continuum of adaptation (Gerday et al. 2000). The two main environmental factors thought to control competition between psychrophilic and psychrotolerant groups are nutrient availability and temperature (Sieburth 1967; Herbert 1986; Deming 2002). In culture, temperature and nutrients were found to determine the outcome of competition between a psychrophilic and a psychrotolerant spe1 Present address: Memorial University, Ocean Sciences Centre, St. John’s, Newfoundland and Labrador, A1C 5S7 Canada. 2 Corresponding author ([email protected]).