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Darling, Joshua ¹ ; Garland, Deena ² ; Stanish, Lee ³ ; Esposito, Rhea ⁴ ;Sokol, Eric ⁵ ; McKnight, Diane ⁶

¹ ENVS, INSTAAR
² INSTAAR
³ INSTAAR
⁴ INSTAAR
⁵ INSTAAR
⁶ CEAE, ENVS, INSTAAR

Benthic microbial mats are the dominant primary producers in the McMurdo Dry Valleys in Antarctica, which make up one of the world’s most extreme desert ecosystems. These perennial mats are found in streambeds and are metabolically resurrected for short growing seasons (4-10 weeks) during the austral summer when glacial meltwater rehydrates the stream channel. Diverse diatom assemblages, including many endemic species, are resident to these mat colonies and able to tolerate the harsh Antarctic habitat; with long periods of desiccation, intense solar radiation, variable flow, and—when streams do flow—large diel fluctuations in water temperature. To understand how diatoms have adapted physiologically to this extreme habitat, we measured the intrinsic growth rates of several species collected from McMurdo Dry Valley streams and grown in lab microcosms with controlled temperature treatments. We hypothesized that Antarctic endemic diatom species would illustrate psychrophilic behavior and grow faster at colder temperatures and that their intrinsic growth rates would vary by temperature. The taxa investigated were isolates of Psammothidium papilio (Antarctic endemic), Hantzschia abundans (widespread), Hantzschia amphioxys (widespread), and Hantzschia amphioxys fo. muelleri (South Victoria Land endemic). These four pennate diatom species were cultured in liquid medium and incubated at 7.6° C, 10° C, and 15° C with a 12:12 light:dark cycle. Growth rates of replicate cultures were estimated by in-vivo chlorophyll a fluorescence measurements over time. Each of the four diatom species examined here exhibited slow growth rates in all temperatures (< 0.22 d-1). And we found that psychrophilic behavior was not a trait only limited to—and by—species considered endemic to Antarctica. Mean growth rates of P. papilio remained statistically similar and did not vary in the three temperature treatments, with an indicated growth tolerance to the temperatures tested. Whereas H. amphioxys fo. muelleri exhibited optimal growth at 10° C and could be considered psychrophilic. Conversely, for the widespread H. amphioxys species, low survival was determined at 15° C but showed stimulated growth at lower temperatures. Although this species is presumably widespread, our results indicate psychrophilic behavior with possible growth adaptations to the isolated polar habitat. In contrast, the widespread species, H. abundans, showed a preference for warmer temperatures and grew fastest at 15° C. These results present the idea that temperature growth rate relationships may provide a better understanding of diatom habitat preferences and variation in abundance by Dry Valley streams.