| Summary: | Filamentous algal endophytes are common in many species of macroalgae along the Antarctic Peninsula, but their influence on host physiology is unknown. However, worldwide endophyte–macroalgae symbioses are known to be detrimental to vital functions of a host. The growth and survival of four Antarctic rhodophyte species were examined in situ under varying loads of endophyte infection. Growth was measured through relative growth rate and surface-area-corrected growth rate, and survivorship of individuals was documented throughout the experiment. The relationship between hosts and their endophytes was best described as innocuous in Myriogramme manginii, mildly pathogenic in Gymnogongrus turquetii and Trematocarpus antarcticus, and pathogenic in Iridaea cordata. Deterioration of thalli and decreased growth rates may be natural in the late austral summer when this experiment took place; however, the effects of increased infection probably expedited deterioration of the host. Endophytes in this study were pigmented green and brown filamentous algae, most of which are never seen as free-living thalli, and some of which may be obligate endophytes.
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