Tissue-Specific Tolerance to High-Temperature and Nutrient-Poor Conditions in a Canopy-Forming Macroalga, Surviving at an Ocean Warming Hotspot
Hikaru Endo | Masafumi Kodama | Ryoya Kawashima | Momochika Kumagai | Midori Matsuoka | Keigo Ebata | Suguru Okunishi
Most canopy-forming macroalgae have disappeared from temperate reefs in southern Japan, one of the ocean warming hotspots, but <i>Sargassum nipponicum</i> is surviving in this region. As this species’ annual shoots emerge from holdfasts during summer, both plant components may be highly tolerant to warm and nutrient-poor conditions in this season. The present study examined the effects of temperature and nutrient conditions on holdfast growth, shoot emergence from holdfasts, and shoot growth in <i>S</i>. <i>nipponicum</i> samples collected in Tanegashima Island, southern Japan. The summer temperature in this region (30 °C) allowed holdfast growth and shoot emergence but inhibited shoot growth. Nutrient-poor conditions had limited effects on the first two parameters but suppressed shoot growth. These results suggested that during warm summers and under nutrient-poor conditions in southern Japan, shoots can emerge from <i>S. nipponicum</i> holdfasts but cannot further grow. Additionally, nutrient loading from a nearby river was higher at the only site dominated by <i>S. nipponicum,</i> than at the other sites where this species was absent on Tanegashima Island. This was observed especially between autumn and winter, implying that such a nutrient-rich environment may contribute to shoot growth in <i>S</i>. <i>nipponicum</i> and to the persistence of its population in the area.
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