Giant kelp ( <i>Macrocystis pyrifera</i> ) gametophytes grow endophytically in red macroalgae: Implications for kelp restoration

Giant kelp (Macrocystis pyrifera) forests have declined by 95% in Australia since the 1970s due to climate change. Despite expanding restoration efforts, long-term recovery is still lacking. Factors influencing recruitment by the microscopic gametophyte phase may represent a key mechanism underpinning the successful recovery and restoration of M. pyrifera. In the northern hemisphere, kelp gametophytes grow epiphytically and endophytically in red macroalgae and may represent an important "propagule bank" for kelp gametophyte recruitment. Our goal was to determine if M. pyrifera recruitment might depend on endo- or epiphytic gametophyte growth with understory red macroalgae on reefs in Tasmania, Australia. We first examined the composition and biomass of understory macroalgae living under kelp forests in Tasmania. We then tested if M. pyrifera gametophytes were located epiphytically or endophytically within specific red macroalgal hosts of 13 common species. Digital polymerase chain reaction (PCR) with M. pyrifera primers found M. pyrifera gametophytes in eight species of red macroalgae (Ballia callitricha, Gloiocladia halymenioides, Halopeltis australis, Phacelocarpus pepperocarpos, Plocamium angustum, Plocamium dilatatumi, Ptilonia australasica, and Ptilonia subulifera), with Ballia callitricha being the most common host (66.7% positive detections). By sterilizing the surface of host tissue, the detection of M. pyrifera DNA was consistent with endophytic occurrence of kelp gametophytes within the red macroalgal hosts but did not exclude the possibility that they also live epiphytically. These results suggest that red macroalgae may provide an important propagule bank for M. pyrifera gametophytes, and maintaining healthy understory assemblages may facilitate recovery and restoration efforts of M. pyrifera.