Macrocystis pyrifera gametophytes growing under controlled conditions produce juveniles with different numbers of fronds, i.e. few (≤ 2) or multiple fronds (≥ 3). We describe these morphotypes and test whether the number of fronds may impact juvenile growth by performing laboratory and field experiments. Kelp reproductive material was collected in southern Chile and 133 female gametophytes were individually monitored for 90 days until they become fertilized and formed sporophytes. Samples of each individual frond were taken for genetic analysis to determine genetic homogeneity. A factorial experiment combining temperature (12 and 18 °C) and N-NO3 (N-NO3 limited and N + NO3 non-limited concentrations) was conducted using juvenile sporophytes. Growth was assessed by measuring individual surface area and wet weight, and the physiological status by quantifying photosynthetic pigments, nitrate reductase activity and photosynthetic parameters. Field experiments with both morphotypes were carried out to measure growth, on the seabed within a natural kelp bed, and a farm using sporophytes. Following the development of female gametophytes with different numbers of oogonia, we found that those with a higher number of oogonia produced a sporophytic unit united by a single holdfast (i.e. multiple fronds morphotype). These sporophytic units were genetically diverse suggesting that oogonia were fertilized by antherozoids from different male gametophytes. The laboratory experiment indicated that these sporophytic units can withstand better nitrogen limiting conditions at 12 °C as shown by a significantly higher growth rate. This trend was also found for chlorophyll and partially supported by the photosynthetic parameters (higher light compensation point). Nitrate reductase activity revealed that under N limitation its activity increased only in the multiple frond morphotype, independent of temperature. M. pyrifera multiple frond morphotype out-planted to on-growing sites showed a significant higher biomass. These results indicate that morphotype is as important as genetic management when pursuing kelp growth.
