The Washington Nationals addressed their struggling bullpen by acquiring All-Star closer Mark Melancon from the Pittsburgh Pirates, the teams announced Saturday.The Pirates received left-handed reliever Felipe Rivero and minor league left-hander Taylor Hearn as part of the trade.The 31-year-old, right-handed Melancon is 1-1 with a 1.51 ERA and 30 saves in 45 appearances this season. He is a three-time All-Star with a 2.65 career ERA and 151 saves in eight seasons with the New York Yankees, Houston Astros, Boston Red Sox and Pirates. He saved a league-high 51 games in 2015. Melancon is expected to serve as the closer for the Nationals after veteran closer Jonathan Papelbon struggled to close out games in the ninth over the past week.Rivero, 25, went 0-3 with one save and a 4.53 ERA in 47 relief appearances this season with the Nationals. He was 2-4 with a 3.67 ERA in 96 career games for Washington.Hearn, 21, went a combined 1-0 with a 2.79 ERA in 10 games (four starts) between the Rookie-level Gulf Coast League Nationals and Class A Hagerstown.--The San Francisco Giants activated outfielder Hunter Pence from the 15-day disabled list after being out for two months with a torn hamstring.Pence tore his right hamstring off the bone while running the bases during a game on June 1 and had surgery a week later. Before going on the disabled list, Pence was batting .298 with seven home runs and 36 RBIs.The Giants designated infielder Ramiro Pena for assignment to make room for Pence on the active roster.--The Seattle Mariners placed right-handed pitcher Nathan Karns on the 15-day disabled list with a low back strain.Karns, 28, is 6-2 with a 5.15 ERA over 94 1/3 innings with 101 strikeouts and 45 walks in 22 games this season. Karns was 6-2 with a 4.56 ERA over 81 innings in 15 starts before being demoted to the bullpen. In relief, he has recorded one save while posting an 8.78 ERA over 13 1/3 innings with 17 strikeouts and seven walks in seven appearances.The Mariners also recalled right-hander Cody Martin from Triple-A Tacoma.Martin, 26, is 9-6 with a 3.95 ERA over 98 innings with 96 strikeouts and 30 walks in 22 games (17 starts) with Tacoma. In one prior stint with Seattle this season, he is 1-0 with a 2.25 ERA over four innings with two strikeouts and one walk in two games.--Chicago Cubs first baseman Anthony Rizzo was held out of the starting lineup Saturday after starting the previous 30 games in a row.Rizzo has played 99 games this season, which is second on the team to third baseman Kris Bryant."He has really been needing a day (off)," Cubs manager Joe Maddon said. "We rely on him so heavily."