While it's powered by DC, the signals are high frequency differential pairs on transformers to prevent the need for dc coupled devices. The AC capacitors couple the differential pairs between the transceiver and receiver, and managed the dc bias blocking.
“To elaborate on @Majenko's answer, both SGMII and 1000Base-x are dual 1000Mbps SERDES pairs (one in each direction), at least until the 1000Base-X signals reach the optical transceiver. The main difference is in the auto-negotiation capabilities.”
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Where is the Ground in a Simple Electronic Circuit
Electrically Grounding a Large Heatsink to PCB Layer Ground
Why I need those capacitors in USB 3.0 transmission lines?
What is the exact difference between SGMII and 1000Base-X?
